Electrostatics AP Study guide

1) Know how charges interact – Coulomb’s Law (pg. 565 – 570) (Practice questions Ch. 21: Q1-4, Q6-10) (Practice problems Ch. 21: P1 – P23)

• Positive and negative • How to calculate the force on one charge from another • How to calculate the distance between 2 charged objects given the charges and the force between them • How to calculate the charge of one of the objects given the other objects charge, the force and the distance

2) Know how to use the shell theorem (pg. 567) (Practice questions: Ch. 21: Q5)

• A shell of charge of uniform charge density can be crushed down to a point charge located at the center of the shell for objects outside it and is rendered non-existent for charged objects within it

3) Know the charge of an electron and proton and know what ‘k’ stands for (pg. 571, 566) 4) Know what the electric field is, how to interpret E-field lines, and how it is defined (pg. 581 – 584, 591-593) (Practice questions Ch. 22: Q1- Q6) (Practice problems Ch. 22: P1 – P17, P39 – P55)

• How is the electric field defined with reference to a test charge • What do the electric field lines for various charge distributions look like • How can one calculate the electric field at a particular “Place” in space from a single or multiple point

charges • How can one calculate the force on a charged particle at a “Place” in space if the E-field there is known • Is E-field a scalar or vector

5) Know how to use an integral and symmetry to find the E-field at a “Place” in space from a continuous charge

distribution (straight line, circular arc, circular ring, disc, etc.) (pg. 586 – 591) (Practice questions Ch. 22: Q7, Q8, Q11) (Practice problems Ch. 22: P22 – P38) 6) Know when and how to use Gauss’ Law to find the electric field of a charge distribution (pg. 606 – 610, 615-620) (Practice questions Ch. 23: Q1 – Q5) (Practice problems Ch. 23: P1 – P16, P22 – P48, P53 – P55)

• Know what flux is • How to pick the correct symmetrical shape to use Gauss’ Law • How to use symmetry to make the E-field constant so you can pull it out of the integral • How to use Gauss’ Law when there is more than one charged distribution

7) Know the properties of charged, isolated conductors and insulators (pg. 612 – 614) (Practice questions Ch. 23: Q6 – Q10) (Practice problems Ch. 23: P17 – P21, P49 – P52)

• E-field inside a charged conductor • How charges distribute themselves on a conductor • Volume Charge densities for an insulator and why volume charge densities for conductors is an oxymoron

8) Know about Electrical Potential Energy – Ue (pg. 629, 641 -643) (Practice questions Ch. 24: Q7, Q9, Q10) (Practice problems Ch. 24: P41 – P 61)

• How is electrical potential energy defined • Is Electrical potential energy a scalar or vector • How is the electrical potential energy of a finite number of point particles calculated

9) Know about Electric Potential – V for point individual or groups of point charges, continuous distributions of

charge, and how V relates to Ue (pg. 630 – 632, 635 – 636, 638 – 640) (Practice questions Ch. 24: Q1 – Q3, Q5, Q8) (Practice problems Ch. 24: P1- P3, P12 – P20, P23 – P33)

• How is electric potential defined • Is electric potential a scalar or vector • How is electric potential, V, related to electric potential energy, Ue • What is an electron volt (eV)

10) Know how electric potential and the E-field are related (pg. 632 – 634, 640 – 641) (Practice questions Ch 24: Q4, Q6) (Practice problems Ch 24: P4 – P11, P34 – P40)

• What is an equipotential surface and how do the lines of equipotential surfaces relate to E-field lines • How to calculate the potential difference (ΔV) (note: ΔV = Vf – Vi ) through an E-field given the E-field

function • How to calculate the E-field from the potential difference given the potential function

11) Know how potential is related to conductors – especially the potential inside a conductor and the E-field inside a conductor suspended in an external E-field (pg. 644 – 645) (Practice problems Ch. 24: P62 – P67)

• How does the potential change throughout a conductor • What is the E-field inside a conductor (always)

1. Eq. 21-1 gives Coulomb’s Law, F k q qr

1 22 , which we solve for the distance:

9 2 2 6 61 2

8.99 10 N m C 26.0 10 C 47.0 10 C| || | 1.39m.5.70N

k q qrF

2. (a) With a understood to mean the magnitude of acceleration, Newton’s second and third laws lead to

m a m a m2 2 1 1 2

77

6 3 10 7 09 0

4 9 10. .

..

kg m sm s

kg.2

2

c hc h

(b) The magnitude of the (only) force on particle 1 is

21 2 9 2 2

1 1 2 28.99 10 N m C .(0.0032 m)

q q qF m a k

r

Inserting the values for m1 and a1 (see part (a)) we obtain |q| = 7.1 10–11 C.

3. The magnitude of the mutual force of attraction at r = 0.120 m is

6 61 2 9 2 2

2 2

3.00 10 C 1.50 10 C8.99 10 N m C 2.81N.

(0.120 m)q q

F kr

4. The fact that the spheres are identical allows us to conclude that when two spheres are in contact, they share equal charge. Therefore, when a charged sphere (q) touches an uncharged one, they will (fairly quickly) each attain half that charge (q/2). We start with spheres 1 and 2 each having charge q and experiencing a mutual repulsive force

2 2/F kq r . When the neutral sphere 3 touches sphere 1, sphere 1’s charge decreases to q/2. Then sphere 3 (now carrying charge q/2) is brought into contact with sphere 2, a total amount of q/2 + q becomes shared equally between them. Therefore, the charge of sphere 3 is 3q/4 in the final situation. The repulsive force between spheres 1 and 2 is finally

2

2 2

( / 2)(3 / 4) 3 3 ' 3 0.375.8 8 8

q q q FF k k Fr r F

Fq Q q

r1

4 02

b g

where r is the distance between the charges. We want the value of q that maximizes the function f(q) = q(Q – q). Setting the derivative /dF dq equal to zero leads to Q – 2q = 0, or q = Q/2. Thus, q/Q = 0.500.

5. The magnitude of the force of either of the charges on the other is given by 6. The unit Ampere is discussed in §21-4. Using i for current, the charge transferred is

4 62.5 10 A 20 10 s 0.50 C.q it

7. We assume the spheres are far apart. Then the charge distribution on each of them is spherically symmetric and Coulomb’s law can be used. Let q1 and q2 be the original charges. We choose the coordinate system so the force on q2 is positive if it is repelled by q1. Then, the force on q2 is

F q qr

k q qra

14 0

1 22

1 22

where r = 0.500 m. The negative sign indicates that the spheres attract each other. After the wire is connected, the spheres, being identical, acquire the same charge. Since charge is conserved, the total charge is the same as it was originally. This means the charge on each sphere is (q1 + q2)/2. The force is now one of repulsion and is given by

Fr

kq q

rb

q q q q1

4 40

2 22

1 22

2

1 2 1 2d id i b g .

We solve the two force equations simultaneously for q1 and q2. The first gives the product

q q r Fk

a1 2

2 2

9120500 0108

8 99 10300 10

. ..

. ,m N

N m CC2 2

2b g b g

and the second gives the sum

q q r Fk

b1 2

62 2 0 500 0 0360 2 00 10. . .m N8.99 10 N m C

C9 2 2b g

where we have taken the positive root (which amounts to assuming q1 + q2 0). Thus, the product result provides the relation

12 2

21

3.00 10 Cq

q

which we substitute into the sum result, producing

qq1

12

1

6300 10 2 00 10. .C C.2

Multiplying by q1 and rearranging, we obtain a quadratic equation

q q12 6

1122 00 10 300 10 0. . .C C2c h

The solutions are

q1

6 6 2 122 00 10 2 00 10 4 300 10

2

. . ..

C C C2c h c h

If the positive sign is used, q1 = 3.00 10–6 C, and if the negative sign is used, 6

1 1.00 10 Cq .

(a) Using q2 = (–3.00 10–12)/q1 with q1 = 3.00 10–6 C, we get 62 1.00 10 Cq .

(b) If we instead work with the q1 = –1.00 10–6 C root, then we find 62 3.00 10 Cq .

Note that since the spheres are identical, the solutions are essentially the same: one sphere originally had charge –1.00 10–6 C and the other had charge +3.00 10–6 C.

What if we had not made the assumption, above, that q1 + q2 0? If the signs of the charges were reversed (so q1 + q2 < 0), then the forces remain the same, so a charge of +1.00 10–6 C on one sphere and a charge of –3.00 10–6 C on the other also satisfies the conditions of the problem.

2 2

2 2 2 20 0

| |1 | | | | 1sin 454 4 | |2 22

y

q Qq q QFa a qa

which (if we demand F2y = 0) leads to / 1/ 2 2Q q . The result is inconsistent with that obtained in part (a). Thus, we are unable to construct an equilibrium configuration with this geometry, where the only forces present are given by Eq. 21-1.

8. For ease of presentation (of the computations below) we assume Q > 0 and q < 0 (although the final result does not depend on this particular choice).

(a) The x-component of the force experienced by q1 = Q is

1 2 2 20 0

| |1 | | / | |cos 45 14 4 2 22

x

Q Q q Q Q q Q qFa aa

which (upon requiring F1x = 0) leads to / | | 2 2Q q , or / 2 2 2.83.Q q

(b) The y-component of the net force on q2 = q is

9. The force experienced by q3 is

3 1 3 2 3 43 31 32 34 2 22

0

| || | | || | | || |1 ˆ ˆ ˆ ˆj (cos45 i sin 45 j) i4 ( 2 )

q q q q q qF F F Fa aa

(a) Therefore, the x-component of the resultant force on q3 is

279 2 23 2

3 42 20

2 1.0 10 C| | | | 1| | 8.99 10 N m C 2 0.17N.4 (0.050 m)2 2 2 2x

q qF qa

(b) Similarly, the y-component of the net force on q3 is

279 2 23 2

3 12 20

2 1.0 10 C| | | | 1| | 8.99 10 N m C 1 0.046N.4 (0.050 m)2 2 2 2y

q qF qa

10. (a) The individual force magnitudes (acting on Q) are, by Eq. 21-1,

1 22 2

0 0

1 14 4/ 2 / 2

q Q q Qa a a a

which leads to |q1| = 9.0 |q2|. Since Q is located between q1 and q2, we conclude q1 and q2are like-sign. Consequently, q1/q2 = 9.0.

(b) Now we have 1 2

2 20 0

1 14 43 / 2 3 / 2

q Q q Qa a a a

which yields |q1| = 25 |q2|. Now, Q is not located between q1 and q2, one of them must push and the other must pull. Thus, they are unlike-sign, so q1/q2 = –25.

1 3 2 33 13 23 2 2

2312 23

.q q q qF F F k kLL L

We note that each term exhibits the proper sign (positive for rightward, negative for leftward) for all possible signs of the charges. For example, the first term (the force exerted on q3 by q1) is negative if they are unlike charges, indicating that q3 is being pulled toward q1, and it is positive if they are like charges (so q3 would be repelled from q1). Setting the net force equal to zero L23= L12 and canceling k, q3 and L12 leads to

1 12

2

0 4.00.4.00q qq

q

11. With rightwards positive, the net force on q3 is

12. As a result of the first action, both sphere W and sphere A possess charge 12 qA , where qA is the initial charge of sphere A. As a result of the second action, sphere W has charge

1 322 2

Aq e .

As a result of the final action, sphere W now has charge equal to

1 1 32 482 2 2

Aq e e .

Setting this final expression equal to +18e as required by the problem leads (after a couple of algebra steps) to the answer: qA = +16e.

13. (a) Eq. 21-1 gives

269 2 21 2

12 22

20.0 10 C8.99 10 N m C 1.60N.

1.50mq qF kd

(b) On the right, a force diagram is shown as well as our choice of yaxis (the dashed line).

The y axis is meant to bisect the line between q2 and q3 in order to make use of the symmetry in the problem (equilateral triangle of side length d, equal-magnitude charges q1 = q2 = q3 = q). We see that the resultant force is along this symmetry axis, and we obtain

2629 2 2

22

20.0 10 C2 cos30 2 8.99 10 N m C cos30 2.77 N

1.50myqF kd

.

14. (a) According to the graph, when q3 is very close to q1 (at which point we can consider the force exerted by particle 1 on 3 to dominate) there is a (large) force in the positive x direction. This is a repulsive force, then, so we conclude q1 has the same sign as q3. Thus, q3 is a positive-valued charge.

(b) Since the graph crosses zero and particle 3 is between the others, q1 must have the same sign as q2, which means it is also positive-valued. We note that it crosses zero at r= 0.020 m (which is a distance d = 0.060 m from q2). Using Coulomb’s law at that point, we have

2 21 3 3 2

2 1 1 12 20 0

1 1 0.060 m 9.04 4 0.020 m

q q q q dq q q qr d r

,

or q2/q1 = 9.0.

15. (a) There is no equilibrium position for q3 between the two fixed charges, because it is being pulled by one and pushed by the other (since q1 and q2 have different signs); in this region this means the two force arrows on q3 are in the same direction and cannot cancel. It should also be clear that off-axis (with the axis defined as that which passes through the two fixed charges) there are no equilibrium positions. On the semi-infinite region of the axis which is nearest q2 and furthest from q1 an equilibrium position for q3 cannot be found because |q1| < |q2| and the magnitude of force exerted by q2 is everywhere (in that region) stronger than that exerted by q1 on q3. Thus, we must look in the semi-infinite region of the axis which is nearest q1 and furthest from q2, where the net force on q3 has magnitude

1 3 2 322

0 0

q q q qk k

L L L

with L = 10 cm and 0L is assumed to be positive. We set this equal to zero, as required by the problem, and cancel k and q3. Thus, we obtain

21 2 0 2

220 0 10

3.0 C0 3.01.0 C

q q L L qL L qL L

which yields (after taking the square root)

00

0

10 cm3 14cm3 1 3 1

L L LLL

for the distance between q3 and q1. That is, 3q should be placed at 14 cmx along the x-axis.

(b) As stated above, y = 0.

16. Since the forces involved are proportional to q, we see that the essential difference between the two situations is Fa qB + qC (when those two charges are on the same side) versus Fb qB + qC (when they are on opposite sides). Setting up ratios, we have

Fa Fb

=qB + qC

- qB + qC

23

24

1 /2.014 10 N2.877 10 N 1 /

C B

C B

q qq q

.

After noting that the ratio on the left hand side is very close to – 7, then, after a couple of algebra steps, we are led to

7 1 8 1.333.7 1 6

C

B

qq

17. (a) The distance between q1 and q2 is

2 2 2 212 2 1 2 1 0.020 m 0.035 m 0.015 m 0.005 m 0.056 m.r x x y y

The magnitude of the force exerted by q1 on q2 is

9 2 2 6 61 2

21 2 212

8.99 10 N m C 3.0 10 C 4.0 10 C| | 35 N.(0.056 m)

q qF kr

(b) The vector F21 is directed towards q1 and makes an angle with the +x axis, where

1 12 1

2 1

1.5 cm 0.5 cmtan tan 10.3 10 .2.0 cm 3.5 cm

y yx x

(c) Let the third charge be located at (x3, y3), a distance r from q2. We note that q1, q2 and q3 must be collinear; otherwise, an equilibrium position for any one of them would be impossible to find. Furthermore, we cannot place q3 on the same side of q2 where we also find q1, since in that region both forces (exerted on q2 by q3 and q1) would be in the same direction (since q2 is attracted to both of them). Thus, in terms of the angle found in part (a), we have x3 = x2 – r cos and y3 = y2 – r sin (which means y3 > y2 since is negative). The magnitude of force exerted on q2 by q3 is 2

23 2 3| |F k q q r , which must equal that of the force exerted on it by q1 (found in part (a)). Therefore,

2 3 1 2 3122 2

12 1

0.0645m 6.45 cm .q q q q qk k r rr r q

Consequently, x3 = x2 – r cos = –2.0 cm – (6.45 cm) cos(–10°) = –8.4 cm,

(d) and y3 = y2 – r sin = 1.5 cm – (6.45 cm) sin(–10°) = 2.7 cm.

