Practice Problems: Coulomb’s Law and Electric Field Strength
F
K  q1  q2
r2
q2 
r
K  q1  q 2
F
F r2
K  q1
K  8.99 x10 9
qe  1.6 x10 19 C
1C  6.24 x1018 e 
F
W
V
V  Ed
q
q
q = charge that is in the electric
field, not the one creating it
E
q = ___________; unit is ___________
V = ___________; unit is ___________
E = ___________; unit is ___________
W = ___________; unit is ___________
d and r = ___________; unit is ___________
F = ___________; unit is ___________
Example: A Van de Graaff generator has a potential difference of 3,250,000 V. It exerts 1.95 N of
force on a balloon in its electric field. The balloon has a charge of 0.475 C.
a. What is the strength of the electric field around the generator?
b. How far was the balloon moved?
c. When the balloon was 15 cm from the generator, the force between them was 3.8 N. What
was the charge on the generator? Give your answer in microcoulombs.
1. A balloon is given a charge of –1.5 C by rubbing it on some hair.
a. If the hair and balloon are 0.200 m apart, how much force is between them?
b. Will they attract or repel each other?
c. How much strength does the electric field have around the balloon?
d. How much potential difference will be present if the hair is pulled away from the
balloons field a distance of 5.01 cm?
e. How much work was done to move the hair?
f. How many electrons were transferred to the balloon?
2. A dime and a nickel are electrically charged. The electric field strength around the dime is
4002 N/C. The nickel has a charge of +21.9 C. They are 20.0 cm apart.
a. How much force is exerted on the nickel by the dime’s electric field?
b. How big is the charge on the dime?
c. How much work would you do on the nickel to obtain a potential difference of 50 V?
3. A cloud might have –5.00 C of charge and be 200.0 m above the earth. The ground gains
+3.000 C because of induction.
a. How many electrons make up the -5.00 C charge?
b. How much force will be unleashed by the bolt of lightening that follows?
c. How strong is the electric field around the cloud?
d. What potential difference was created when the electrons moved the 200 meters to
the ground?
e. How much work was done to move the electrons to the ground?
4. If the clouds charge was doubled, what would happen to the force of the bolt? (Do a
calculation to check your prediction.)
5. If the clouds were half as high up off the ground, what would happen to the force of the bolt?
(Do a calculation to check your prediction.)
Answers:
Example
a. 4.11 x 106 N/C
b. 0.792 m
c. -20. C
1. Remember that the balloon received its charge from the hair.
a. -0.51 N
b. (hint: what kind of charges do they have?
c. -3.4 x 105 N/C
d. 17,000 V
e. 0.025 J
f. 9.4 x 1012 electrons
2.
a. 0.0876 N
b. 1.8 x 10-8 C
c. 0.001 J
3.
a.
b.
c.
d.
e.
3.13 x 1019 electrons
-3.37 MN
-1.12 x 106 N/C
2.25 x 108 V
1.01 x 107 J