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6.2 Electric Force and Field (2 hr)
Name _____________________
Activity 621 Investigating Electric Force
Activity 622 Electric Field Hockey
Read Tsokos pp 279-287, 289-291
Read Cutnell pp 533 to 552
Assignment A621 -- Tsokos pp 287, 288 #3-10, Holt SG pp91-96,
Assignment A622 -- Cutnell p563 #8,9,12,13,17,
Assignment A623 -- Ranking pp38,39(?), 125, 126, 134, 135, 136,
137137, 138, 140, 141,142, 145, 146, 148, 149
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6.2 Electric Force and Field
Electric Charge
A. Properties of Charges
(a) Two types of charge:
Positive: proton, quarks (up, charmed, and top)
Negative: electron, quarks (down, strange, and bottom)
(b) Charges are measured in a unit called Coulomb (C).
The charge on an electron e– = –1.602x10–19 C
The charge on a proton p+ = + 1.602x10–19 C
The charge on up, charmed, and top quarks = + e
The charge on down, strange, and bottom = - e
The symbol e = 1.6x10–19 C
(c) An atom is neutral when
Number of e- = number of p+
An object becomes positively charged when it loses
electrons; an object becomes negatively charged when it
gains electrons.
(d) Electric charge, which can be positive or negative,
occurs in discrete natural units (Charge always comes in
an integral multiple of a basic unit - it is quantized.)
(e) Electric charge is neither created nor destroyed.
(f) The total electric charge of the universe is constant. No
physical process can make this happen
Summary
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(g) Any kind of charge attracts neutral object because of
polarization. (Polarization is the process of separating
opposite charges within an object. The object still
remains neutral while the electrons are pulled toward
one of the ends by an external electrical attraction
causing both ends with different excessive charges)
(h) This basic unit of charge is conventionally denoted by e :
e = 1.602 x 10- 19 Coulombs (C)
(i) All natural charged objects must have a net charge that
is an integral of e, which was confirmed by American
Physicist Robert A. Millikan.(1868-1953)
<Sample question 1>
How much charge does a lithium ion, carries? There
are 3 protons and 4 neutrons in the lithium nucleus.
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<Sample question 2>
An electronically neutral object acquires a net electric
charge. Which one of the following statements
concerning the mass of the object is true? ______
(i) The mass does not change
(ii) The mass decreases if the charge is negative and
increases if the charge is positive.
(iii) The mass increases if the charge is negative and
decreases if the charge is positive.
Summary
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Electrostatic Phenomenon:
a. Charged objects may bend water, attract dust, attract
small pieces of paper, cause electric shock.
b. Colliding crystals of ice in a rain cloud can cause charge
separation – result in lightning bolts, to bring unlike
charges together again.
c. Voyager spacecraft observed electric discharges between
the particles in the rings of Saturn. The particles are
constantly undergoing collision and becoming charged.
Application of Electrostatic
d. Air filter
e. metal coating
f. CO2 fire extinguisher
g. Photocopiers and printers
h. Disordering tools (to apply strong suction to remove
excess molten solder from a printed circuit board or
connection terminal.)
B. Law of Conservation of Electric Charges
During any process, the net electric charge of an
isolated system remains constant.
<Sample process>
When Kim uses a piece of nylon cloth to wipe a Teflon frying
pan, the nylon becomes positively charged while the Teflon
becomes negatively charge because the
electrons on the nylon are transferred to
the Teflon.
<Sample process>
HeHH 3
2
2
1
1
1 (Fusion)
nRbCsUUn 1
0
90
37
143
55
*236
92
235
92
1
0 3 (Fission)
Summary
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C. Conductors and Insulators
Conductors: Materials, in which electric charges move about
more or less freely. Copper and gold are called electrical
conductors
Insulators: Materials, in which electric charges do not move
freely because of the strong bonding. Glass, rubbers, silk,
and plastic, are called electrical insulators
Both conductors and insulators may be charged by
induction and contact.
Most metals are good conductors and most non-metals
are not.
Most electric insulators are also good thermal
insulators.
Semiconductors: the materials that has properties between
conductors and insulators; the materials that can be “fine-
tuned” to display desired conductivity by controlling the
concentration of “impurities” inside of the materials.
http://superconductors.org/INdex.htm
Photoconductive materials: the materials that conducts
electricity when exposed to light (or due to the absorption of
electromagnetic radiation/wave but are insulators when in the
dark.
Summary
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Coulomb’s Law (Electric Force)
Coulomb’s Law states that the electric force between two
point charges is directly proportional to the charges and
inversely proportional to the distance between two charges.