18. (a) For the net force to be in the +x direction, the y components of the individual forces must cancel. The angle of the force exerted by the q1 = 40 C charge on

3 20q C is 45°, and the angle of force exerted on q3 by Q is at – where

1 2.0 cmtan 33.7 .3.0 cm

Therefore, cancellation of y components requires

1 3 32 2

2 2

| |sin 45 sin0.02 2 m (0.030 m) (0.020 m)

q q Q qk k

from which we obtain |Q| = 83 C. Charge Q is “pulling” on q3, so (since q3 > 0) we conclude Q = –83 C.

(b) Now, we require that the x components cancel, and we note that in this case, the angle of force on q3 exerted by Q is + (it is repulsive, and Q is positive-valued). Therefore,

1 3 32 2

2 2cos 45 cos

0.02 2 m (0.030 m) (0.020 m)

q q Qqk k

from which we obtain Q = 55.2 C 55 C .

23

2 20

1 4.00 .4q

qq qFx L

The signs are chosen so that a negative force value would cause q to move leftward. We require Fq = 0 and solve for q3:

23

3 2

4 4 4 0.4449 9

qqxq qL q

where x = L/3 is used. Note that we may easily verify that the force on 4.00q also vanishes:

22 2 2 20

4 22 2 2 2 20 0 0

4 4 941 4 1 4 1 4 4 04 4 4 9 4q

qqqq q q qFL L L L LL x

.

19. (a) If the system of three charges is to be in equilibrium, the force on each charge must be zero. The third charge q3 must lie between the other two or else the forces acting on it due to the other charges would be in the same direction and q3 could not be in equilibrium. Suppose q3 is at a distance x from q, and L – x from 4.00q. The force acting on it is then given by

3 33 22

0

414

qq qqFx L x

where the positive direction is rightward. We require F3 = 0 and solve for x. Canceling common factors yields 1/x2 = 4/(L – x)2 and taking the square root yields 1/x = 2/(L – x). The solution is x = L/3. With L = 9.00 cm, we have x = 3.00 cm.

(b) Similarly, the y coordinate of q3 is y = 0.

(c) The force on q is

20. (a) We note that cos(30º) = 12 3 , so that the dashed line distance in the figure is

2 / 3r d . We net force on q1 due to the two charges q3 and q4 (with |q3| = |q4| = 1.60 10 19 C) on the y axis has magnitude

1 3 1 32 2

0 0

| | 3 3 | |2 cos(30 )4 16

q q q qr d

.

This must be set equal to the magnitude of the force exerted on q1 by q2 = 8.00 10 19 C = 5.00 |q3| in order that its net force be zero:

1 3 1 22 2

0 0

3 3 | | | |16 4 ( )

q q q qd D d

D = d 2 53 3

1 = 0.9245 d .

Given d = 2.00 cm, this then leads to D = 1.92 cm.

(b) As the angle decreases, its cosine increases, resulting in a larger contribution from the charges on the y axis. To offset this, the force exerted by q2 must be made stronger, so that it must be brought closer to q1 (keep in mind that Coulomb’s law is inverselyproportional to distance-squared). Thus, D must be decreased.

We note that, due to the symmetry in the problem, there is no y component to the net force on the third particle. Thus, F represents the magnitude of force exerted by q1 or q2

on q3. Let e = +1.60 10 19 C, then q1 = q2 = +2e and q3 = 4.0e and we have

Fnet = 2F cos =2(2e)(4e)

4 o (x2 + d2)x

x2 + d2 =4e2 x

o (x2 + d2 )3/2 .

(a) To find where the force is at an extremum, we can set the derivative of this expression equal to zero and solve for x, but it is good in any case to graph the function for a fuller understanding of its behavior – and as a quick way to see whether an extremum point is a maximum or a miminum. In this way, we find that the value coming from the derivative procedure is a maximum (and will be presented in part (b)) and that the minimum is found at the lower limit of the interval. Thus, the net force is found to be zero at x = 0, which is the smallest value of the net force in the interval 5.0 m x 0.

(b) The maximum is found to be at x = d/ 2 or roughly 12 cm.

(c) The value of the net force at x = 0 is Fnet = 0.

(d) The value of the net force at x = d/ 2 is Fnet = 4.9 10 26 N.

21. If is the angle between the force and the x-axis, then

cos =x

x2 + d2 .

53 =

1 + ¼1 ¼ = 16 .

Of course, this could also be figured as illustrated in part (a), looking at the maximum force ratio by itself and solving, or looking at the minimum force ratio (¾) at = 180º and solving for .

22. We note that the problem is examining the force on charge A, so that the respective distances (involved in the Coulomb force expressions) between B and A, and between Cand A, do not change as particle B is moved along its circular path. We focus on the endpoints ( = 0º and 180º) of each graph, since they represent cases where the forces (on A) due to B and C are either parallel or antiparallel (yielding maximum or minimum force magnitudes, respectively). We note, too, that since Coulomb’s law is inversely proportional to r² then the (if, say, the charges were all the same) force due to C would be one-fourth as big as that due to B (since C is twice as far away from A). The charges, it turns out, are not the same, so there is also a factor of the charge ratio (the charge of Cdivided by the charge of B), as well as the aforementioned ¼ factor. That is, the force exerted by C is, by Coulomb’s law equal to ±¼ multiplied by the force exerted by B.

(a) The maximum force is 2F0 and occurs when = 180º (B is to the left of A, while C is the right of A). We choose the minus sign and write

2 F0 = (1 ¼ ) F0 = – 4 .

One way to think of the minus sign choice is cos(180º) = –1. This is certainly consistent with the minimum force ratio (zero) at = 0º since that would also imply

0 = 1 + ¼ = – 4 .

(b) The ratio of maximum to minimum forces is 1.25/0.75 = 5/3 in this case, which implies

23. The charge dq within a thin shell of thickness dr is dq dV Adr where A = 4 r2.Thus, with = b/r, we have

q dq b r dr b r rr

rzz 4 21

2

22

12c h.

With b = 3.0 C/m2, r2 = 0.06 m and r1 = 0.04 m, we obtain q = 0.038 C = 3.8 10 8 C.

1. (a) We note that the electric field points leftward at both points. Using F q E0 , and orienting our x axis rightward (so i points right in the figure), we find

19 18N ˆ ˆ1.6 10 C 40 i ( 6.4 10 N) iC

F

which means the magnitude of the force on the proton is 6.4 10–18 N and its direction ˆ( i) is leftward.

(b) As the discussion in §22-2 makes clear, the field strength is proportional to the“crowdedness” of the field lines. It is seen that the lines are twice as crowded at A than at B, so we conclude that EA = 2EB. Thus, EB = 20 N/C.

2. We note that the symbol q2 is used in the problem statement to mean the absolute value of the negative charge which resides on the larger shell. The following sketch is for

1 2q q .

The following two sketches are for the cases q1 > q2 (left figure) and q1 < q2 (right figure).

3. Since the magnitude of the electric field produced by a point charge q is given by 2

0| | / 4E q r , where r is the distance from the charge to the point where the field has magnitude E, the magnitude of the charge is

22 11

0 9 2 2

0.50m 2.0 N C4 5.6 10 C.

8.99 10 N m Cq r E

4. We find the charge magnitude |q| from E = |q|/4 0r2:

22 10

0 9 2 2

1.00 N C 1.00m4 1.11 10 C.

8.99 10 N m Cq Er

5. Since the charge is uniformly distributed throughout a sphere, the electric field at the surface is exactly the same as it would be if the charge were all at the center. That is, the magnitude of the field is

E qR4 0

2

where q is the magnitude of the total charge and R is the sphere radius.

(a) The magnitude of the total charge is Ze, so

E ZeR4

8 99 10 94 160 10

6 64 103 07 10

02

9 2 2 19

15 221

. .

.. .

N m C C

mN C

c hb gc hc h

(b) The field is normal to the surface and since the charge is positive, it points outward from the surface.

6. With x1 = 6.00 cm and x2 = 21.00 cm, the point midway between the two charges is located at x = 13.5 cm. The values of the charge are q1 = –q2 = – 2.00 10–7 C, and the magnitudes and directions of the individual fields are given by:

9 2 2 751

1 220 1

9 2 2 752

2 220 2

| | (8.99 10 N m C )| 2.00 10 C|ˆ ˆ ˆi i (3.196 10 N C)i4 ( ) 0.135 m 0.060 m

(8.99 10 N m C )(2.00 10 C)ˆ ˆ ˆi i (3.196 10 N C)i4 ( ) 0.135 m 0.210 m

qEx x

qEx x

Thus, the net electric field is

5net 1 2

ˆ(6.39 10 N C)iE E E

Let x be the coordinate of P, the point where the field vanishes. Then, the total electric field at P is given by

2 122

0 2 1

| | | |14 ( )

q qEx x x x

.

If the field is to vanish, then

22 1 2 2

2 222 11 1

| | | | | | ( ) .( ) | |

q q q x xx x qx x x x

Taking the square root of both sides, noting that |q2|/|q1| = 4, we obtain

70 cm 2.020 cm

xx

.

Choosing –2.0 for consistency, the value of x is found to be x = 30 cm.

7. At points between the charges, the individual electric fields are in the same direction and do not cancel. Since charge q2= 4.00 q1 located at x2 = 70 cm has a greater magnitude than q1 = 2.1 10 8 C located at x1 = 20 cm, a point of zero field must be closer to q1 than to q2. It must be to the left of q1.

Thus, we obtain 2.721 2 5

Lx L .

(b) A sketch of the field lines is shown in the figure below:

8. (a) The individual magnitudes E1 and E2 are figured from Eq. 22-3, where the absolute value signs for q2 are unnecessary since this charge is positive. Whether we add the magnitudes or subtract them depends on if E1 is in the same, or opposite, direction as E2 . At points left of q1 (on the –x axis) the fields point in opposite directions, but there is no possibility of cancellation (zero net field) since E1 is everywhere bigger than E2 in this region. In the region between the charges (0 < x < L) both fields point leftward and there is no possibility of cancellation. At points to the right of q2 (where x > L), E1 points leftward and E2 points rightward so the net field in this range is

net 2 1ˆ| | | | iE E E .

Although |q1| > q2 there is the possibility of Enet 0 since these points are closer to q2

than to q1. Thus, we look for the zero net field point in the x > L region:

1 21 2 22

0 0

| |1 1| | | | 4 4

q qE Ex x L

which leads to

2

1

2 .| | 5qx L

x q

9. The x component of the electric field at the center of the square is given by

31 2 42 2 2 2

0

1 2 3 420

| || | | | | |1 cos 454 ( / 2) ( / 2) ( / 2) ( / 2)

1 1 1| | | | | | | |4 / 2 20.

xqq q qE

a a a a

q q q qa

Similarly, the y component of the electric field is

31 2 42 2 2 2

0

1 2 3 420

9 2 2 85

2

| || | | | | |1 cos 454 ( / 2) ( / 2) ( / 2) ( / 2)

1 1 1| | | | | | | |4 / 2 2

8.99 10 N m / C (2.0 10 C) 1 1.02 10 N/C.(0.050 m) / 2 2

yqq q qE

a a a a

q q q qa

Thus, the electric field at the center of the square is 5ˆ ˆj (1.02 10 N/C)j.yE E

| | | |E E1 2 ), and the net field (if there is any) should be along the y axis, with magnitude equal to

Eqd

qd

qd

qdnet j j

FHG

IKJ

FHG

IKJ

14 2

14

124

3

0

42

32

02 2b g

which is seen to be zero. A rough sketch of the field lines is shown below:

10. We place the origin of our coordinate system at point P and orient our y axis in the direction of the q4 = –12q charge (passing through the q3 = +3q charge). The x axis is perpendicular to the y axis, and thus passes through the identical q1 = q2 = +5q charges. The individual magnitudes | |, | |, | |,E E E1 2 3 and | |E4 are figured from Eq. 22-3, where the absolute value signs for q1, q2, and q3 are unnecessary since those charges are positive (assuming q > 0). We note that the contribution from q1 cancels that of q2 (that is,

11. (a) The vertical components of the individual fields (due to the two charges) cancel, by symmetry. Using d = 3.00 m and y = 4.00 m, the horizontal components (both pointing to the –x direction) add to give a magnitude of

9 2 2 19

,net 2 2 3/ 2 2 2 3/ 20

10

2 | | 2(8.99 10 N m C )(3.20 10 C)(3.00 m)4 ( ) [(3.00 m) (4.00 m) ]

1.38 10 N/C .

xq dE

d y .

(b) The net electric field points in the –x direction, or 180 counterclockwise from the +xaxis.

(a) If we differentiate Enet with respect to x and set equal to zero (in order to find where it is maximum), we obtain (after some simplification) that location:

x = 23

32 +

13

34 +

13 L = 1.70(20 cm) = 34 cm.

We note that the result for part (a) does not depend on the particular value of .

(b) Now we are asked to set = 3e, where e = 1.60 10 19 C, and evaluate Enet at the value of x (converted to meters) found in part (a). The result is 2.2 10 8 N/C .

12. For it to be possible for the net field to vanish at some x > 0, the two individual fields (caused by q1 and q2) must point in opposite directions for x > 0. Given their locations in the figure, we conclude they are therefore oppositely charged. Further, since the net field points more strongly leftward for the small positive x (where it is very close to q2) then we conclude that q2 is the negative-valued charge. Thus, q1 is a positive-valued charge. We write each charge as a multiple of some positive number (not determined at this point). Since the problem states the absolute value of their ratio, and we have already inferred their signs, we have q1 = 4 and q2 = . Using Eq. 22-3 for the individual fields, we find

Enet = E1 + E2 =4

4 o (L + x)2 – 4 o x2

for points along the positive x axis. Setting Enet = 0 at x = 20 cm (see graph) immediately leads to L = 20 cm.

13. By symmetry we see the contributions from the two charges q1 = q2 = +e cancel each other, and we simply use Eq. 22-3 to compute magnitude of the field due to q3 = +2e.

(a) The magnitude of the net electric field is

net 2 220 0 0

199 2 2

6 2

1 2 1 2 1 4| |4 4 4( / 2)

4(1.60 10 C)(8.99 10 N m C ) 160 N/C.(6.00 10 m)

e e eEr aa

(b) This field points at 45.0°, counterclockwise from the x axis.

14. The field of each charge has magnitude

199 2 2 6

22 2

1.60 10 C(8.99 10 N m C ) 3.6 10 N C.(0.020 m)0.020m

kq eE kr

The directions are indicated in standard format below. We use the magnitude-angle notation (convenient if one is using a vector-capable calculator in polar mode) and write (starting with the proton on the left and moving around clockwise) the contributions to Enet as follows:

E E E E E20 130 100 150 0b g b g b g b g b g.

This yields 393 10 76 46. .c h , with the N/C unit understood.

(a) The result above shows that the magnitude of the net electric field is 6

net| | 3.93 10 N/C.E

(b) Similarly, the direction of Enet is –76.4 from the x axis.

15. (a) The electron ec is a distance r = z = 0.020 m away. Thus,

9 2 2 196

2 20

(8.99 10 N m C )(1.60 10 C) 3.60 10 N/C4 (0.020 m)C

eEr

.

(b) The horizontal components of the individual fields (due to the two es charges) cancel, and the vertical components add to give

9 2 2 19

s,net 2 2 3/ 2 2 2 3/ 20

6

2 2(8.99 10 N m C )(1.6 10 C)(0.020 m)4 ( ) [(0.020 m) (0.020 m) ]

2.55 10 N/C .

ezER z

(c) Calculation similar to that shown in part (a) now leads to a stronger field 43.60 10 N/CcE from the central charge.

(d) The field due to the side charges may be obtained from calculation similar to that shown in part (b). The result is Es, net = 7.09 10 7 N/C.

(e) Since Ec is inversely proportional to z2, this is a simple result of the fact that z is now much smaller than in part (a). For the net effect due to the side charges, it is the “trigonometric factor” for the y component (here expressed as z/ r ) which shrinks almost linearly (as z decreases) for very small z, plus the fact that the x components cancel, which leads to the decreasing value of Es, net .

16. The net field components along the x and y axes are

1 2 2net, net, 2 2 2

0 0 0

cos sin, .4 4 4x y

q q qE ER R R

The magnitude is the square root of the sum of the components-squared. Setting the magnitude equal to E = 2.00 105 N/C, squaring and simplifying, we obtain

2 22 1 1 1 2

2 20

2 cos(4 )

q q q qER

.

