Key
Static Electricity Conceptual Questions
1. A positively charged rod is brought near, but does not touch, a neutral metal ball sitting on a table. The ball
rolls towards the rod.
a. Is the ball charged (+), (-) or 0? 0
b. Describe the motion of charges on the ball when the rod is brought near. Be specific with both sign
and direction. Electrons in ball move towards the rod. Protons cannot move, so they stay
where they are even though they repel the rod.
c. Why is the ball attracted to the rod? Charge polarization
2. Why will a neutral object never be repelled by a charged object? The opposite charge will always be
attracted to the charged object and want to minimize the distance between it and the charged
object. The like charge will never move closer to the charged object.
3. A positive rod touches a neutral pith ball on a string. As a result of being touched, the ball flies away from
the rod.
a. What is the method of charging? (Induction, Contact, or Friction) Contact
b. Describe the flow of charge when the ball touches the rod. electrons flow from ball to rod,
leaving a (+) charge
c. Explain why the pith ball flies away from the rod. Both now have the same charge (+).
4. A balloon is rubbed against your hair. The balloon sticks to your hair.
a. What is the method of charging? Friction
b. Why does the balloon stick to your hair? Charge (equal in magnitude, opposite in sign) is
transferred between balloon and hair, so they are attracted.
5. A neutral pith ball on a string is held near a positively charged Van de Graff globe, without touching. You
touch the ball and then remove your hand. Now, the ball is attracted to the globe.
a. Is the pith ball now charged (+), (-), or 0? (-)
b. What is the method of charging? Induction
c. Describe the flow of charge when you touched the ball. Electrons flowed up from the ground,
through my hand, to the ball.
6. Describe the flow of charge in the following two examples.
a. You ground (touch) a positively
charged globe. (e-) flow up hand
b. You ground a neutral globe that is near
a negatively charged rod. (e-) flow down hand
7. You rub your feet back and forth on the carpet, acquiring a charge. You know you're charged, because you
then shock your friend who's standing next to you. While you were rubbing your feet, were you grounded
or not grounded? Explain how you know. Not grounded. The carpet insulated you from the ground.
Your friend acts as the ground.
8. The correct answer to question #3(b) is that electrons flow from the ball into the rod. Why didn't protons
flow from the rod into the ball? More generally, why do electrons flow (move from one place to another)
but protons do not flow?
Protons can’t leave the nucleus, but since electrons are outside of the nucleus they are free to leave
the atom / move.
9. (a) What does it mean for a substance to be a good electrical conductor? A good heat conductor? Heat /
electricity can flow easily through the object.
b) Metals are good electrical conductors and good heat conductors for the same reason. What is that reason?
Electrons can move easily through the metal.
q1 q2
.
d2
mm
Newton's Law for gravitational force: Fg = G 1 2 2
d
10. Coulomb's Law for electrostatic force: FE = k
Which is the true statement? C
(a) G is a much larger number than k.
(c) K is a much larger number than G.
(b) G is larger, but not by a lot.
(d) K is larger, but not by a lot.
11. Explain the significance of your answer to the previous question.
The electrostatic force is a much larger force than the gravitational force.
12. If electrostatic force is so much stronger than gravitation, why do we notice gravitation every day, but static
electricity only occasionally?
Charges cancel out, but masses don’t.
13. Why do Rice Krispies placed in a paper cup on top of a charged Van de Graff globe fly up?
They all have the same charge and repel one another
14. Why do Rice Krispies placed in a metal cup on top of a charged Van de Graff globe not fly up?
The metal acts as a conductor, so the charge is all on the outside of the metal cup. The net charge inside the
cup is zero.
15. You walk close to a charged Van de Graff globe without touching it. If you walk too close, a spark might fly
from the globe to you and shock you. Are you more likely to get sparked if you are wearing metal-soled
shoes or rubber-soled shoes? Why?
With a high enough charge, it won’t matter because anything can act as a conductor. But, if it is not a very
high charge, you will be more likely charged with metal shoes because they act as better grounds.
16. You are sitting in a car during a thunder and lightning storm. True or false: your car is unlikely to get struck
by lightning because of the rubber tires? Explain.
False. With high enough charge anything will act as a conductor.
17. Why are you safe in the car, even if it is struck by lightning? Explain.
The charge is distributed all along the outside of the metal car, not the inside. The net charge on the inside, as
a result, is zero.
18. Which is a better conductor of electricity: humid air that has water in it or dry air? Air with water in it.
19. You charge up the Van de Graff globe and then stop cranking. Will the globe stay charged longer on a dry
day or a humid day? Explain. Dry day. If it were humid, the water molecules in the air would absorb
the charge, so the charge would leak off the Van de Graff into the air.