Q: A $$2.0 \ \mu\text{F}$$ capacitor is charged to $$100 \ \text{V}. $$How much energy is stored in the capacitor?

$$U = \frac{1}{2} C V^2$ = 0.01 \ \text{J}$$

Question 1

A plastic rod is rubbed with wool and then brought near a neutral metal sphere. What happens to the charges in the metal sphere, and what is the name of this phenomenon?

Accepted Answer

Negative charges in the sphere are attracted to the rod, causing a net attractive force; this is called polarization.

Question 2

Which of the following best describes the law of conservation of charge?

Accepted Answer

Charge is neither created nor destroyed, only transferred from one place to another.

Question 3

Two identical metal spheres are connected by a metal rod. A charged plastic rod touches one sphere. Afterward, both spheres repel a charged hanging plastic rod. What property of metals explains this observation?

Accepted Answer

Metals are conductors, so charge moves freely and distributes over both spheres.

Question 4

A glass rod rubbed with silk is brought near a hanging plastic rod rubbed with wool. The rods attract each other. What can you conclude about the charges on the rods?

Accepted Answer

The glass rod is positively charged and the plastic rod is negatively charged.

Question 5

Which of the following statements about electric forces is correct?

Accepted Answer

Electric forces can be both attractive and repulsive, depending on the charges involved.

Question 6

A neutral object is brought near a negatively charged rod. What happens to the charges in the neutral object?

Accepted Answer

Positive and negative charges in the object separate slightly, with positive charges moving closer to the rod.

Question 7

Which of the following best describes the process of charging by induction?

Accepted Answer

Separating charges in a conductor by bringing a charged object near and then separating the conductor while polarized.

Question 8

A point charge $$q$$ creates an electric field at a distance $$r. $$What is the magnitude of the electric field at that point?

Accepted Answer

$$E = \frac{1}{4\pi\$$epsilon_0$$} \frac{q}{r^2$}$$

Question 9

A parallel-plate capacitor has plates of area $$A$$ and separation $$d. $$What is the capacitance of this capacitor?

Accepted Answer

$$C = \$$epsilon_0$$ \frac{A}{d}$$

Question 10

A $$2.0 \ \mu	ext{F}$$ capacitor is charged to $$100 \ 	ext{V}. $$How much energy is stored in the capacitor?

Accepted Answer

$$U = \frac{1}{2} C V^2$ = 0.01 \ 	ext{J}$$

Question 11

A proton is released from rest at a point where the electric potential is $$+100 \ 	ext{V}. It $$moves to a point where the potential is $$-150 \ 	ext{V}. $$What happens to its kinetic energy?

Accepted Answer

It increases.

Question 12

If the distance between the plates of a parallel-plate capacitor is doubled while the charge remains constant, what happens to the electric field between the plates?

Accepted Answer

It remains the same.

Question 13

A $$10 \ 	ext{nC}$$ charge is placed at a point where the electric potential is $$500 \ 	ext{V}. $$What is the electric potential energy of the charge at that point?

Accepted Answer

$$U = 5 	imes 10$$^{-6}$$ \ 	ext{J}$$

Question 14

A dielectric with dielectric constant $$\kappa is $$inserted between the plates of a capacitor after it has been charged and disconnected from the battery. What happens to the voltage across the plates?

Accepted Answer

It decreases by a factor of $$\kappa.$$

Question 15

Which of the following is true about the electric field inside a conductor in electrostatic equilibrium?

Accepted Answer

The electric field is zero everywhere inside the conductor.

Question 16

A $$1.0 \ 	ext{nC}$$ point charge is located at the origin. What is the electric potential at a point $$2.0 \ 	ext{cm}$$ away from the charge?

Accepted Answer

$$V = 4.5 	imes 10$$^{2}$$ \ 	ext{V}$$

Question 17

Which of the following best describes the energy stored in a capacitor?

Accepted Answer

It is stored as electric potential energy in the electric field between the plates.

Question 18

A DNA molecule is placed in a gel and an electric field is applied. What happens to the DNA fragments?

Accepted Answer

They move toward the positive electrode because DNA is negatively charged.

Electric Fields and Forces: Study Notes for College Physics

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