With R = 0.500 m, q1 = 2.00 10 6 C and q2 = 6.00 10 6 C, we can solve this expression for cos and then take the inverse cosine to find the angle:

2 2 2 2 21 1 1 0

1 2

(4 )cos2

q q R Eq q

.

There are two answers.

(a) The positive value of angle is = 67.8 .

(b) The positive value of angle is = 67.8 .

17. We make the assumption that bead 2 is in the lower half of the circle, partly because it would be awkward for bead 1 to “slide through” bead 2 if it were in the path of bead 1 (which is the upper half of the circle) and partly to eliminate a second solution to the problem (which would have opposite angle and charge for bead 2). We note that the net y component of the electric field evaluated at the origin is negative (points down) for all positions of bead 1, which implies (with our assumption in the previous sentence) that bead 2 is a negative charge.

(a) When bead 1 is on the +y axis, there is no x component of the net electric field, which implies bead 2 is on the –y axis, so its angle is –90°.

(b) Since the downward component of the net field, when bead 1 is on the +y axis, is of largest magnitude, then bead 1 must be a positive charge (so that its field is in the same direction as that of bead 2, in that situation). Comparing the values of Ey at 0° and at 90° we see that the absolute values of the charges on beads 1 and 2 must be in the ratio of 5 to 4. This checks with the 180° value from the Ex graph, which further confirms our belief that bead 1 is positively charged. In fact, the 180° value from the Ex graph allows us to solve for its charge (using Eq. 22-3):

q1 = 4 or²E = 4 ( 8.854 10 12 C2

N m2 )(0.60 m)2 (5.0 104 NC ) = 2.0 10 6 C .

(c) Similarly, the 0° value from the Ey graph allows us to solve for the charge of bead 2:

q2 = 4 or²E = 4 ( 8.854 10 12 C2

N m2 )(0.60 m)2 (– 4.0 104 NC ) = –1.6 10 6 C .

L = (0.0400 m)(0.698 rad) = 0.0279 m.

With q = 300(1.602 10 19 C), we obtain = 1.72 10 15 C/m.

(b) We consider the same charge distributed over an area A = r2 = (0.0200 m)2 and obtain = q/A = 3.82 10 14 C/m².

(c) Now the area is four times larger than in the previous part (Asphere = 4 r2) and thus obtain an answer that is one-fourth as big:

= q/Asphere = 9.56 10 15 C/m².

(d) Finally, we consider that same charge spread throughout a volume of V = 4 r3/3 and obtain the charge density = /q V = 1.43 10 12 C/m3.

22. (a) We use the usual notation for the linear charge density: = q/L. The arc length is L = r with is expressed in radians. Thus,

23. We use Eq. 22-3, assuming both charges are positive. At P, we have

1 2left ring right ring 3/ 2 2 2 3/ 22 2

00

(2 )4 [(2 ) ]4

q R q RE ER RR R

Simplifying, we obtain 3/ 2

1

2

22 0.506.5

qq

net, 2 20 0

9 2 2 12

2 2

1 2 2 | | 1 4| |2 cos 454 4

(8.99 10 N m C )4(4.50 10 C) 20.6 N/C.(5.00 10 m)

xq qE

r r

(b) By symmetry, the net field points vertically downward in the j direction, or 90counterclockwise from the +x axis.

24. Studying Sample Problem 22-3, we see that the field evaluated at the center of curvature due to a charged distribution on a circular arc is given by

0

sin4

Er

along the symmetry axis, with = q/r with in radians. In this problem, each charged quarter-circle produces a field of magnitude

/ 4

2/ 40 0

| | 1 1 2 2 | || | sin ./ 2 4 4

q qEr r r

That produced by the positive quarter-circle points at – 45°, and that of the negative quarter-circle points at +45°.

(a) The magnitude of the net field is

25. From symmetry, we see that the net field at P is twice the field caused by the upper semicircular charge q R (and that it points downward). Adapting the steps leading to Eq. 22-21, we find

90

net 2 2900 0

ˆ ˆ2 j sin j.4

qER R

(a) With R = 8.50 10 2 m and q = 1.50 10 8 C, net| | 23.8 N/C.E

(b) The net electric field netE points in the j direction, or 90 counterclockwise from the +x axis.

26. We find the maximum by differentiating Eq. 22-16 and setting the result equal to zero.

ddz

qz

z R

q R z

z R4 42 0

02 2 3 2

0

2 2

2 2 5 2

FHGG

IKJJc h c h/ /

which leads to z R / 2 . With R = 2.40 cm, we have z = 1.70 cm.

Let dx be an infinitesimal length of rod at x. The charge in this segment is dq dx . The charge dq may be considered to be a point charge. The electric field it produces at point Phas only an x component and this component is given by

dE dxL a xx

14 0

2b g .

The total electric field produced at P by the whole rod is the integral

20 00 0 0

0 0

1 1 14 4 4

,4 4

LL

xdxE

L a x a L aL a xL q

a L a a L a

upon substituting q L . With q = 4.23 10 15 C, L =0.0815 m and a = 0.120 m, we obtain 31.57 10 N/CxE , or 3| | 1.57 10 N/CxE .

(c) The negative sign in xE indicates that the field points in the –x direction, or 180counterclockwise form the +x axis.

(d) If a is much larger than L, the quantity L + a in the denominator can be approximated by a and the expression for the electric field becomes

E qax 4 0

2 .

Since 50 m 0.0815 m,a L the above approximation applies and we have 81.52 10 N/CxE , or 8| | 1.52 10 N/CxE .

(e) For a particle of charge 154.23 10 C,q the electric field at a distance a = 50 m away has a magnitude 8| | 1.52 10 N/CxE .

27. (a) The linear charge density is the charge per unit length of rod. Since the charge is uniformly distributed on the rod,

15144.23 10 C 5.19 10 C/m.

0.0815 mq

L

(b) We position the x axis along the rod with the origin at the left end of the rod, as shown in the diagram.

28. We use Eq. 22-16, with “q” denoting the charge on the larger ring:

3/ 2

2 2 3/ 2 2 2 3/ 20 0

130 4.194 ( ) 4 [ (3 ) ] 5

qz qz q Q Qz R z R

.

Note: we set z = 2R in the above calculation.

length for each arc is = q/L where each charge q is specified in the figure. Following the steps that lead to Eq. 22-21 in Sample Problem 22-3, we find

31 2net 2 2

0 1 0 2 0 3 0

(2sin 45 )(2sin 45 ) (2sin 45 )4 4 4 2

QEr r r R

which yields Enet = 1.62 10 N/C .

(b) The direction is – 45º, measured counterclockwise from the +x axis.

29. The smallest arc is of length L1 = r1 /2 = R/2; the middle-sized arc has length 2 2 / 2 (2 ) / 2L r R R ; and, the largest arc has L3 = R)/2. The charge per unit

30. (a) It is clear from symmetry (also from Eq. 22-16) that the field vanishes at the center.

(b) The result (E = 0) for points infinitely far away can be reasoned directly from Eq. 22-16 (it goes as 1/z² as z ) or by recalling the starting point of its derivation (Eq. 22-11, which makes it clearer that the field strength decreases as 1/r² at distant points).

(c) Differentiating Eq. 22-16 and setting equal to zero (to obtain the location where it is maximum) leads to

2 2

3/ 2 5/ 22 2 2 200

2 0 0.7074 24

d qz q R z Rz Rdz z R z R

.

(d) Plugging this value back into Eq. 22-16 with the values stated in the problem, we find Emax = 3.46 107 N/C.

2particle 0

2arc 0

/ 42 sin( / 2) / 4 2sin( / 2)

E Q RE Q R

.

With , we have particle

arc

1.57.2

EE

31. First, we need a formula for the field due to the arc. We use the notation for the charge density, = Q/L. Sample Problem 22-3 illustrates the simplest approach to circular arc field problems. Following the steps leading to Eq. 22-21, we see that the general result (for arcs that subtend angle ) is

arc0 0

2 sin( / 2)sin( / 2) sin( / 2)4 4

Er r

.

Now, the arc length is L = r if is expressed in radians. Thus, using R instead of r, we obtain

arc 20 0 0

2( / )sin( / 2) 2( / )sin( / 2) 2 sin( / 2)4 4 4

Q L Q R QEr r R

.

The problem asks for the ratio Eparticle / Earc where Eparticle is given by Eq. 22-3:

32. We assume q > 0. Using the notation = q/L we note that the (infinitesimal) charge on an element dx of the rod contains charge dq = dx. By symmetry, we conclude that all horizontal field components (due to the dq’s) cancel and we need only “sum” (integrate) the vertical components. Symmetry also allows us to integrate these contributions over only half the rod (0 x L/2) and then simply double the result. In that regard we note that sin = R/r where 2 2r x R .

(a) Using Eq. 22-3 (with the 2 and sin factors just discussed) the magnitude is

2 2

2 2 2 2 20 00 0

/ 22

3 2 2 2 20 2 20 0 0

2 2 220 0

22 sin4 4

2 2

2 12 2 42

L L

LL

dq dx yEr x R x R

q L RR dx xR x Rx R

q L qLR R L RL R

where the integral may be evaluated by elementary means or looked up in Appendix E (item #19 in the list of integrals). With 127.81 10 Cq , 0.145 mL and R = 0.0600 m, we have | | 12.4 N/CE .

(b) As noted above, the electric field E points in the +y direction, or 90 counterclockwise from the +x axis.

The x and the y components are

2

1 sin4x

dxdEr

and

2

1 cos4y

dxdEr

,

respectively. We use as the variable of integration and substitute r = R/cos ,tanx R and dx = (R/cos2 ) d . The limits of integration are 0 and /2 rad. Thus,

0 00 0 0

sin cos4 4 4xE d

R R Rand

/ 2

0 00 0 0

cos sin .4 4 4yE d

R R R

We notice that Ex = Ey no matter what the value of R. Thus, E makes an angle of 45° with the rod for all values of R.

33. Consider an infinitesimal section of the rod of length dx, a distance x from the left end, as shown in the following diagram. It contains charge dq = dx and is a distance r from P. The magnitude of the field it produces at P is given by

20

1 .4

dxdEr

34. From Eq. 22-26, we obtain

263

12 2 22 2 2 20

5.3 10 C m 12cm1 1 6.3 10 N C.2 2 8.85 10 C /N m 12cm 2.5cm

zEz R

where R is the radius of the disk and is the surface charge density on the disk. See Eq. 22-26. The magnitude of the field at the center of the disk (z = 0) is Ec = /2 0. We want to solve for the value of z such that E/Ec = 1/2. This means

2 2 2 2

1 11 .2 2

z zz R z R

Squaring both sides, then multiplying them by z2 + R2, we obtain z2 = (z2/4) + (R2/4).Thus, z2 = R2/3, or z R 3 . With R = 0.600 m, we have z = 0.346 m.

35. At a point on the axis of a uniformly charged disk a distance z above the center of the disk, the magnitude of the electric field is

E zz R

LNM

OQP2

10

2 2

36. We write Eq. 22-26 as

2 2 1/ 2max

1( )

E zE z R

and note that this ratio is 12 (according to the graph shown in the figure) when z = 4.0 cm.

Solving this for R we obtain R = z 3 = 6.9 cm.

37. We use Eq. 22-26, noting that the disk in figure (b) is effectively equivalent to the disk in figure (a) plus a concentric smaller disk (of radius R/2) with the opposite value of

. That is,

E(b) = E(a) – 2 o1

2R(2R)2 + (R/2)2

where

E(a) = 2 o1

2R(2R)2 + R2 .

We find the relative difference and simplify:

E(a) – E(b) E(a)

= 1 2 / 4 1/ 4 1 2 / 17 / 4 0.0299 0.2830.10561 2 / 4 1 1 2 / 5

or approximately 28%.

38. From dA = 2 r dr (which can be thought of as the differential of A = r²) and dq = dA (from the definition of the surface charge density ), we have

dq = QR2 2 r dr

where we have used the fact that the disk is uniformly charged to set the surface charge density equal to the total charge (Q) divided by the total area ( R2). We next set r = 0.0050 m and make the approximation dr 30 10 6 m. Thus we get dq 2.4 10 16 C.

39. The magnitude of the force acting on the electron is F = eE, where E is the magnitude of the electric field at its location. The acceleration of the electron is given by Newton’s second law:

a Fm

eEm

160 10

911 10351 10

19

3115

.

.. .

C 2.00 10 N C

kgm s

42c hc h

40. Eq. 22-28 gives

E Fq

mae

me

aFHGIKJb g

using Newton’s second law.

(a) With east being the i direction, we have

3129

19

9.11 10 kg ˆ ˆ1.80 10 m s i ( 0.0102 N C) i1.60 10 C

E

which means the field has a magnitude of 0.0102 N/C

(b) The result shows that the field E is directed in the –x direction, or westward.

41. We combine Eq. 22-9 and Eq. 22-28 (in absolute values).

F q E q pz

kepz

FHG

IKJ2

2

03 3

where we have used Eq. 21-5 for the constant k in the last step. Thus, we obtain

9 2 2 19 2915

39

2 8.99 10 N m C 1.60 10 C 3.6 10 C m6.6 10 N

25 10 mF .

If the dipole is oriented such that p is in the +z direction, then F points in the –zdirection.

27 27

19

(6.64 10 kg)(9.8 m/s ) 2.03 10 N C2 2(1.6 10 C)mgE

e.

(b) Since the force of gravity is downward, then qE must point upward. Since q > 0 in this situation, this implies E must itself point upward.

42. (a) Vertical equilibrium of forces leads to the equality

.2mgq E mg E

e

Substituting the values given in the problem, we obtain

43. (a) The magnitude of the force on the particle is given by F = qE, where q is the magnitude of the charge carried by the particle and E is the magnitude of the electric field at the location of the particle. Thus,

E Fq

30 102 0 10

15 106

93.

..N

CN C.

The force points downward and the charge is negative, so the field points upward.

(b) The magnitude of the electrostatic force on a proton is

19 3 161.60 10 C 1.5 10 N C 2.4 10 N.elF eE

(c) A proton is positively charged, so the force is in the same direction as the field, upward.

(d) The magnitude of the gravitational force on the proton is

227 261.67 10 kg 9.8 m s 1.6 10 N.gF mg

The force is downward.

(e) The ratio of the forces is

1610

26

2.4 10 N 1.5 10 .1.64 10 N

el

g

FF

44. (a) Fe = Ee = (3.0 106 N/C)(1.6 10–19 C) = 4.8 10 – 13 N.

(b) Fi = Eqion = Ee = (3.0 106 N/C)(1.6 10–19 C) = 4.8 10 – 13 N.

a160 10

167 10192 10

19

2712

.

.. .

C 2.00 10 N C

kgm s

42c hc h

(b) We assume the proton starts from rest and use the kinematic equation v v ax202 2

(or else x at12

2 and v = at) to show that

v ax2 2 192 10 0 0100 196 1012 5. . .m s m m s.2d ib g

45. (a) The magnitude of the force acting on the proton is F = eE, where E is the magnitude of the electric field. According to Newton’s second law, the acceleration of the proton is a = F/m = eE/m, where m is the mass of the proton. Thus,

46. (a) The initial direction of motion is taken to be the +x direction (this is also the direction of E ). We use v v a xf i

2 2 2 with vf = 0 and a F m eE me to solve for distance x:

xva

m veE

i e i2 2 31

192

2 2911 10

2 160 10712 10

.

..

kg 5.00 10 m s

C 1.00 10 N Cm.

6 2

3

c hc hc hc h

(b) Eq. 2-17 leads to

t xv

xviavg

m

m ss.2 2 7 12 10

5 00 102 85 10

2

68

.

..

c h

(c) Using v2 = 2a x with the new value of x, we find

21 22

2 2 2 212

19 3 3

231 6

2 2

2 1.60 10 C 1.00 10 N C 8.00 10 m0.112.

9.11 10 kg 5.00 10 m s

e

i e i i i e i

m vK v a x eE xK m v v v m v

Thus, the fraction of the initial kinetic energy lost in the region is 0.112 or 11.2%.