The electric force between Q1 and Q2 is given by
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r
qqkF or
Where Q1, Q2 are amount of charges,
r is the separation between them, and
k is Coulomb’s constant;
k =
= 8.99x109 N m2 C–2
is the permittivity of free space
– N–1 m2 C–2
<Sample Question>
Q2 Two charges, q 1 = 4μC and q2 = 6 μC, are placed
along a straight line and separated by a distance of 2
cm. Find the force exerted on each other.
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Q3 At what distance from q1 of example question Q2
would a third positive charge experienced no net
electric force?
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Q4 Three charges, q 1 = 2μC and q2 = 2 μC and q3 = –3 μC,
are at the vertices of an equilateral triangle, of side 3
cm. Find the force on q1.
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Q5 Two equal charges q are suspended from string as
shown. Show that tan =
,
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Q6 Two identical conducting spheres are kept a certain
distance “r” apart. One sphere has a positive charge Q on
the surface and the other is neutral. The spheres are
allowed to touch and then separated. Write down an
expression for the electric force between the spheres. One
sphere is discharged. The spheres are allowed to touch and
then separated again. Write down the expression for the
electric force between the spheres now.
Summary
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Q 7 Compare the net electric force acting on charge #1, 2,
3, and 4 as shown in the diagram
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Electric Field, E
Electric field is defined as the force per unit charge
experienced by a small positive test charge.
The strength of an electric field, E, at a certain point in the space
is defined to be the magnitude of the electric force, F, acting on
a small test charge divided by the magnitude of its charge, q0:
0q
FE
Where E is the electric field, F is the electric force acting on the text
charge, qo is the amount of charge on the text charge
SI unit of electric field is newton per coulomb (NC– -1 ) or volt
per meter (voltm–1)
Direction of E at a point is defined to be the direction of the
electric force that would be exerted on a small positive charge
placed at that point.
The test charge should be small, both in physical size and charge,
to accurately measure the electric field. Strong test charge may
influence the source of the electric field to be determined.
Acting through space – no physical contact. An electric field
exists in the region of space around a charged object. Electric
forces arise when another charged object enters this electric
field.
Consider the electric field due to a point charge, Q. The
magnitude of the force exerted on a test charge, q, at a distance
r away is
The magnitude of the electrical field due to charge Q at the
position of q is F/q
Summary
2r
QkE
2r
qQkF
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Q7 To determine the direction and the strength of an uniform
electric field, Tom places a very small particle charge carrying
+2.0 μC in the field. The electric force acting on the test charge
is detected as 26 pN, south. What is the direction and the
magnitude of the electric field?
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Q8 A + 20 nC positively charge creates an electric field in
space around it. What is the electric field 0.4 m from the
center of the charge?
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Q9 A charge, q1 = 5.00 μC, is at the origin, and a second
charge, q2 = –3.00 μC, is on the x-axis 0.800 m from the
origin. Find the electric field at a point on x = 0.200m.
Summary
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Q10 An electric field of 2.0x104 NC –1 is directed along the
positive x-axis. What is the electric force on an electron in this
field?
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Q11 Describe the motion of a proton in an uniform electric
field as shown.
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Summary
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Q12 A small 2.0 g plastic ball is suspected by a 20.0 cm string in a
uniform electric field of 1.0 x 104 NC–1, as shown below.
(a) In the ball’s charge positive or negative?
(b) If the ball is in equilibrium when the string makes a 15o
angle with the vertical as indicated, what is the net
charge on the ball?
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Summary
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Electric Field Lines
The following rules apply to electric field lines:
1. Lines begin and end only at charges (beginning at +
charges, ending at - charges) or at Infinity.
2. Lines are closer together where the field is stronger.
3. Larger charges have more field lines beginning or ending
on them.
4. Electric Field lines never cross (since E must point in a
definite direction unless it is zero).
5. At any location, the direction of the electric field is
tangent to the electric field line that passes through
that location.
6. Electric file lines are perpendicular to surface of
conductors.
Examples:
1. Positive point charge 2. Negative point charge
3. Electric dipole 4. Electric dipole (Two equal and opposite point charges) (Two equal and same point charges)
5. Two parallel charged plates 6. Two unequal charges
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Activity 621 Investigating Electric Force
Coulomb's law, or Coulomb's inverse-square law, is a law of physics describing the
electrostatic interaction between electrically charged particles. The magnitude of
the electric force is directly proportional to the charges and inversely proportional
to distance square.
Essential Question:
How do you determine the electric force between two charged objects?
1. Identify the chosen variables.
2. Decide the procedure and divide the work among partners
3. Carry it out collaboratively
4. Check your findings and analyze it.
Activity 622 Electric Field Hockey
Go to the following link http://phet.colorado.edu/en/simulation/electric-hockey, play all
three levels of the electric field games, copy (control + C) and paste (control +V)the wining
image for the proof.
