Physics C
Electric Fields
Name:_________________
AP Review
Charge
Net – charge: excess electrons
Net + charge: excess “holes”
SI unit: coulomb (C)
Quantum of charge:
the proton charge (e)
the electron charge (e)
e = 6.02
×
10
19
Coulombs
Conservation of Charge
The net charge in any process or reaction
remains unchanged.
Coulomb’s Law
Calculates magnitude of force between charges
Force is repulsive if charges have the same sign.
Force is attractive if charges have opposite signs.
F =
kq
1
q
2
/r
2
F: force (N)
k: constant 9.0
×
10
9
N m
2
/C
2
q
1
,
q
2
: charges (C)
r: distance between charge centers (m)
NOTE:
k =
1/(4
πε
o
)
where
ε
o
= 8.85
×
10
12
C
2
/ N m
2
Multiple Forces on a Charges
F
tot
=
Σ
F
i
Problems: Electric Force (1998)
Questions 39

40
As shown above, two particles, each of charge
+Q, are fixed at opposite corners of a square
that lies in the plane of the page. A positive
test charge +q is placed at a third corner.
39. What is the direction of the force on the test
charge due to the two other charges?
(A)
(B)
(C)
(D)
(E)
Explain your reasoning
40. If F is the magnitude of the force on the test
charge due to only one of the other charges,
what is the magnitude of the net force acting on
the test charge due to both of these charges?
(A) Zero
(B)
2
F
(C) F
(D)
F
2
(E) 2
Show your work
Problem: Electric Force (1998)
55.
Suppose that an electron (charge e) could
orbit a proton (charge +e) in a circular orbit of
constant radius R. Assuming that the proton is
stationary and only electrostatic forces act on the
particles, which of the following represents the
kinetic energy of the twoparticle system?
e
0
4
1
πε
e
2
0
8
1
πε
e
2
0
8
1
πε

2
0
4
1
R
e
πε
2
2
0
4
1
R
e
πε

Show your work
2/26/2012
1
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Problem: Electric Force (1988)
36
Two small spheres have equal charges q and
are separated by a distance d.
The force
exerted on each sphere by the other has
magnitude F.
If the charge on each sphere is
doubled and d is halved, the force on each
sphere has magnitude
(A) F
(B) 2F
(C) 4F
(D) 8F
(E) 16F
Show your work
Electric Field
Exists in space due to the presence of charge.
Predicts what will happen to a charged particle
put in that location in space.
Field is directed outward from the positive
charges creating it.
Field is directed inward toward the negative
charge creating it.
This equation calculates magnitude of field at point
in space near one spherically symmetric charge:
E =
kq/r
2
(magnitude calculation)
E: field (N/C)
k: constant 9.0
×
10
9
N m
2
/C
2
q: charge (C)
r: distance between center of charge and point
in space (m)
The equation above works only for spherically
symmetric charge distributions.
NOTE:
Positive charges experience a force in the same
direction as the electric field is pointing.
Negative charges experience a force in the
opposite direction as the electric field.
Problem: Electric Field (1998)
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 Spring '09
 IRENEPOLYCARPOU
 Charge, Electric Fields, Electric charge

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