practice midterm 1.2 v2

7/24/2019 Practice Midterm 1.2 v2

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Department of PhysicsUniversity of California, San Diego

Physics 2B Summer Session I, 2014Instructor: Brian Shotwell

Practice Midterm 1.2, TEST FORM xx

July 10, 2014

Reminder: make sure you do the following or you will be penalized up to 20% on this test:

Write down AND bubble in your 3-digit exam code number under EXAM NUMBER.Do NOT write or bubble in anything under ID NUMBER.Bubble in the TEST FORM (A-D) that appears at the top of your midterm/final.Write your name on your scantron.Turn in your scantron to the correct pile (A-D). Use the correct scantron form: X-101864-PAR-L. Use a pencil!

1. Four charges are located at the vertices of a square as shown in the figure below. What is the magnitudeof the force acting on the 4q charge?

A) 16.3 kq2/a2

B) 7.4 kq2/a2

C) 21.0 kq2/a2

D) 14.5 kq2/a2

E) 11.0 kq2/a2

2. An electric field has the following properties:At x = 5m, the electric field points in the (+x)-direction and has magnitude 5.0V/m.At x = 4m, the electric field points in the (+x)-direction and has magnitude 20.V/m.

If the above electric field is due to a single charge, what is the electric charge and its location?

A) Charge: +0.55nC; Location: x= 6mB) Charge: +2.2nC; Location: x= 6mC) Charge: +2.2nC; Location: x= 3mD) Charge: 2.2nC; Location: x= 6m

E) Charge:

2.2nC; Location: x=

3m

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3. Charge is distributed non-uniformly on a infinitely-long (but finite radius R) solid cylinder. Inside thecylinder, the volume charge density is proportional to r2. If the electric field at r = R/2 has magnitude E0,what is the electric field at r = R?

A) E0B) 2E0C) 4E0

D) 8E0E) 16E0

4. An electron (charge 1.602 1019 C, mass 9.11 1031 kg) is to be released from rest on the central axisof a uniformly charged disk of radius R = 10 cm and charge 1nC. What is the magnitude of the electronsinitial acceleration if it is released at a distance R/100? (picture is NOT drawn to scale.)

A) 3 106 m/s2

B) 3 108 m/s2

C) 3 1010 m/s2

D) 3 1012 m/s2

E) 3 1014 m/s2

5. A positive charge is placed next to a solid metal sphere, hanging by a string from the ceiling, that iselectrically neutral. Despite the fact that the metal sphere has no net charge, the two ob jects attract eachother. Which of the following best describes the reason for the attraction?

A) A Gaussian sphere around the positive charge requires a non-zero electric field throughout the metalsphere.

B) The capacitance of the metal sphere creates an electric field which attracts positive charges and repels

negative charges.C) The metal sphere picked up a negative charge from the string and the negative charge on the sphere is

attracted to the positive charge.D) Electric induction causes a rearrangement of charges on the sphere such that the positive charge is

attracted to the sphere.E) Since the metal sphere has no net charge, the effect cannot be of electric origin (it is due to magnetism,

instead).

6. The magnitude Eof an electric field depends on the radial distance r according to E=A/r5, where Ais a constant with the unit volt meter4. As a multiple ofA, what is the magnitude of the electric potentialdifference betweenr = 2.00m andr = 3.00m?

A) (0.0293)A meter3

B) (0.0125)A meter3

C) (0.0271)A meter3

D) (0.0417)A meter3

E) (0.0355)A meter3

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7. PointsA,B , andCare in a uniform electric field as shown below. Consider the following four statementsabout the change in electric potential:

Statement I: Valong the path from A B is negative.Statement II: Valong the closed loop from A B C A is zero.Statement III: Vis the same along the path from A B and the path from A C.Statement IV: Valong a straight line path from B Cis zero.

How many of the above statements are TRUE?