47. When the drop is in equilibrium, the force of gravity is balanced by the force of the electric field: mg = qE, where m is the mass of the drop, q is the charge on the drop, and E is the magnitude of the electric field. The mass of the drop is given by m = (4 /3)r3 ,where r is its radius and is its mass density. Thus,

3 3 26319

5

4 1.64 10 m 851kg m 9.8m s4 8.0 10 C3 3 1.92 10 N C

mg r gqE E

and q/e = ( 8.0 10–19 C)/(1.60 10–19 C) = 5, or 5q e .

48. We assume there are no forces or force-components along the x direction. We combine Eq. 22-28 with Newton’s second law, then use Eq. 4-21 to determine time tfollowed by Eq. 4-23 to determine the final velocity (with –g replaced by the ay of this problem); for these purposes, the velocity components given in the problem statement are re-labeled as v0x and v0y respectively.

(a) We have / ( / )a qE m e m E which leads to

19213

31

1.60 10 C N ˆ ˆ120 j (2.1 10 m s ) j.9.11 10 kg C

a

(b) Since vx = v0x in this problem (that is, ax = 0), we obtain

m1.5 10 m s

s

m s m s 1.3 10 s

5

2

t xv

v v a tx

y y y

0

7

03 13 7

0 020 13 10

30 10 21 10

. .

. .d ic h

which leads to vy = –2.8 106 m/s. Therefore, the final velocity is

5 6ˆ ˆ(1.5 10 m/s) i (2.8 10 m/s) j.v

49. (a) We use x = vavgt = vt/2:

v xt

2 2 2 0 1015 10

2 7 102

86

..

.ms

m s.c h

(b) We use x at12

2 and E = F/e = ma/e:

E mae

xmet

2 2 2 0 10

160 1010 102

2 31

19 8

3.

..

m 9.11 10 kg

C 1.5 10 sN C.2

c hc hc hc h

This leads (using Eq. 4-23) to

6 11 2 60 0

5

sin (2.00 10 m/s)sin40.0 (8.78 10 m/s )(1.96 10 s)

4.34 10 m/s .yv v at

Since the x component of velocity does not change, then the final velocity is

v = (1.53 106 m/s) i^ (4.34 105 m/s) j^ .

50. Due to the fact that the electron is negatively charged, then (as a consequence of Eq. 22-28 and Newton’s second law) the field E pointing in the +y direction (which we will call “upward”) leads to a downward acceleration. This is exactly like a projectile motion problem as treated in Chapter 4 (but with g replaced with a = eE/m = 8.78 1011 m/s2).Thus, Eq. 4-21 gives

66

0 0

3.00 m 1.96 10 scos (2.00 10 m/s)cos40.0xt

v.

51. We take the positive direction to be to the right in the figure. The acceleration of the proton is ap = eE/mp and the acceleration of the electron is ae = –eE/me, where E is the magnitude of the electric field, mp is the mass of the proton, and me is the mass of the electron. We take the origin to be at the initial position of the proton. Then, the coordinate of the proton at time t is x a tp

12

2 and the coordinate of the electron is x L a te12

2 .

They pass each other when their coordinates are the same, or 12

2 12

2a t L a tp e . This means t2 = 2L/(ap – ae) and

31

31 27

5

9.11 10 kg 0.050m9.11 10 kg 1.67 10 kg

2.7 10 m.

p p e

p e e pp e

a eE m mx L L La a m meE m eE m

52. We are given = 4.00 10 6 C/m2 and various values of z (in the notation of Eq. 22-26 which specifies the field E of the charged disk). Using this with F = eE (the magnitude of Eq. 22-28 applied to the electron) and F = ma, we obtain / /a F m eE m .

(a) The magnitude of the acceleration at a distance R is

a = e ( )

4 m o = 1.16 1016 m/s2 .

(b) At a distance R/100, a =e (10001 10001 )

20002 m o = 3.94 1016 m/s2 .

(c) At a distance R/1000, a =e (1000001 1000001 )

2000002 m o = 3.97 1016 m/s2 .

(d) The field due to the disk becomes more uniform as the electron nears the center point. One way to view this is to consider the forces exerted on the electron by the charges near the edge of the disk; the net force on the electron caused by those charges will decrease due to the fact that their contributions come closer to canceling out as the electron approaches the middle of the disk.

53. (a) Using Eq. 22-28, we find

F 8 00 10 8 00 10 600

0 240 0 0480

5 5. .

. .

C 3.00 10 N C i C N C j

N i N j.

3c hc h c hb gb g b g

Therefore, the force has magnitude equal to

2 22 2 0.240N 0.0480N 0.245N.x yF F F

(b) The angle the force F makes with the +x axis is

1 1 0.0480Ntan tan 11.30.240N

y

x

FF

measured counterclockwise from the +x axis.

(c) With m = 0.0100 kg, the (x, y) coordinates at t = 3.00 s can be found by combining Newton’s second law with the kinematics equations of Chapters 2–4. The x coordinate is

222 0.240 N 3.00 s1 108m.

2 2 2 0.0100 kgx

xF tx a t

m

(d) Similarly, the y coordinate is

222 0.0480 N 3.00 s1 21.6m.

2 2 2 0.0100 kgy

y

F ty a t

m

(b) The displacement is equal to the distance since the electron does not change its direction of motion. The field is uniform, which implies the acceleration is constant. Thus,

50 5.0 10 m.2

v vd t

54. (a) Due to the fact that the electron is negatively charged, then (as a consequence of Eq. 22-28 and Newton’s second law) the field E pointing in the same direction as the velocity leads to deceleration. Thus, with t = 1.5 10 9 s, we find

194 9

0 0 31

4

(1.6 10 C)(50 N/C)| | 4.0 10 m/s (1.5 10 s)9.11 10 kg

2.7 10 m/s .

eEv v a t v tm

55. We take the charge 45.0 pCQ of the bee to be concentrated as a particle at the center of the sphere. The magnitude of the induced charges on the sides of the grain is | | 1.000 pC.q

(a) The electrostatic force on the grain by the bee is

2 2 2 2

( ) 1 1| |( / 2) ( / 2) ( / 2) ( / 2)

kQq kQ qF kQ qd D D D d D

where 1.000 cmD is the diameter of the sphere representing the honeybee, and 40.0 md is the diameter of the grain. Substituting the values, we obtain

9 2 2 12 123 2 3 2

10

1 18.99 10 N m C (45.0 10 C)(1.000 10 C)(5.00 10 m) (5.04 10 m)

2.56 10 N .

F

The negative sign implies that the force between the bee and the grain is attractive. The magnitude of the force is 10| | 2.56 10 NF .

(b) Let | | 45.0 pCQ be the magnitude of the charge on the tip of the stigma. The force on the grain due to the stigma is

2 2 2 2

| | | | ( ) 1 1| || |( ) ( ) ( ) ( )k Q q k Q qF k Q qd D D D d D

where 1.000 mmD is the distance between the grain and the tip of the stigma. Substituting the values given, we have

9 2 2 12 123 2 3 2

8

1 18.99 10 N m C (45.0 10 C)(1.000 10 C)(1.000 10 m) (1.040 10 m)

3.06 10 N .

F

The negative sign implies that the force between the grain and the stigma is attractive. The magnitude of the force is 8| | 3.06 10 NF .

(c) Since | | | | ,F F the grain will move to the stigma.

1. The vector area A and the electric field E are shown on the diagram below. The angle between them is 180° – 35° = 145°, so the electric flux through the area is

23 2 2cos 1800 N C 3.2 10 m cos145 1.5 10 N m C.E A EA

(e) We now have to add the flux through all six faces. One can easily verify that the flux through the front face is zero, while that through the right face is the opposite of that through the left one, or +16 N·m2/C. Thus the net flux through the cube is

= (–72 + 24 – 16 + 0 + 0 + 16) N·m2/C = – 48 N·m2/C.

2. We use z E dA and note that the side length of the cube is (3.0 m–1.0 m) = 2.0 m.

(a) On the top face of the cube y = 2.0 m and jdA dA . Therefore, we have 2ˆ ˆ ˆ ˆ4i 3 2.0 2 j 4i 18jE . Thus the flux is

2 2 2

top top topˆ ˆ ˆ4i 18j j 18 18 2.0 N m C 72 N m C.E dA dA dA

(b) On the bottom face of the cube y = 0 and dA dAb ge jj . Therefore, we have

E 4 3 0 2 4 62i j i jc h . Thus, the flux is

2 2 2

bottom bottom bottomˆ ˆ ˆ4i 6 j j 6 6 2.0 N m C 24 N m C.E dA dA dA

(c) On the left face of the cube idA dA . So

2 2 2

left left bottom ˆ ˆ ˆˆ 4i j i 4 4 2.0 N m C 16 N m C.yE dA E dA dA

(d) On the back face of the cube kdA dA . But since E has no z component

0E dA . Thus, = 0.

3. We use E A , where A A .j m j2140b g .

(a) 2ˆ ˆ6.00 N C i 1.40 m j 0.

(b) 2 2ˆ ˆ2.00 N C j 1.40 m j 3.92 N m C.

(c) 2ˆ ˆ ˆ3.00 N C i 400 N C k 1.40 m j 0 .

(d) The total flux of a uniform field through a closed surface is always zero.

4. There is no flux through the sides, so we have two “inward” contributions to the flux, one from the top (of magnitude (34)(3.0)2) and one from the bottom (of magnitude (20)(3.0)2). With “inward” flux being negative, the result is = – 486 N m2/C. Gauss’ law then leads to

12 2 2 2 9enc 0 (8.85 10 C /N m )( 486 N m C) 4.3 10 C.q

5. We use Gauss’ law: 0 q , where is the total flux through the cube surface and qis the net charge inside the cube. Thus,

65 2

12 2 20

1.8 10 C 2.0 10 N m C.8.85 10 C N m

q

6. The flux through the flat surface encircled by the rim is given by 2 .a E Thus, the flux through the netting is

2 3 4 2(0.11 m) (3.0 10 N/C) 1.1 10 N m /Ca E .

7. To exploit the symmetry of the situation, we imagine a closed Gaussian surface in the shape of a cube, of edge length d, with a proton of charge 191.6 10 Cq situated at the inside center of the cube. The cube has six faces, and we expect an equal amount of flux through each face. The total amount of flux is net = q/ 0, and we conclude that the flux through the square is one-sixth of that. Thus,

199 2

12 2 20

1.6 10 C 3.01 10 N m C.6 6(8.85 10 C N m )q

8. We note that only the smaller shell contributes a (non-zero) field at the designated point, since the point is inside the radius of the large sphere (and E = 0 inside of a spherical charge), and the field points towards the x direction. Thus, with R = 0.020 m (the radius of the smaller shell), L = 0.10 m and x = 0.020 m, we obtain

2 22 2

2 2 20 0 0

2 6 24

12 2 2 2

4ˆ ˆ ˆ ˆ( j) j j j4 4 ( ) ( )

(0.020 m) (4.0 10 C/m ) ˆ ˆj 2.8 10 N/C j .(8.85 10 C /N m )(0.10 m 0.020 m)

R RqE Er L x L x

9. Let A be the area of one face of the cube, Eu be the magnitude of the electric field at the upper face, and El be the magnitude of the field at the lower face. Since the field is downward, the flux through the upper face is negative and the flux through the lower face is positive. The flux through the other faces is zero, so the total flux through the cube surface is ( ).uA E E The net charge inside the cube is given by Gauss’ law:

12 2 2 20 0

6

( ) (8.85 10 C / N m )(100 m) (100 N/C 60.0 N/C)

3.54 10 C 3.54 C.uq A E E

10. (a) The total surface area bounding the bathroom is

22 2.5 3.0 2 3.0 2.0 2 2.0 2.5 37 m .A

The absolute value of the total electric flux, with the assumptions stated in the problem, is

2 3 2| | | | | | (600 N/C)(37 m ) 22 10 N m / C.E A E A

By Gauss’ law, we conclude that the enclosed charge (in absolute value) is 7

enc 0| | | | 2.0 10 C.q Therefore, with volume V = 15 m3, and recognizing that we are dealing with negative charges, the charge density is

78 3enc

3

| | 2.0 10 C 1.3 10 C/m .15 m

qV

(b) We find (|qenc|/e)/V = (2.0 10–7 C/1.6 10–19 C)/15 m3 = 8.2 1010 excess electrons per cubic meter.

2 2

=0 1.40

ˆ ˆ ˆ ˆ3.00 j j 3.00 j A j 3.00 1.40 1.40 8.23 N m C.y y

y A y

(b) The charge is given by

12 2 2 2 11enc 0 8.85 10 C / N m 8.23 N m C 7.29 10 Cq .

(c) The electric field can be re-written as 0ˆ3.00 jE y E , where 0

ˆ ˆ4.00i 6.00jE is a constant field which does not contribute to the net flux through the cube. Thus is still 8.23 N m2/C.

(d) The charge is again given by

12 2 2 2 11enc 0 8.85 10 C / N m 8.23 N m C 7.29 10 Cq .

11. (a) Let A = (1.40 m)2. Then

12. Eq. 23-6 (Gauss’ law) gives qenc .

(a) Thus, the value 5 22.0 10 N m /C for small r leads to

12 2 2 5 2 6 6central 0 (8.85 10 C /N m )(2.0 10 N m /C) 1.77 10 C 1.8 10 Cq

(b) The next value that takes is 5 24.0 10 N m /C , which implies 6

enc 3.54 10 C.q But we have already accounted for some of that charge in part (a), so the result for part (b) is

qA = qenc – qcentral = – 5.3 10 6 C.

(c) Finally, the large r value for is 5 26.0 10 N m /C , which implies 6

total enc 5.31 10 C.q Considering what we have already found, then the result is

total enc central 8.9 .Aq q q C

13. The total flux through any surface that completely surrounds the point charge is q/ 0.

(a) If we stack identical cubes side by side and directly on top of each other, we will find that eight cubes meet at any corner. Thus, one-eighth of the field lines emanating from the point charge pass through a cube with a corner at the charge, and the total flux through the surface of such a cube is q/8 0. Now the field lines are radial, so at each of the three cube faces that meet at the charge, the lines are parallel to the face and the flux through the face is zero.

(b) The fluxes through each of the other three faces are the same, so the flux through each of them is one-third of the total. That is, the flux through each of these faces is (1/3)(q/8 0)= q/24 0. Thus, the multiple is 1/24 = 0.0417.

14. None of the constant terms will result in a nonzero contribution to the flux (see Eq. 23-4 and Eq. 23-7), so we focus on the x dependent term only. In Si units, we have

Enon-constant = 3x i^ .

The face of the cube located at x = 0 (in the yz plane) has area A = 4 m2 (and it “faces” the +i^ direction) and has a “contribution” to the flux equal to Enon-constant A = (3)(0)(4) = 0. The face of the cube located at x = 2 m has the same area A (and this one “faces” the –i^

direction) and a contribution to the flux:

Enon-constant A = (3)( 2)(4) = 24 N·m/C2.

Thus, the net flux is = 0 + 24 = 24 N·m/C2. According to Gauss’ law, we therefore have qenc = = 2.13 10 10 C.

15. None of the constant terms will result in a nonzero contribution to the flux (see Eq. 23-4 and Eq. 23-7), so we focus on the x dependent term only:

Enon-constant = ( 4.00y2 ) i^ (in SI units) .

The face of the cube located at y = 4.00 has area A = 4.00 m2 (and it “faces” the +j^

direction) and has a “contribution” to the flux equal to

Enon-constant A = ( 4)(42)(4) = –256 N·m/C2.

The face of the cube located at y = 2.00 m has the same area A (however, this one “faces” the –j^ direction) and a contribution to the flux:

Enon-constant A = ( 4)(22)(4) = N·m/C2.

Thus, the net flux is = ( 256 + 64) N·m/C2 = 192 N·m/C2. According to Gauss’s law, we therefore have

12 2 2 2 9enc 0 (8.85 10 C /N m )( 192 N m C) 1.70 10 C.q

2 2

1 1

2 2

1 1

1 3

2 1 0 1

1 3

0 1

( ) ( ) 10 2(4) 10 2(1)

6 6(1)(2) 12.

y z

yz x x y z

y z

y z

E x x E x x dydz dy dz

dy dz

Similarly, the net flux through the two faces parallel to the xz plane is

2 2

1 1

4 3

2 1 1 1( ) ( ) [ 3 ( 3)] 0

x z

xz y y x zE y y E y y dxdz dy dz ,

and the net flux through the two faces parallel to the xy plane is

2 2

1 1

4 1

2 1 1 0( ) ( ) 3 2 (3)(1) 6 .

x y

xy z z x yE z z E z z dxdy dx dy b b b b

Applying Gauss’ law, we obtain

enc 0 0 0 0( ) (6.00 0 12.0) 24.0xy xz yzq b

which implies that b = 2.00 N/C m .

