Question 1

A metal bucket with a metal lid has zero excess charge. You place a metal ball with an excess of positive charge, $$+q$$, inside the bucket, but the ball does not touch the bucket. At electrostatic equilibrium, what is the excess charge on the inner and outer surfaces of the bucket?

Accepted Answer

The inner surface has charge $$-q$$ and the outer surface has charge $$+q.$$

Question 2

A parallel-plate capacitor is fully charged by a battery and then disconnected. If the separation between the plates is doubled, what happens to the electric field between the plates?

Accepted Answer

The electric field remains the same.

Question 3

The electric potential energy stored in a system of two point charges $$q_1$$ and $$q_2$$ separated by a distance $$r is $$given by which formula?

Accepted Answer

$$U_{elec} = K \frac{q_1$ q_2$}{r}$$

Question 4

An electron is released from rest just above the negative plate of a parallel-plate capacitor with a potential difference of $$2.4 	imes 10^4$ V. What is its speed when it reaches the positive plate? (Electron charge q = -1.6$$ 	imes $$10^{-19} C$$, mass $$m = 9.1 	imes 10$$^{-31}$$ kg)

Accepted Answer

$$v = \sqrt{\frac{-2qV_{cap}}{m}}$$

Question 5

Two metal spheres, A and B, are separated by a large distance. The radius of sphere A is twice that of sphere B. Both spheres have charge uniformly distributed over the surface, and the electric potential at the surface of both spheres is equal. How does the charge on sphere A, $$Q_A$$, compare to that on sphere B, $$Q_B$$?

Accepted Answer

$$Q_A = 2Q_B$$

Question 6

A parallel-plate capacitor is fully charged by a battery and then disconnected. The area of the plates is then halved (with the total charge remaining the same). What happens to the potential difference between the plates?

Accepted Answer

The potential difference doubles.

Question 7

The electric potential in a region of space is described by $$V(y) = A + \frac{B}{y^2$}$$, where $$A = -2 	imes 10$$^{-3}$$ V and B = 1$ m^2$/V. Which of the following statements is TRUE at y = 2$ m?

Accepted Answer

The electric field points in the positive $$y$$ direction.

Question 8

A parallel-plate capacitor is fully charged by a battery and then disconnected. A dielectric is inserted between the plates (without touching them). What happens to the potential difference between the plates?

Accepted Answer

The potential difference decreases.

Question 9

Which of the following changes will increase the current through a wire by a factor of 4, assuming the potential difference across the wire is kept constant?

Accepted Answer

Increase the radius of the wire by a factor of 2.

Question 10

A vertical wire carries current upward through the center of a horizontal table. A compass rests on the table. In which direction does the north pole of the compass point?

Accepted Answer

Tangential to the circle centered on the wire, following the right-hand rule.

Question 11

A parallel-plate capacitor is fully charged by a battery and then disconnected. The plates are then pulled apart until they are separated by a distance $$2d. $$What is the ratio of the final to initial electrostatic energies, $$U_f/U_i$$?

Accepted Answer

$$2$$

Question 12

The electric field between the plates of a parallel-plate capacitor is given by $$E = \frac{Q}{\$$epsilon_0$$ A}. If $$the area $$A is $$halved while the charge $$Q$$ remains constant, what happens to the electric field?

Accepted Answer

The electric field doubles.

Question 13

A battery is connected to a parallel-plate capacitor. If a dielectric is inserted between the plates while the battery remains connected, what happens to the charge on the plates?

Accepted Answer

The charge increases.

Question 14

Which of the following best describes the function of an electrostatic precipitator in a coal-burning power plant?

Accepted Answer

It removes smoke particles from waste gas by charging them and attracting them to collecting plates.

Question 15

The electric potential at a distance $$r$$ from a point charge $$q is $$given by which formula (assuming $$V = 0 at $$infinity)?

Accepted Answer

$$V = \frac{1}{4\pi\$$epsilon_0$$} \frac{q}{r}$$

Question 16

A parallel-plate capacitor is fully charged by a battery. The positive plate has charge $$+q$$ and the negative plate has charge $$-q. $$What happens to the charge on the plates when the separation between plates is doubled while the battery is still connected?

Accepted Answer

The charge halves.

Question 17

Which of the following statements about the electric field and electric potential inside a conductor in electrostatic equilibrium is correct?

Accepted Answer

The electric field is zero and the potential is constant everywhere inside the conductor.

Question 18

A system consists of two charges, $$+q$$ and $$-q$$, fixed in position. If a third charge is added at a point equidistant from both, how does the total electric potential energy of the system change?

Accepted Answer

The potential energy does not change.

Question 19

The current density $$J in a $$wire is related to the electric field $$E$$ and the conductivity $$\sigma by $$which equation?

Accepted Answer

$$J = \sigma E$$

Electrostatics, Electric Potential, and Capacitance: Study Notes for Physics College Students

Study Guide - Practice Questions

Study Guide - Flashcards