A) 0B) 1C) 2D) 3E) 4

8. You have two parallel-plate capacitors of equal area. The first capacitor has plate-separation 1 mm andhas air between the metal plates. The second capacitor has plate-separation 2 mm and is filled with a materialof dielectric constant 4.0. If we apply an electric potential difference of magnitude Vto both capacitors, whatis the ratio of the charge on the second capacitor to the charge on the first capacitor?

A) 2.0B) 0.50C) 8.0D) 0.125E) 4.0

9. What is the equivalent capacitance between pointsA & D of the circuit shown below?Other questions that could be asked about this arrangement of capacitors:

i) If the potential difference between pointsA & D has magnitudeV, what is the charge on the 4Ccapacitor?ii) If VAD has magnitudeV , what is the (magnitude of the) potential difference across the 4Ccapacitor?iii) If the charge on the capacitor with capacitanceC isQ, what is the energy stored in all of the capacitors?Answers: i) (8/13)CV , ii) (2/13)V, and iii) (13/22)Q2/C

A) (23/11)CB) 11CC) 13CD) (11/13)CE) (11/23)C

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10. Two identical conducting spheres, fixed in place, attract each other with an electrostatic force of 9 Nwhen their center-to-center separation is 1 m. The spheres are then connected by a thin conducting wire.When the wire is removed, the spheres repel each other with an electrostatic force of 0.9 N. What is the totalcharge on the two spheres?

A) 500CB) 100C

C) 1CD) 0.2CE) 20C

11. The figure below shows three sets of cross sections of equipotential surfaces in uniform electric fields; allthree cover the same size region of space. The electric potential is indicated for each equipotential surface.Rank the arrangements according to the electric field present, greatest electric field upwards first and greatestelectric field downwards last.

A) (1), (2), (3).B) (1), (3), (2).C) (2), (3), (1).D) (3), (2), (1).E) (3), (1), (2).

12. An isolated air-filled parallel-plate capacitor has initial chargeqand an initial voltageV. The separationof the plates is tripled, and wax is inserted between them. The voltage difference between the plates is still Vafter the change. What are (i) the final charge on the capacitor, and (ii) the dielectric constant of the wax?

A) (i) q, (ii) 9.B) (i) 2q, (ii) 9.

C) (i) 2q, (ii) 3.D) (i) q, (ii) 3.E) (i) 3q, (ii) 3.

13. The figure below shows short sections of two very long parallel lines of charge, fixed in place, separatedbyL. The uniform linear charge densities are 6.0 mC/m for line 1 and 2.0 mC/m for line 2. Where alongthex-axis shown is the net electric field from the two lines zero?

A) x= 2L/3B) x= L/3C) x= L/3D) x= 2L/3E) None of the above.

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14. Two electrons (massm, charge e) are separated by a distance d. Another electron is shot from infinityand stops midway between the two. What is its initial speed?

A) 2ke2/dB) ke2/d

C)

2ke/(dm)D)

ke/(2dm)

E)

8ke2/(dm)

15. The potential energies associated with four orientations of an electric dipole in an electric field are:(i) 9U0, (ii) 7U0, (iii) 3U0, and (iv) 7U0. Which orientation has the greatest torque acting on the dipole?(the dipole moment and electric field are the same for all four situations; only the orientation is different.)

A) (i)B) (ii)C) (iii)

D) (iv)E) Two configurations are equal and have a greater torque acting on them than the other two.

16. An electron on the axis of an electric dipole is 25 nm from the center of the dipole (assume the dipoleis much smaller than 25 nm). A vector pointing from the dipole to the electron is opposite the direction ofthe dipole moment vector. What is the magnitude and direction of the force on the electron if the dipolemoment is 2 1030 C m?

A) 9.2 1024 N towards the dipole.B) 9.2 1024 N away from the dipole.C) 3.7 1016 N towards the dipole.D) 3.7 1016 N away from the dipole.

E) 4.6 1024 N away from the dipole.

End of Exam

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practice midterm 1.2 v2

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