16. The total electric flux through the cube is E dA . The net flux through the two faces parallel to the yz plane is

17. (a) The area of a sphere may be written 4 R2= D2. Thus,

67 2

22

2.4 10 C 4.5 10 C/m .1.3 m

qD

(b) Eq. 23-11 gives 7 2

412 2 2

0

4.5 10 C/m 5.1 10 N/C.8.85 10 C / N m

E

18. Eq. 23-6 (Gauss’ law) gives qenc.

(a) The value 5 29.0 10 N m /C for small r leads to qcentral = – 7.97 10 6 C or roughly – 8.0 C.

(b) The next (non-zero) value that takes is 5 24.0 10 N m /C , which implies 6

enc 3.54 10 C.q But we have already accounted for some of that charge in part (a), so the result is

qA = qenc – qcentral = 11.5 10 6 C 12 C .

(c) Finally, the large r value for is 5 22.0 10 N m /C, which implies 6

total enc 1.77 10 C.q Considering what we have already found, then the result is

qtotal enc – qA qcentral = –5.3 C.

19. (a) The charge on the surface of the sphere is the product of the surface charge density and the surface area of the sphere (which is 24 ,r where r is the radius). Thus,

22 6 2 5 m4 4 8.1 10 C/m 3.7 10 C.

2q r

(b) We choose a Gaussian surface in the form of a sphere, concentric with the conducting sphere and with a slightly larger radius. The flux is given by Gauss’s law:

56 2

12 2 20

3.66 10 C 4.1 10 N m / C .8.85 10 C / N m

q

20. Using Eq. 23-11, the surface charge density is

5 12 2 2 6 20 2.3 10 N C 8.85 10 C / N m 2.0 10 C/m .E

21. (a) Consider a Gaussian surface that is completely within the conductor and surrounds the cavity. Since the electric field is zero everywhere on the surface, the net charge it encloses is zero. The net charge is the sum of the charge q in the cavity and the charge qw

on the cavity wall, so q + qw = 0 and qw = –q = –3.0 10–6C.

(b) The net charge Q of the conductor is the sum of the charge on the cavity wall and the charge qs on the outer surface of the conductor, so Q = qw + qs and

6 6 510 10 C 3.0 10 C 1.3 10 C.sq Q q

Here, the maximum value is

84

max 12 2 20

2.0 10 C/m1.2 10 N/C.

2 2 0.030 m 8.85 10 C / N mE

r

22. We imagine a cylindrical Gaussian surface A of radius r and unit length concentric

with the metal tube. Then by symmetry enc

0

2 .A

qE dA rE

(a) For r < R, qenc = 0, so E = 0.

(b) For r > R, qenc = , so 0( ) / 2 .E r r With 82.00 10 C/m and r = 2.00R = 0.0600 m, we obtain

83

12 2 2

2.0 10 C/m5.99 10 N/C.

2 0.0600 m 8.85 10 C / N mE

(c) The plot of E vs. r is shown below.

23. The magnitude of the electric field produced by a uniformly charged infinite line is E= /2 0r, where is the linear charge density and r is the distance from the line to the point where the field is measured. See Eq. 23-12. Thus,

12 2 2 4 602 2 8.85 10 C / N m 4.5 10 N/C 2.0 m 5.0 10 C/m.Er

24. We combine Newton’s second law (F = ma) with the definition of electric field ( F qE ) and with Eq. 23-12 (for the field due to a line of charge). In terms of magnitudes, we have (if r = 0.080 m and 66.0 10 C/m )

ma = eE =e

2 o r a = e

2 o r m = 2.1 1017 m/s2 .

25. (a) The side surface area A for the drum of diameter D and length h is given by A Dh . Thus,

12 2 2 50

7

8.85 10 C /N m 2.3 10 N/C 0.12 m 0.42 m

3.2 10 C.

q A Dh EDh

(b) The new charge is

7 78.0 cm 28 cm3.2 10 C 1.4 10 C.

12 cm 42 cmA D hq q qA Dh

1 2net 1 2

0 0

2 24 ( / 2) 4 ( / 2)

E E Ex L x L

.

Setting this equal to zero and solving for x we find

1 2

1 2

6.0 C/m ( 2.0 C/m) 8.0 cm 8.0 cm2 6.0 C/m ( 2.0 C/m) 2Lx .

26. We reason that point P (the point on the x axis where the net electric field is zero) cannot be between the lines of charge (since their charges have opposite sign). We reason further that P is not to the left of “line 1” since its magnitude of charge (per unit length) exceeds that of “line 2”; thus, we look in the region to the right of “line 2” for P.Using Eq. 23-12, we have

27. We assume the charge density of both the conducting cylinder and the shell are uniform, and we neglect fringing effect. Symmetry can be used to show that the electric field is radial, both between the cylinder and the shell and outside the shell. It is zero, of course, inside the cylinder and inside the shell.

(a) We take the Gaussian surface to be a cylinder of length L, coaxial with the given cylinders and of larger radius r than either of them. The flux through this surface is

2 ,rLE where E is the magnitude of the field at the Gaussian surface. We may ignore any flux through the ends. Now, the charge enclosed by the Gaussian surface is qenc = Q1 + Q2 = –Q1= –3.40 10 12 C. Consequently, Gauss’ law yields 0 enc2 ,r LE qor

12enc

12 2 2 30

3.40 10 C 0.214 N/C,2 2 (8.85 10 C / N m )(11.0 m)(20.0 1.30 10 m)

qELr

or | | 0.214 N/C.E

(b) The negative sign in E indicates that the field points inward.

(c) Next, for r = 5.00 R1, the charge enclosed by the Gaussian surface is qenc = Q1 = 3.40 10 12 C. Consequently, Gauss’ law yields 0 enc2 ,r LE q or

12enc

12 2 2 30

3.40 10 C 0.855 N/C.2 2 (8.85 10 C / N m )(11.0 m)(5.00 1.30 10 m)

qELr

(d) The positive sign indicates that the field points outward.

(e) we consider a cylindrical Gaussian surface whose radius places it within the shell itself. The electric field is zero at all points on the surface since any field within a conducting material would lead to current flow (and thus to a situation other than the electrostatic ones being considered here), so the total electric flux through the Gaussian surface is zero and the net charge within it is zero (by Gauss’ law). Since the central rod has charge Q1, the inner surface of the shell must have charge Qin = –Q1= –3.40 10 12 C.

(f) Since the shell is known to have total charge Q2 = –2.00Q1, it must have charge Qout = Q2 – Qin = –Q1= –3.40 10 12 C on its outer surface.

28. As we approach r = 3.5 cm from the inside, we have

internal0

2 1000 N/C4

Er

.

And as we approach r = 3.5 cm from the outside, we have

external0 0

2 2 3000 N/C4 4

Er r

.

Considering the difference (Eexternal – Einternal ) allows us to find (the charge per unit length on the larger cylinder). Using r = 0.035 m, we obtain = –5.8 10 9 C/m.

29. We denote the inner and outer cylinders with subscripts i and o, respectively.

(a) Since ri < r = 4.0 cm < ro,

66

12 2 2 20

5.0 10 C/m( ) 2.3 10 N/C.2 2 (8.85 10 C / N m )(4.0 10 m)

iE rr

(b) The electric field E r( ) points radially outward.

(c) Since r > ro,

6 65

12 2 2 20

5.0 10 C/m 7.0 10 C/m( 8.0 cm) 4.5 10 N/C,2 2 (8.85 10 C / N m )(8.0 10 m)

i oE rr

or 5| ( 8.0 cm) | 4.5 10 N/C.E r

(d) The minus sign indicates that ( )E r points radially inward.

(b) Since the field is zero inside the conductor (in an electrostatic configuration), then there resides on the inner surface charge –q, and on the outer surface, charge +q (where qis the charge on the rod at the center). Therefore, with ri = 0.05 m, the surface density of charge is

99 2

inner2.0 10 C/m 6.4 10 C/m

2 2 2 (0.050 m)i i

qr L r

for the inner surface.

(c) With ro = 0.10 m, the surface charge density of the outer surface is

9 2outer 3.2 10 C/m .

2 2o o

qr L r

30. (a) In Eq. 23-12, = q/L where q is the net charge enclosed by a cylindrical Gaussian surface of radius r. The field is being measured outside the system (the charged rod coaxial with the neutral cylinder) so that the net enclosed charge is only that which is on the rod. Consequently,

92

0 0

2(2.0 10 C/m) 2.4 10 N/C.4 4 (0.15 m)

Er

31. We denote the radius of the thin cylinder as R = 0.015 m. Using Eq. 23-12, the net electric field for r > R is given by

net wire cylinder0 02 2

E E Er r

where – = –3.6 nC/m is the linear charge density of the wire and ' is the linear charge density of the thin cylinder. We note that the surface and linear charge densities of the thin cylinder are related by

cylinder (2 ) (2 ).q L RL R

Now, Enet outside the cylinder will equal zero, provided that 2 R = , or

68 23.6 10 C/m 3.8 10 C/m .

2 (2 )(0.015 m)R

(b) Once outside the cylinder, Eq. 23-12 is obeyed. To find = q/L we must find the total charge q. Therefore,

0.04 2 11

0

1 2 1.0 10 C/m.q Ar r L drL L

And the result, for r = 0.050 m, is 0| | /2 3.6 N/C.E r

32. To evaluate the field using Gauss’ law, we employ a cylindrical surface of area 2 r Lwhere L is very large (large enough that contributions from the ends of the cylinder become irrelevant to the calculation). The volume within this surface is V = r2 L, or expressed more appropriate to our needs: dV = 2 r L dr. The charge enclosed is, with

6 52.5 10 C/mA ,2 4

enc 02 .

2r

q Ar r L dr ALr

By Gauss’ law, we find enc 0| | (2 ) / ;E rL q we thus obtain 3

0

.4ArE

(a) With r = 0.030 m, we find | | 1.9 N/C.E

33. In the region between sheets 1 and 2, the net field is E1 – E2 + E3 = 2.0 105 N/C .

In the region between sheets 2 and 3, the net field is at its greatest value:

E1 + E2 + E3 = 6.0 105 N/C .

The net field vanishes in the region to the right of sheet 3, where E1 + E2 = E3 . We note the implication that 3 is negative (and is the largest surface-density, in magnitude). These three conditions are sufficient for finding the fields:

E1 = 1.0 105 N/C , E2 = 2.0 105 N/C , E3 = 3.0 105 N/C .

From Eq. 23-13, we infer (from these values of E)

| 3|| 2|

= 3.0 x 105 N/C2.0 x 105 N/C = 1.5 .

Recalling our observation, above, about 3, we conclude 3

2 = –1.5 .

34. According to Eq. 23-13 the electric field due to either sheet of charge with surface charge density 10 22 C/m2 is perpendicular to the plane of the sheet (pointing away from the sheet if the charge is positive) and has magnitude E = /2 0. Using the superposition principle, we conclude:

(a) E = / 0 = ( 10 22 C/m2)/(8.85 10 12 2 2C /N m ) = 2.00 10 11 N/C, pointing in the upward direction, or 11 ˆ(2.00 10 N/C)jE .

(b) E = 0;

(c) and, E = / 0, pointing down, or 11 ˆ(2.00 10 N/C)jE .

distributed uniformly over both sides of the original plate, with half being on the side near the field point. Thus,

64 2

2

6.0 10 C 4.69 10 C/m .2 2(0.080 m)qA

The magnitude of the field is

4 27

12 2 20

4.69 10 C/m 5.3 10 N/C.8.85 10 C / N m

E

The field is normal to the plate and since the charge on the plate is positive, it points away from the plate.

(b) At a point far away from the plate, the electric field is nearly that of a point particle with charge equal to the total charge on the plate. The magnitude of the field is

2 20/ 4 /E q r kq r , where r is the distance from the plate. Thus,

9 2 2 6

2

8.99 10 N m / C 6.0 10 C60 N/C.

30 mE

35. (a) To calculate the electric field at a point very close to the center of a large, uniformly charged conducting plate, we may replace the finite plate with an infinite plate with the same area charge density and take the magnitude of the field to be E = / 0,where is the area charge density for the surface just under the point. The charge is

36. The charge distribution in this problem is equivalent to that of an infinite sheet of charge with surface charge density 4.50 10 12 C/m2 plus a small circular pad of radius R = 1.80 cm located at the middle of the sheet with charge density – . We denote the electric fields produced by the sheet and the pad with subscripts 1 and 2, respectively. Using Eq. 22-26 for 2E , the net electric field E at a distance z = 2.56 cm along the central axis is then

1 2 2 2 2 20 0 0

12 2 2

12 2 2 2 2 2 2

ˆ ˆ ˆk 1 k k2 2 2

(4.50 10 C/m )(2.56 10 m) ˆ ˆk (0.208 N/C) k2(8.85 10 C /N m ) (2.56 10 m) (1.80 10 m)

z zE E Ez R z R 0

ˆ/ 2 iE (from the right plate) 0ˆ/ 2 ( i) (from the left one) = 0.

(c) Between the plates:

22 211

12 2 20 0 0

7.00 10 C/mˆ ˆ ˆ ˆ ˆ( i) i ( i) i 7.91 10 N/C i.2 2 8.85 10 C /N m

E

37. We use Eq. 23-13.

(a) To the left of the plates:

0ˆ/ 2 ( i)E (from the right plate) 0

ˆ( / 2 )i (from the left one) = 0.

(b) To the right of the plates:

38. We use the result of part (c) of Problem 23-35 to obtain the surface charge density.

12 2 2 10 20 0/ 8.85 10 C /N m (55 N/C) 4.9 10 C/m .E E

Since the area of the plates is 21.0 mA , the magnitude of the charge on the plate is 104.9 10 C.Q A

39. The charge on the metal plate, which is negative, exerts a force of repulsion on the electron and stops it. First find an expression for the acceleration of the electron, then use kinematics to find the stopping distance. We take the initial direction of motion of the electron to be positive. Then, the electric field is given by E = / 0, where is the surface charge density on the plate. The force on the electron is F = –eE = –e / 0 and the acceleration is

0

F eam m

where m is the mass of the electron. The force is constant, so we use constant acceleration kinematics. If v0 is the initial velocity of the electron, v is the final velocity, and x is the distance traveled between the initial and final positions, then 2 2

0 2 .v v ax Set v = 0 and replace a with –e / 0m, then solve for x. We find

2 20 0 0 .

2 2v mvxa e

Now 2102 mv is the initial kinetic energy K0, so

12 2 2 1740 0

19 6 2

8.85 10 C / N m 1.60 10 J4.4 10 m.

1.60 10 C 2.0 10 C/mKx

e

a = e

2 o m = slope of the graph ( = 2.0 105 m/s divided by 7.0 10 12 s) .

Thus we obtain = 2.9 10 6 C/m2.

40. The field due to the sheet is E = 2 . The force (in magnitude) on the electron (due to

that field) is F = eE, and assuming it’s the only force then the acceleration is

41. The forces acting on the ball are shown in the diagram on the right. The gravitational force has magnitude mg, where m is the mass of the ball; the electrical force has magnitude qE, where q is the charge on the ball and E is the magnitude of the electric field at the position of the ball; and, the tension in the thread is denoted by T. The electric field produced by the plate is normal to the plate and points to the right. Since the ball is positively charged, the electric force on it also points to the right. The tension in the thread makes the angle (= 30°) with the vertical.

Since the ball is in equilibrium the net force on it vanishes. The sum of the horizontal components yields

qE – T sin = 0

and the sum of the vertical components yields

cos 0T mg .

The expression T = qE/sin , from the first equation, is substituted into the second to obtain qE = mg tan . The electric field produced by a large uniform plane of charge is given by E = /2 0, where is the surface charge density. Thus,

0

tan2q mg

and12 2 2 6 2

08

9 2

2 8.85 10 C / N.m 1.0 10 kg 9.8 m/s tan 302 tan2.0 10 C

5.0 10 C/m .

mgq

If d = 0.20 m (which is less than the magnitude of r found above), then neither of the points (x ± 0.691 m) is in the “forbidden region” between the particle and the sheet. Thus, both values are allowed. Thus, we have

(a) x = 0.691 m on the positive axis, and

(b) x = 0.691 m on the negative axis.

(c) If, however, d = 0.80 m (greater than the magnitude of r found above), then one of the points (x 0.691 m) is in the “forbidden region” between the particle and the sheet and is disallowed. In this part, the fields cancel only at the point x +0.691 m.

42. The point where the individual fields cancel cannot be in the region between the sheet and the particle ( d < x < 0) since the sheet and the particle have opposite-signed charges.The point(s) could be in the region to the right of the particle (x > 0) and in the region to the left of the sheet (x < d); this is where the condition

20 0

| |2 4

Qr

must hold. Solving this with the given values, we find r = x = ± 3/2 ± 0.691 m.

(d) For x = 26.0 mm = 2.60 10 2 m, we take a Gaussian surface of the same shape and orientation, but with x > d/2, so the left and right faces are outside the slab. The total flux through the surface is again 22Ea but the charge enclosed is now q = a2d . Gauss’ law yields 2 0Ea2 = a2d , so

15 3 36

12 2 20

(5.80 10 C/m )(9.40 10 m) 3.08 10 N/C.2 2(8.85 10 C /N m )

dE

43. We use a Gaussian surface in the form of a box with rectangular sides. The cross section is shown with dashed lines in the diagram below. It is centered at the central plane of the slab, so the left and right faces are each a distance x from the central plane. We take the thickness of the rectangular solid to be a, the same as its length, so the left and right faces are squares.

The electric field is normal to the left and right faces and is uniform over them. Since = 5.80 fC/m3 is positive, it points outward at both faces: toward the left at the left face and toward the right at the right face. Furthermore, the magnitude is the same at both faces. The electric flux through each of these faces is Ea2. The field is parallel to the other faces of the Gaussian surface and the flux through them is zero. The total flux through the Gaussian surface is

22 .Ea The volume enclosed by the Gaussian surface is 2a2xand the charge contained within it is 22q a x . Gauss’ law yields

2 0Ea2 = 2a2x .

We solve for the magnitude of the electric field: 0/ .E x

(a) For x =0, E =0.

(b) For x = 2.00 mm = 2.00 10 3 m,

15 3 36

12 2 20

(5.80 10 C/m )(2.00 10 m) 1.31 10 N/C.8.85 10 C /N m

xE

(c) For x = d/2 = 4.70 mm = 4.70 10 3 m,

15 3 36

12 2 20

(5.80 10 C/m )(4.70 10 m) 3.08 10 N/C.8.85 10 C /N m

xE

44. (a) The flux is still 2750 N m /C , since it depends only on the amount of charge enclosed.

(b) We use 0/q to obtain the charge q:

12 2 2 2 90 8.85 10 C /N m 750 N m / C 6.64 10 C.q

45. Charge is distributed uniformly over the surface of the sphere and the electric field it produces at points outside the sphere is like the field of a point particle with charge equal to the net charge on the sphere. That is, the magnitude of the field is given by E = |q|/4 0r2, where |q| is the magnitude of the charge on the sphere and r is the distance from the center of the sphere to the point where the field is measured. Thus,

2 32 9

0 9 2 2

0.15 m 3.0 10 N/C| | 4 7.5 10 C.

8.99 10 N m / Cq r E

The field points inward, toward the sphere center, so the charge is negative, i.e., 97.5 10 C.q

46. We determine the (total) charge on the ball by examining the maximum value (E = 5.0 107 N/C) shown in the graph (which occurs at r = 0.020 m). Thus, from

20/ 4 ,E q r we obtain

2 72 6

0 9 2 2

(0.020 m) (5.0 10 N/C)4 2.2 10 C8.99 10 N m C

q r E .

47. (a) Since r1 = 10.0 cm < r = 12.0 cm < r2 = 15.0 cm,

9 2 2 841

220

8.99 10 N m /C 4.00 10 C1( ) 2.50 10 N/C.4 0.120 m

qE rr

(b) Since r1 < r2 < r = 20.0 cm,

9 2 2 841 2

2 20

8.99 10 N m / C 4.00 2.00 1 10 C1( ) 1.35 10 N/C.4 0.200 m

q qE rr

48. The point where the individual fields cancel cannot be in the region between the shells since the shells have opposite-signed charges. It cannot be inside the radius R of one of the shells since there is only one field contribution there (which would not be canceled by another field contribution and thus would not lead to zero net field). We note shell 2 has greater magnitude of charge (| 2|A2) than shell 1, which implies the point is not to the right of shell 2 (any such point would always be closer to the larger charge and thus no possibility for cancellation of equal-magnitude fields could occur). Consequently, the point should be in the region to the left of shell 1 (at a distance r > R1 from its center); this is where the condition

1 21 2 2 2

0 0

| | | |4 4 ( )

q qE Er r L

or1 1 2 2

2 20 0

| |4 4 ( )

A Ar r L

.

Using the fact that the area of a sphere is A = 4 R2 , this condition simplifies to

r = L

(R2 /R1) | 2|/ 1 1 = 3.3 cm .

We note that this value satisfies the requirement r > R1. The answer, then, is that the net field vanishes at x = r = 3.3 cm.

The electric field is radial, so the flux through the Gaussian surface is 4 2r Eg , where E is the magnitude of the field. Gauss’ law yields

4 202 2 2Er q A r ag gd i.

We solve for E:

E qr

A Aarg g

LNMM

OQPP

14

2 2

02

2

2 .

For the field to be uniform, the first and last terms in the brackets must cancel. They do if q – 2 Aa2 = 0 or A = q/2 a2. With a = 2.00 10 2 m and q = 45.0 10 15 C, we have

11 21.79 10 C/m .A

49. To find an expression for the electric field inside the shell in terms of A and the distance from the center of the shell, select A so the field does not depend on the distance. We use a Gaussian surface in the form of a sphere with radius rg, concentric with the spherical shell and within it (a < rg < b). Gauss’ law will be used to find the magnitude of the electric field a distance rg from the shell center. The charge that is both in the shell and within the Gaussian sphere is given by the integral q dVs z over the portion of the shell within the Gaussian surface. Since the charge distribution has spherical symmetry, we may take dV to be the volume of a spherical shell with radius r and infinitesimal thickness dr: dV r dr4 2 . Thus,

2 2 2 24 4 4 2 .g g gr r r

s ga a a

Aq r dr r dr A r dr A r ar

The total charge inside the Gaussian surface is

q q q A r as g2 2 2d i .

(f) For r b we have 2total / 4E q r or

3

20

.3

b aEr

Thus, for r = 3.00b = 6.00a, the electric field is

3 3 9 3

2 12 2 20 0

(2.00 ) 7 (1.84 10 C/m )(0.100 m) 7 1.35 N/C.3 (6.00 ) 3 36 3(8.85 10 C /N m ) 36

a a aEa

50. The field is zero for 0 r a as a result of Eq. 23-16. Thus,

(a) E = 0 at r = 0,

(b) E = 0 at r = a/2.00, and

(c) E = 0 at r = a.

For a r b the enclosed charge qenc (for a r b) is related to the volume by

q r aencFHG

IKJ

43

43

3 3

.

Therefore, the electric field is

E qr r

r a r ar

FHG

IKJ

14 4

43

43 30

20

2

3 3

0

3 3

2enc

for a r b.

(d) For r =1.50a, we have

3 3 9 3

2 12 2 20 0

(1.50 ) 2.375 (1.84 10 C/m )(0.100 m) 2.375 7.32 N/C.3 (1.50 ) 3 2.25 3(8.85 10 C /N m ) 2.25

a a aEa

(e) For r = b=2.00a, the electric field is

3 3 9 3

2 12 2 20 0

(2.00 ) 7 (1.84 10 C/m )(0.100 m) 7 12.1 N/C.3 (2.00 ) 3 4 3(8.85 10 C /N m ) 4

a a aEa

(e) In the region b < r < c, since the shell is conducting, the electric field is zero. Thus, for r = 2.30a, we have E = 0.

(f) For r > c, the charge enclosed by the Gaussian surface is zero. Gauss’ law yields 4 0 02r E E . Thus, E = 0 at r = 3.50a.

(g) Consider a Gaussian surface that lies completely within the conducting shell. Since the electric field is everywhere zero on the surface, E dAz 0 and, according to Gauss’

law, the net charge enclosed by the surface is zero. If Qi is the charge on the inner surface of the shell, then q1 + Qi = 0 and Qi = –q1 = –5.00 fC.

(h) Let Qo be the charge on the outer surface of the shell. Since the net charge on the shell is –q, Qi + Qo = –q1. This means

Qo = –q1 – Qi = –q1 –(–q1) = 0.

51. At all points where there is an electric field, it is radially outward. For each part of the problem, use a Gaussian surface in the form of a sphere that is concentric with the sphere of charge and passes through the point where the electric field is to be found. The field is uniform on the surface, so 24E dA r E , where r is the radius of the Gaussian surface.

For r < a, the charge enclosed by the Gaussian surface is q1(r/a)3. Gauss’ law yields

32 1 1

30 0

4 .4

q q rrr E Ea a

(a) For r = 0, the above equation implies E = 0.

(b) For r = a/2, we have

9 2 2 1521

3 2 20

( / 2) (8.99 10 N m /C )(5.00 10 C) 5.62 10 N/C.4 2(2.00 10 m)q aE

a

(c) For r = a, we have

9 2 2 151

2 2 20

(8.99 10 N m /C )(5.00 10 C) 0.112 N/C.4 (2.00 10 m)

qEa

In the case where a < r < b, the charge enclosed by the Gaussian surface is q1, so Gauss’ law leads to

2 1 12

0 0

4 .4

q qr E Er

(d) For r = 1.50a, we have

9 2 2 151

2 2 20

(8.99 10 N m /C )(5.00 10 C) 0.0499 N/C.4 (1.50 2.00 10 m)

qEr

52. Let EA designate the magnitude of the field at r = 2.4 cm. Thus EA = 2.0 107 N/C, and is totally due to the particle. Since 2

particle 0/ 4 ,E q r then the field due to the particle at any other point will relate to EA by a ratio of distances squared. Now, we note that at r = 3.0 cm the total contribution (from particle and shell) is 8.0 107 N/C.Therefore,

Eshell + Eparticle = Eshell + (2.4/3)2 EA = 8.0 107 N/C .

Using the value for EA noted above, we find Eshell = 6.6 107 N/C. Thus, with r = 0.030 m, we find the charge Q using 2

shell 0/ 4E Q r :

2 2 72 6shell

0 shell 9 2 2

(0.030 m) (6.6 10 N/C)4 6.6 10 C8.99 10 N m C

r EQ r Ek

.

53. We use 2enc

2 2 00 0

1( ) ( )44 4

rqE r r r drr r

to solve for (r) and obtain

( ) ( ) .rr

ddr

r E rr

ddr

Kr K r02

2 02

60

36c h

Also, outside sphere 2 we have

2 22 2 2

0 0

| | | |4 4 (1.50 )

q qEr R

.

Equating these and solving for the ratio of charges, we arrive atq2q1

=98 = 1.125.

54. Applying Eq. 23-20, we have

1 1 11 13 3 2

0 0 0

| | | | | |14 4 2 2 4

q q qRE rR R R

.

(d) For r = R, the electric field is

29 2 2 12 3

0 02

1 (8.99 10 N m C ) (14.1 10 C/m )(0.0560 m)4 4

2.23 10 N/C.

s sR RER

55. (a) We integrate the volume charge density over the volume and require the result be equal to the total charge:

2

04 .

Rdx dy dz dr r Q

Substituting the expression = sr/R, with s= 14.1 pC/m3, and performing the integration leads to

4

44

s R QR

or3 12 3 3 15(14.1 10 C/m )(0.0560 m) 7.78 10 C.sQ R

(b) At r = 0, the electric field is zero (E = 0) since the enclosed charge is zero.

At a certain point within the sphere, at some distance r from the center, the field (see Eq. 23-8 through Eq. 23-10) is given by Gauss’ law:

enc2

0

14

qEr

where qenc is given by an integral similar to that worked in part (a):

42

enc 04 4 .

4r s rq dr r

RTherefore,

4 2

20 0

1 14 4

s sr rERr R

.

(c) For r = R/2.00, where R = 5.60 cm, the electric field is

2 9 2 2 12 3

0 03

( / 2.00)1 1 (8.99 10 N m C ) (14.1 10 C/m )(0.0560 m)4 4 4.00 4.00

5.58 10 N/C.

s sR RER

(e) The electric field strength as a function of r is depicted below: 1. If the electric potential is zero at infinity then at the surface of a uniformly charged sphere it is V = q/4 0R, where q is the charge on the sphere and R is the sphere radius. Thus q = 4 0RV and the number of electrons is

65

9 2 2 19

1.0 10 m 400V42.8 10 .

8.99 10 N m C 1.60 10 Cq R V

ne e

2. The magnitude is U = e V = 1.2 109 eV = 1.2 GeV. 3. (a) An Ampere is a Coulomb per second, so

84 84 3600 30 105 A h C hs

sh

CFHG

IKJFHG

IKJ . .

(b) The change in potential energy is U = q V = (3.0 105 C)(12 V) = 3.6 106 J.

4. (a) VB – VA = U/q = –W/(–e) = – (3.94 10–19 J)/(–1.60 10–19 C) = 2.46 V.

(b) VC – VA = VB – VA = 2.46 V.

(c) VC – VB = 0 (Since C and B are on the same equipotential line).

5. The electric field produced by an infinite sheet of charge has magnitude E = /2 0,where is the surface charge density. The field is normal to the sheet and is uniform. Place the origin of a coordinate system at the sheet and take the x axis to be parallel to the field and positive in the direction of the field. Then the electric potential is

V V E dx V Exs

x

sz0 ,

where Vs is the potential at the sheet. The equipotential surfaces are surfaces of constant x;that is, they are planes that are parallel to the plane of charge. If two surfaces are separated by x then their potentials differ in magnitude by

V = E x = ( /2 0) x.Thus,

x V2 2 885 10 50010 10

88 10012 2

63

..

. .C N m V

C mm

2

2

c hb g

6. (a) 15 19 4 43.9 10 N 1.60 10 C 2.4 10 N C 2.4 10 V/m.E F e

(b) V E s 2 4 10 012 2 9 104 3. . . .N C m Vc hb g

7. (a) The work done by the electric field is

19 12 20 0

0 12 2 200 0

21

(1.60 10 C)(5.80 10 C/m )(0.0356 m)2 2 2(8.85 10 C /N m )

1.87 10 J.

f d

i

q q dW q E ds dz

(b) Since V – V0 = –W/q0 = – z/2 0, with V0 set to be zero on the sheet, the electric potential at P is

12 22

12 2 20

(5.80 10 C/m )(0.0356 m) 1.17 10 V.2 2(8.85 10 C /N m )

zV

8. (a) By Eq. 24-18, the change in potential is the negative of the “area” under the curve. Thus, using the area-of-a-triangle formula, we have

V E dsxz10 1

22 20

0

2 b gb g

which yields V = 30 V.

(b) For any region within 0 3 zx E dsm, is positive, but for any region for which

x > 3 m it is negative. Therefore, V = Vmax occurs at x = 3 m.

V E dsxz10 1

23 20

0

3 b gb gwhich yields Vmax = 40 V.

(c) In view of our result in part (b), we see that now (to find V = 0) we are looking for some X > 3 m such that the “area” from x = 3 m to x = X is 40 V. Using the formula for a triangle (3 < x < 4) and a rectangle (4 < x < X), we require

12

1 20 4 20 40b gb g b gb gX .

Therefore, X = 5.5 m.

9. We connect A to the origin with a line along the y axis, along which there is no change of potential (Eq. 24-18: E dsz 0). Then, we connect the origin to B with a line along

the x axis, along which the change in potential is

V E ds x dxx F

HGIKJzz 4 00 4 00 4

2

2

0

4

0

4. .

which yields VB – VA = –32.0 V.

10. In the “inside” region between the plates, the individual fields (given by Eq. 24-13) are in the same direction ( i ):

9 2 9 23

in 12 2 2 12 2 2

50 10 C/m 25 10 C/m ˆ ˆi (4.2 10 N/C)i2(8.85 10 C /N m ) 2(8.85 10 C /N m )

E .

In the “outside” region where x > 0.5 m, the individual fields point in opposite directions:

9 2 9 23

out 12 2 2 12 2 2

50 10 C/m 25 10 C/mˆ ˆ ˆi i (1.4 10 N/C)i .2(8.85 10 C /N m ) 2(8.85 10 C /N m )

E

Therefore, by Eq. 24-18, we have

0.8 0.5 0.8 3 3in out0 0 0.5

3

4.2 10 0.5 1.4 10 0.3

2.5 10 V.

V E ds E dx E dx

11. (a) The potential as a function of r is

2

3 30 00 0

9 2 2 15 24

3

0 04 8

(8.99 10 N m C )(3.50 10 C)(0.0145 m) 2.68 10 V.2(0.0231 m)

r r qr qrV r V E r dr drR R

(b) Since V = V(0) – V(R) = q/8 0R, we have

9 2 2 154

0

(8.99 10 N m C )(3.50 10 C) 6.81 10 V.8 2(0.0231 m)

qV RR

6 9 2 2

0 0

3

1 11.0 10 C 8.99 10 N m C4 4 2.0 m 1.0 m

4.5 10 V.

A BA B

q qV Vr r

(b) Since V(r) depends only on the magnitude of r , the result is unchanged.

12. (a) The potential difference is 13. (a) The charge on the sphere is

90 9 2 2

(200 V)(0.15 m)4 3.3 10 C.8.99 10 N m C

q VR

(b) The (uniform) surface charge density (charge divided by the area of the sphere) is

qR4

3 3 104 015

12 102

9

28.

.. . C

m C / m2

b g

14. The charge is 9

0 9 2 2m /C

(10m)( 1.0V)4 1.1 10 C.8.99 10 N

q RV

15. A charge –5q is a distance 2d from P, a charge –5q is a distance d from P, and two charges +5q are each a distance d from P, so the electric potential at P is

9 2 2 15

20 0

4

1 1 1 1 (8.99 10 N m C )(5.00 10 C)4 2 8 2(4.00 10 m)

5.62 10 V.

q qVd d d d d

The zero of the electric potential was taken to be at infinity.

16. Since according to the problem statement there is a point in between the two charges on the x axis where the net electric field is zero, the fields at that point due to q1 and q2must be directed opposite to each other. This means that q1 and q2 must have the same sign (i.e., either both are positive or both negative). Thus, the potentials due to either of them must be of the same sign. Therefore, the net electric potential cannot possibly be zero anywhere except at infinity.

V x kqd

qd

qx d x

( )FHG

IKJ

FHG

IKJ

1

1

2

2 041 3 0

and solve: x = d/4. With d = 24.0 cm, we have x = 6.00 cm.

(b) Similarly, for x < 0 the separation between q1 and a point on the x axis whose coordinate is x is given by d1 = –x; while the corresponding separation for q2 is d2 = d – x.We set

V x kqd

qd

qx d x

( )FHG

IKJ

FHG

IKJ

1

1

2

2 041 3 0

to obtain x = –d/2. With d = 24.0 cm, we have x = –12.0 cm.

17. First, we observe that V (x) cannot be equal to zero for x > d. In fact V (x) is always negative for x > d. Now we consider the two remaining regions on the x axis: x < 0 and0 < x < d.

(a) For 0 < x < d we have d1 = x and d2 = d – x. Let

18. In applying Eq. 24-27, we are assuming V 0 as r . All corner particles are equidistant from the center, and since their total charge is

2q1– 3q1+ 2 q1– q1 = 0,

then their contribution to Eq. 24-27 vanishes. The net potential is due, then, to the two +4q2 particles, each of which is a distance of a/2 from the center:

9 2 2 122 2 2

0 0 0

4 4 161 1 16(8.99 10 N m C )(6.00 10 C) 2.21 V.4 / 2 4 / 2 4 0.39 m

q q qVa a a

19. (a) The electric potential V at the surface of the drop, the charge q on the drop, and the radius R of the drop are related by V = q/4 0R. Thus

R qV4

8 99 10 30 10

50054 10

0

9 124

. /.

N m C C

V m.

2 2c hc h

(b) After the drops combine the total volume is twice the volume of an original drop, so the radius R' of the combined drop is given by (R')3 = 2R3 and R' = 21/3R. The charge is twice the charge of original drop: q' = 2q. Thus,

V qR

qR

V14

14

22

2 2 500 7900 0

1 32 3 2 3

// / ( V) V.

V1 = 1

04q

d = 5.76 × 10 7 V.

Thus, q1/d = 6.41 × 10 17 C/m. Next, we note that when q2 is located at x = 0.080 m, the net potential vanishes (V1 + V2 = 0). Therefore,

2 100.08 m

kq kqd

Thus, we find q2 = 1( / )(0.08 m)q d = –5.13 × 10 18 C = –32 e.

20. When the charge q2 is infinitely far away, the potential at the origin is due only to the charge q1 :

23. (a) All the charge is the same distance R from C, so the electric potential at C is

9 2 2 121 1 1

20 0

6 51 5(8.99 10 N m C )(4.20 10 C) 2.30 V,4 4 8.20 10 m

Q Q QVR R R

where the zero was taken to be at infinity.

(b) All the charge is the same distance from P. That distance is 2 2 ,R D so the electric potential at P is

2

1 1 12 2 2 2 20 0

9 2 2 12

2 2 2 2

6 514 4

5(8.99 10 N m C )(4.20 10 C) 1.78 V.(8.20 10 m) (6.71 10 m)

Q Q QVR D R D R D

9 2 2 12

2 rod rod0 0 0

1 1 (8.99 10 N m C )(25.6 10 C)4 4 4 3.71 10 m6.20 V.

Pdq QV dqR R R

We note that the result is exactly what one would expect for a point-charge –Q at a distance R. This “coincidence” is due, in part, to the fact that V is a scalar quantity.

24. The potential is 25. (a) From Eq. 24-35, we find the potential to be

2 2

0

2 29 2 2 12

2

/ 2 ( / 4)2 ln

4

(0.06 m / 2) (0.06 m) / 4 (0.08 m)2(8.99 10 N m C )(3.68 10 C/m) ln

0.08 m

2.43 10 V.

L L dV

d

(b) The potential at P is V = 0 due to superposition.

26. Using Gauss’ law, q = nC. Consequently,

9 2 2 74

0

(8.99 10 N m C )(4.958 10 C) 3.71 10 V.4 0.120 m

qVr

27. Since the charge distribution on the arc is equidistant from the point where V is evaluated, its contribution is identical to that of a point charge at that distance. We assume V 0 as r and apply Eq. 24-27:

1 1 1 1

0 0

9 2 2 122

4 21 1 1 14 4 2 4 4

(8.99 10 N m C )(7.21 10 C) 3.24 10 V.2.00 m

Q Q Q QVR R R R

28. The dipole potential is given by Eq. 24-30 (with = 90º in this case)

2 20 0

cos cos90 04 4p pV

r r

since cos(90º) = 0 . The potential due to the short arc is 1 0 1/ 4q r and that caused by the long arc is 2 0 2/ 4q r . Since q1 = +2 C, r1 = 4.0 cm, q2 = 3 C, and r2 = 6.0 cm, the potentials of the arcs cancel. The result is zero.

29. The disk is uniformly charged. This means that when the full disk is present each quadrant contributes equally to the electric potential at P, so the potential at P due to a single quadrant is one-fourth the potential due to the entire disk. First find an expression for the potential at P due to the entire disk. We consider a ring of charge with radius r and (infinitesimal) width dr. Its area is 2 r dr and it contains charge dq = 2 r dr. All the charge in it is a distance 2 2r D from P, so the potential it produces at P is

2 2 2 20 0

1 2 .4 2

rdr rdrdVr D r D

The total potential at P is

2 2 2 2

2 20 00 0 0

.2 2 2

RR rdrV r D R D Dr D

The potential Vsq at P due to a single quadrant is

15 22 2 2 2

12 2 20

5

(7.73 10 C/m ) (0.640 m) (0.259 m) 0.259 m4 8 8(8.85 10 C /N m )

4.71 10 V.

sqVV R D D

30. Consider an infinitesimal segment of the rod, located between x and x + dx. It has length dx and contains charge dq = dx, where = Q/L is the linear charge density of the rod. Its distance from P1 is d + x and the potential it creates at P1 is

dV dqd x

dxd x

14

140 0

.

To find the total potential at P1, we integrate over the length of the rod and obtain:

0 00 0 09 2 2 15

3

ln( ) ln 14 4 4

(8.99 10 N m C )(56.1 10 C) 0.12 mln 1 7.39 10 V.0.12 m 0.025 m

LL dx Q LV d xd x L d

31. Letting d denote 0.010 m, we have

9 2 2 941 1 1 1

0 0 0 0

3 3 (8.99 10 N m C )(30 10 C) 1.3 10 V.4 2(0.01 m)

Q Q Q QVd d d d

(a) Here r = x > 0, so that

V bx dxx

bz14

0 2040

0 20 .. b g = 36 V.

(b) Now r x d2 2 where d = 0.15 m, so that

0.200.20 2 2

2 200

14 4

bxdx bV x dx d

= 18 V.

32. Eq. 24-32 applies with dq = dx = bx dx (along 0 x 0.20 m). 33. Consider an infinitesimal segment of the rod, located between x and x + dx. It has length dx and contains charge dq = dx = cx dx. Its distance from P1 is d + x and the potential it creates at P1 is

dV dqd x

cx dxd x

14

140 0

.

To find the total potential at P1, we integrate over the length of the rod and obtain

0 00 0 0

9 2 2 12 2

2

[ ln( )] ln 14 4 4

0.120 m(8.99 10 N m C )(28.9 10 C/m ) 0.120 m (0.030 m) ln 10.030 m

1.86 10 V.

LLc xdx c c LV x d x d L dd x d

34. We use Eq. 24-41. This is an ordinary derivative since the potential is a function of only one variable.

2 2ˆ ˆ ˆ ˆ ˆi (1500 )i ( 3000 )i ( 3000V/m )(0.0130m)i ( 39V/m)i.dV dE x xdx dx

(a) Thus, the magnitude of the electric field is E = 39 V/m.

(b) The direction of E is i , or toward plate 1. We evaluate at x = 3.0 m and y = 2.0 m to obtain

ˆ ˆ( 12 V/m)i (12 V/m)jE .

35. We use Eq. 24-41:

E x y Vx x

x y x

E x y Vy y

x y y

x

y

( , ) ( . ) . ) ( . ) ;

( , ) ( . ) . ) ( . ) .

2 0 30 2 2 0

2 0 30 2 30

2 2

2 2

V / m V / m V / m

V / m V / m V / m

2 2 2

2 2 2

c h

c h

36. The magnitude of the electric field is given by

22(5.0V)| | 6.7 10 V m.0.015m

VEx

At any point in the region between the plates, E points away from the positively charged plate, directly towards the negatively charged one.

37. The electric field (along some axis) is the (negative of the) derivative of the potential V with respect to the corresponding coordinate. In this case, the derivatives can be read off of the graphs as slopes (since the graphs are of straight lines). Thus,

500 V 2500 V/m 2500 N/C0.20 m

300 V 1000 V/m 1000 N/C.0.30 m

x

y

VEx

VEy

These components imply the electric field has a magnitude of 2693 N/C and a direction of –21.8º (with respect to the positive x axis). The force on the electron is given by F qE where q = –e. The minus sign associated with the value of q has the implication

that F points in the opposite direction from E (which is to say that its angle is found by adding 180º to that of E ). With e = 1.60 × 10–19 C, we obtain

19 16 16ˆ ˆ ˆ ˆ( 1.60 10 C)[(2500 N/C)i (1000 N/C)j] ( 4.0 10 N)i (1.60 10 N)jF .

(b) We differentiate the potential with respect to x to find the x component of the electric field:

20 0 0

9 2 2 15 4 2

1ln4 4 4 ( )

(8.99 10 N m C )(43.6 10 C) (3.92 10 N m C) ,( 0.135 m) ( 0.135 m)

xV Q x L Q x x L QEx L x x L x L x x x x L

x x x x

or4 2(3.92 10 N m C)| | .

( 0.135 m)xEx x

(c) Since 0xE , its direction relative to the positive x axis is 180 .

(d) At x = d = 6.20 cm, we obtain

4 2(3.92 10 N m C)| | 0.0321 N/C.(0.0620 m)(0.0620 m 0.135 m)xE

(e) Consider two points an equal infinitesimal distance on either side of P1, along a line that is perpendicular to the x axis. The difference in the electric potential divided by their separation gives the transverse component of the electric field. Since the two points are situated symmetrically with respect to the rod, their potentials are the same and the potential difference is zero. Thus, the transverse component of the electric field Ey is zero.

38. (a) From the result of Problem 24-30, the electric potential at a point with coordinate x is given by

0

ln .4

Q x LVL x

At x = d we obtain

9 2 2 15

0

3

(8.99 10 N m C )(43.6 10 C) 0.135 mln ln 14 0.135 m

0.135 m(2.90 10 V) ln 1 .

Q d LVL d d

d

39. We apply Eq. 24-41: 2

2

2.00

2.00

4.00

x

y

z

VE yzxVE xzyVE xyzz

which, at (x, y, z) = (3.00 m, –2.00 m, 4.00 m), gives

(Ex, Ey, Ez) = (64.0 V/m, –96.0 V/m, 96.0 V/m).

The magnitude of the field is therefore

2 2 2 150V m 150 N C.x y zE E E E

40. (a) Consider an infinitesimal segment of the rod from x to x + dx. Its contribution to the potential at point P2 is

dV x dxx y

cxx y

dx14

140

2 20

2 2

( ) .

Thus,

2 2

2 2rod 00 0

9 2 2 12 2 2 2

2

4 4

(8.99 10 N m C )(49.9 10 C/m ) (0.100 m) (0.0356 m) 0.0356 m

3.16 10 V.

L

Pc x cV dV dx L y y

x y

(b) The y component of the field there is

2 2

2 20 0

9 2 2 12 2

2 2

1 .4 4

0.0356 m(8.99 10 N m C )(49.9 10 C/m ) 1(0.100 m) (0.0356 m)

0.298 N/C.

Py

V c d c yE L y yy dy L y

(c) We obtained above the value of the potential at any point P strictly on the y-axis. In order to obtain Ex(x, y) we need to first calculate V(x, y). That is, we must find the potential for an arbitrary point located at (x, y). Then Ex(x, y) can be obtained from

( , ) ( , ) /xE x y V x y x .

41. We choose the zero of electric potential to be at infinity. The initial electric potential energy Ui of the system before the particles are brought together is therefore zero. After the system is set up the final potential energy is

2 2

0 0

1 1 1 1 1 1 2 1 2 .4 42 2 2f

q qUa a a a aa a

Thus the amount of work required to set up the system is given by

2 9 2 2 12 2

0

13

2 1 2(8.99 10 N m C )(2.30 10 C) 12 24 0.640 m2 2

1.92 10 J.

f i fqW U U U U

a

42. The work done must equal the change in the electric potential energy. From Eq. 24-14 and Eq. 24-26, we find (with r = 0.020 m)

9 2 2 19 225

0

8.99 10 N m C (18)(1.60 10 C)(3 2 2 )(6 ) 2.1 10 J4 0.020 m

e e e eWr

.

43. We apply conservation of energy for the particle with q = 7.5 10 6 C (which has zero initial kinetic energy):

U0 = Kf + Uf ,

where U =q Q

4 or .

(a) The initial value of r is 0.60 m and the final value is (0.6 + 0.4) m = 1.0 m (since the particles repel each other). Conservation of energy, then, leads to Kf = 0.90 J.

(b) Now the particles attract each other so that the final value of r is 0.60 0.40 = 0.20 m. Use of energy conservation yields Kf = 4.5 J in this case.

44. (a) We use Eq. 24-43 with q1 = q2 = –e and r = 2.00 nm:

9 2 2 19 22191 2

9

8.99 10 N m C (1.60 10 C)1.15 10 J.

2.00 10 mq q eU k k

r r

(b) Since U > 0 and U r–1 the potential energy U decreases as r increases.

energy when q3 is at A and UB is the potential energy when q3 is at B, then the work done in moving the charge from B to A is

W = UA – UB = q3(VA – VB) = (3.0 10–6 C)(6.0 104 V + 7.8 105 V) = 2.5 J.

(d) The work done by the external agent is positive, so the energy of the three-charge system increases.

(e) and (f) The electrostatic force is conservative, so the work is the same no matter which path is used.

45. (a) Let 015. m be the length of the rectangle and w = 0.050 m be its width. Charge q1 is a distance from point A and charge q2 is a distance w, so the electric potential at Ais

6 69 2 21 2

0

4

1 5.0 10 C 2.0 10 C(8.99 10 N m / C )4 0.15m 0.050 m

6.0 10 V.

Aq qV

w

(b) Charge q1 is a distance w from point b and charge q2 is a distance , so the electric potential at B is

6 69 2 21 2

0

5

1 5.0 10 C 2.0 10 C(8.99 10 N m / C )4 0.050 m 0.15m

7.8 10 V.

Bq qVw

(c) Since the kinetic energy is zero at the beginning and end of the trip, the work done by an external agent equals the change in the potential energy of the system. The potential energy is the product of the charge q3 and the electric potential. If UA is the potential

46. The work required is

1 2 1 1

0 0

( / 2)1 1 0.4 2 4 2

q Q q Q q Q q QW Ud d d d

47. We use the conservation of energy principle. The initial potential energy is Ui = q2/4 0r1, the initial kinetic energy is Ki = 0, the final potential energy is Uf = q2/4 0r2,and the final kinetic energy is K mvf

12

2 , where v is the final speed of the particle. Conservation of energy yields

qr

qr

mv2

0 1

2

0 2

2

4 412

.

The solution for v is

2 9 2 2 6 2

6 3 30 1 2

3

2 1 1 (8.99 10 N m C )(2)(3.1 10 C) 1 14 20 10 kg 0.90 10 m 2.5 10 m

2.5 10 m s.

qvm r r

48. Let r = 1.5 m, x = 3.0 m, q1 = –9.0 nC, and q2 = –6.0 pC. The work done by an external agent is given by

W U q qr r x

FHG

IKJ

FHG

IKJL

NMM

O

QPP

1 22 2

9 12

10

41

9 0 10 6 0 10 115

1

15 30

18 10

1

C C 8.99 10 N mC m m m

J.

92

2 2 2. .

. . .

.

c hc hb g b g

2

0

12 4

eqmvr

where m = 9.11 10 31 kg, e = 1.60 10 19 C, q = 10000e, and r = 0.010 m. This yields v = 22490 m/s 42.2 10 m/s .

49. The escape speed may be calculated from the requirement that the initial kinetic energy (of launch) be equal to the absolute value of the initial potential energy (compare with the gravitational case in chapter 14). Thus,

50. The change in electric potential energy of the electron-shell system as the electron starts from its initial position and just reaches the shell is U = (–e)(–V) = eV. Thus from

U K m ve i12

2 we find the initial electron speed to be

196

31

2 2 2(1.6 10 C)(125 V) 6.63 10 m/s.9.11 10 kgi

e e

U eVvm m

U m v m vA A B B12

12

2 2 .

Momentum is also conserved, so

0 m v m vA A B B .

These equations may be solved simultaneously for vA and vB. Substituting ( / )B A B Av m m v , from the momentum equation into the energy equation, and collecting

terms, we obtain U m m m m vA B A B A

12

2( / )( ) . Thus,

3

3 3 3

2 2(0.225 J)(10 10 kg) 7.75 m/s.( ) (5.0 10 kg)(5.0 10 kg 10 10 kg)

BA

A A B

Umvm m m

51. (a) The potential energy is

U qd

2 9 6

48 99 10 5 0 10

1000 225

. ..

. N m C C

m J

2 2 2c hc h

relative to the potential energy at infinite separation.

(b) Each sphere repels the other with a force that has magnitude

F qd

2

2

9 6

48 99 10 5 0 10

0 225. .

. N m C C

1.00 m N.

2 2 2

2

c hc hb g

According to Newton’s second law the acceleration of each sphere is the force divided by the mass of the sphere. Let mA and mB be the masses of the spheres. The acceleration of sphere A is

a FmA

A

0 225 4503

. . N5.0 10 kg

m s2

and the acceleration of sphere B is

a FmB

B

0 225 22 53

. . . N10 10 kg

m s2

(c) Energy is conserved. The initial potential energy is U = 0.225 J, as calculated in part (a). The initial kinetic energy is zero since the spheres start from rest. The final potential energy is zero since the spheres are then far apart. The final kinetic energy is 12

2 12

2m v m vA A B B , where vA and vB are the final velocities. Thus,

We thus obtain 3

3

5.0 10 kg (7.75 m/s) 3.87 m/s,10 10 kg

AB A

B

mv vm

or | | 3.87 m/s.Bv

52. When particle 3 is at x = 0.10 m, the total potential energy vanishes. Using Eq. 24-43, we have (with meters understood at the length unit)

1 3 3 21 2

0 0 0

04 4 ( 0.10 m) 4 (0.10 m)

q q q qq qd d

This leads to1 2 1 2

3 0.10 m 0.10 mq q q qq

d d

which yields q3 = 5.7 C.

v edm

44

8 99 10 4 160 10

0 01032 10

2 9 19

312

. .

..

N m C C

m 9.11 10 kgm s.

2 2 2c hb gc hb gc h

53. We use conservation of energy, taking the potential energy to be zero when the moving electron is far away from the fixed electrons. The final potential energy is then

202 / 4fU e d , where d is half the distance between the fixed electrons. The initial

kinetic energy is K mvi12

2 , where m is the mass of an electron and v is the initial speed of the moving electron. The final kinetic energy is zero. Thus,

Ki = Uf2 2

01 2 / 4 .2

mv e d

Hence,

54. (a) When the proton is released, its energy is K + U = 4.0 eV + 3.0 eV (the latter value is inferred from the graph). This implies that if we draw a horizontal line at the 7.0 Volt “height” in the graph and find where it intersects the voltage plot, then we can determine the turning point. Interpolating in the region between 1.0 cm and 3.0 cm, we find the turning point is at roughly x = 1.7 cm.

(b) There is no turning point towards the right, so the speed there is nonzero, and is given by energy conservation:

v = 2(7.0 eV)

m = 2(7.0 eV)(1.6 x 10-19 J/eV)

1.67 x 10-27 kg = 20 km/s.

(c) The electric field at any point P is the (negative of the) slope of the voltage graph evaluated at P. Once we know the electric field, the force on the proton follows immediately from F = q E , where q = +e for the proton. In the region just to the left of x= 3.0 cm, the field is E = (+300 V/m) i and the force is F = +4.8 10 17 N.

(d) The force F points in the +x direction, as the electric field E .

(e) In the region just to the right of x = 5.0 cm, the field is E =(–200 V/m) i and the magnitude of the force is F = 3.2 10 17 N.

(f) The force F points in the x direction, as the electric field E .

55. (a) The electric field between the plates is leftward in Fig, 24-52 since it points towards lower values of potential. The force (associated with the field, by Eq. 23-28) is evidently leftward, from the problem description (indicating deceleration of the rightward moving particle), so that q > 0 (ensuring that F is parallel to E ); it is a proton.

(b) We use conservation of energy:

K0 + U0 = K + U12 mpv

20 + qV1=

12 mpv2 + qV 2 .

Using q = +1.6 10 19 C, mp = 1.67 10 27 kg, v0 = 90 103 m/s, V1 = 70 V and 2 50 VV , we obtain the final speed v = 6.53 104 m/s. We note that the value of d is

not used in the solution.

2 20 0

cos4 4p epU qV e

r r

where r = 0.020 m. Using energy conservation, we set this expression equal to 100 eV and solve for p. The magnitude of the dipole moment is therefore p = 4.5 10 12 C m .

56. From Eq. 24-30 and Eq. 24-7, we have (for = 180º)

57. Let the distance in question be r. The initial kinetic energy of the electron is K m vi e i

12

2 , where vi = 3.2 105 m/s. As the speed doubles, K becomes 4Ki. Thus

U er

K K K K m vi i i e i

22

44 3 3

2( ) ,

or219 9 2 22

922 19 5

0

2 1.6 10 C 8.99 10 N m C2 1.6 10 m.3 4 3 9.11 10 kg 3.2 10 m se i

erm v

58. (a) Using U = qV we can “translate” the graph of voltage into a potential energy graph (in eV units). From the information in the problem, we can calculate its kinetic energy (which is its total energy at x = 0) in those units: Ki = 284 eV. This is less than the “height” of the potential energy “barrier” (500 eV high once we’ve translated the graph as indicated above). Thus, it must reach a turning point and then reverse its motion.

(b) Its final velocity, then, is in the negative x direction with a magnitude equal to that of its initial velocity. That is, its speed (upon leaving this region) is 1.0 107 m/s.

0 0 2 2 2 20 0 0

2 21.2 J4 4

fqq qqK U Ux y x y

.

This yields y = 8.5 m.

59. We apply conservation of energy for particle 3 (with q' = 15 10 6 C):

K0 + U0 = Kf + Uf

where (letting x = 3 m and q1 = q2 = 50 10 6 C = q)

1 22 2 2 2 2 2

0 0 0

24 4 4

q q q q qqUx y x y x y

.

(a) We solve for Kf (with y0 = 4 m):

0 0 2 20 0

2 1 11.2 J 3.0 J4 | |f f

qqK K U Uxx y

.

(b) We set Kf = 0 and solve for y (choosing the negative root, as indicated in the problem statement):

60. (a) The work done results in a potential energy gain:

W = q V = ( e)Q

4 o R = + 2.16 10 13 J .

With R = 0.0800 m, we find Q = –1.20 10 5 C.

(b) The work is the same, so the increase in the potential energy is U = + 2.16 10 13 J.

N 4 5 6 7 8 9 10 11 12 13 14 15 U1 3.83 6.88 10.96 16.13 22.44 29.92 38.62 48.58 59.81 72.35 86.22 101.5 U2 4.73 7.83 11.88 16.96 23.13 30.44 39.92 48.62 59.58 71.81 85.35 100.2

We see that the potential energy for configuration 2 is greater than that for configuration 1 for N < 12, but for N 12 it is configuration 1 that has the greatest potential energy.

(a) N = 12 is the smallest value such that U2 < U1.

61. We note that for two points on a circle, separated by angle (in radians), the direct-line distance between them is r = 2R sin( /2). Using this fact, distinguishing between the cases where N = odd and N = even, and counting the pair-wise interactions very carefully, we arrive at the following results for the total potential energies. We use k 1 4 . For configuration 1 (where all N electrons are on the circle), we have

112 22 2

1, even 1, odd1 1

1 1 1,2 sin 2 2 2 sin 2

N N

N Nj j

Nke NkeU UR j R j

where 2N

. For configuration 2, we find

312 22 2

2, even 2, odd1 1

1 11 1 52 ,2 sin 2 2 sin 2 2

N N

N Nj j

N ke N keU U

R j R j

where 21N

. The results are all of the form

U keR1

2

2or 2 a pure number.

In our table, below, we have the results for those “pure numbers” as they depend on Nand on which configuration we are considering. The values listed in the U rows are the potential energies divided by ke2/2R.

(b) For N = 12, configuration 2 consists of 11 electrons distributed at equal distances around the circle, and one electron at the center. A specific electron e0 on the circle is Rdistance from the one in the center, and is

r R RFHGIKJ2 0 56sin .

distance away from its nearest neighbors on the circle (of which there are two — one on each side). Beyond the nearest neighbors, the next nearest electron on the circle is

r R RFHGIKJ2 11sin .

distance away from e0. Thus, we see that there are only two electrons closer to e0 than the one in the center.

62. Since the electric potential throughout the entire conductor is a constant, the electric potential at its center is also +400 V.

63. If the electric potential is zero at infinity, then the potential at the surface of the sphere is given by V = q/4 0r, where q is the charge on the sphere and r is its radius. Thus,

q rV4015

8 99 102 5 109

8..

. m 1500 V

N m CC.2 2

b gb g

221 1 1 1 2

22 2 2 2 1

4 2.00.4

q R q Rq R q R

64. (a) Since the two conductors are connected V1 and V2 must be equal to each other.

Let V1 = q1/4 0R1 = V2 = q2/4 0R2 and note that q1 + q2 = q and R2 = 2R1. We solve for q1 and q2: q1 = q/3, q2 = 2q/3, or

(b) q1/q = 1/3 = 0.333.

(c) Similarly, q2/q = 2/3 = 0.667.

(d) The ratio of surface charge densities is

65. (a) The electric potential is the sum of the contributions of the individual spheres. Let q1 be the charge on one, q2 be the charge on the other, and d be their separation. The point halfway between them is the same distance d/2 (= 1.0 m) from the center of each sphere, so the potential at the halfway point is

9 2 2 8 821 2

8.99 10 N m C 1.0 10 C 3.0 10 C1.8 10 V.

4 2 1.0 mq qV

d

(b) The distance from the center of one sphere to the surface of the other is d – R, where R is the radius of either sphere. The potential of either one of the spheres is due to the charge on that sphere and the charge on the other sphere. The potential at the surface of sphere 1 is

8 89 2 2 31 2

10

1 1.0 10 C 3.0 10 C8.99 10 N m C 2.9 10 V.4 0.030 m 2.0 m 0.030 m

q qVR d R

(c) The potential at the surface of sphere 2 is

8 89 2 2 31 2

20

1 1.0 10 C 3.0 10 C8.99 10 N m C 8.9 10 V.4 2.0 m 0.030 m 0.030 m

q qVd R R

(c) For R2 > R1 > r, the enclosed charge is zero. Thus, E = 0.

The electric potential may be obtained using Eq. 24-18:

V r V r E r drr

rb g b g b gz .

(d) For r = 4.00 m > R2 > R1, we have

9 2 2 6 631 2

0

(8.99 10 N m C )(2.00 10 C 1.00 10 C) 6.74 10 V.4 (4.00 m)q qV r

r

(e) For r = 1.00 m = R2 > R1, we have

9 2 2 6 641 2

0

(8.99 10 N m C )(2.00 10 C 1.00 10 C) 2.70 10 V.4 (1.00 m)q qV r

r

(f) For R2 > r = 0.700 m > R2,

6 69 2 21 2

0 2

4

1 2.00 10 C 1.00 10 C(8.99 10 N m C )4 0.700 m 1.00 m

3.47 10 V.

q qV rr R

66. Since the charge distribution is spherically symmetric we may write

enc

0

1( ) ,4

qE rr

where qenc is the charge enclosed in a sphere of radius r centered at the origin.

(a) For r = 4.00 m, R2 = 1.00 m and R1 = 0.500 m, with r > R2 > R1 we have

9 2 2 6 631 2

2 20

(8.99 10 N m C )(2.00 10 C 1.00 10 C) 1.69 10 V/m.4 (4.00 m)q qE r

r

(b) For R2 > r = 0.700 m > R2

9 2 2 641

2 20

(8.99 10 N m C )(2.00 10 C) 3.67 10 V/m.4 (0.700 m)

qE rr

(g) For R2 > r = 0.500 m = R2,

6 69 2 21 2

0 2

4

1 2.00 10 C 1.00 10 C(8.99 10 N m C )4 0.500 m 1.00 m

4.50 10 V.

q qV rr R

(h) For R2 > R1 > r,

6 69 2 21 2

0 1 2

4

1 2.00 10 C 1.00 10 C(8.99 10 N m C )4 0.500 m 1.00 m

4.50 10 V.

q qVR R

(i) At r = 0, the potential remains constant, 44.50 10 V.V

(j) The electric field and the potential as a function of r are depicted below:

67. (a) The magnitude of the electric field is

8 9 2 24

220 0

3.0 10 C 8.99 10 N m C1.2 10 N C.

4 0.15mqE

R

(b) V = RE = (0.15 m)(1.2 104 N/C) = 1.8 103 V.

(c) Let the distance be x. Then

V V x V qR x RFHG

IKJb g

41 1 500 V,

which gives

x R VV V

015 5001800 500

58 10 2..

m VV V

m.b gb g