Q: A parallel plate capacitor has an electric field strength of $$1600\ \text{V/m}$$ and plate separation $$0.0075\ \text{m}. $$What is the potential difference across the plates?

$$\Delta V = Ed = 1600 \times 0.0075 = 12\ \text{V}$$

Q: A proton moves from a point at potential $$+125\ \text{V} to a $$point at $$-45\ \text{V}. $$How much work does the electric field do on the proton?

$$W = q \Delta V = (1.6 \times 10$$^{-19}$$\ \text{C}) \times (-170\ \text{V}) = -2.72 \times 10$$^{-17}$$\ \text{J}$$

Q: What is the capacitance $$C of a $$parallel plate capacitor with plate area $$A = 0.0280\ \text{m}^2$$ and separation $$d = 0.550\ \text{mm}$$, using $$\$$epsilon_0$$ = 8.85 \times 10$$^{-12}$$\ \text{F/m}$$?

$$C = \frac{\$$epsilon_0$$ A}{d} = \frac{8.85 \times 10$$^{-12}$$ \times 0.0280}{0.00055} \approx 4.5 \times 10$$^{-10}$$\ \text{F}$$

Question 1

A positive charge is moved from position i to position f near another positive charge. What happens to its electric potential energy?

Accepted Answer

It increases because work is done against the repulsive force.

Question 2

What is the electric potential at a point A a distance $$r$$ from a fixed charge $$+Q$$?

Accepted Answer

It is $$V = \frac{Q}{4\pi\$$epsilon_0$$ r}$$ for both positive and negative test charges.

Question 3

Two charges are brought separately into the vicinity of a fixed charge $$+Q. $$First, $$+q is $$brought to point A, a distance $$r$$ away. Second, $$+q is $$removed and $$-q is $$brought to the same point A. The electric potential energy at point A is:

Accepted Answer

Greater for the $$+q$$ charge in situation 1.

Question 4

If two charges have opposite signs (one positive and one negative), what is the sign of their electric potential energy when placed near each other?

Accepted Answer

Negative, because the force is attractive and work is done in the same direction as the force.

Question 5

A positive and a negative charge are released from rest in vacuum. They move toward each other. As they do:

Accepted Answer

A negative potential energy becomes more negative.

Question 6

The electric potential $$V at a $$distance $$r$$ from a point charge $$Q is $$given by:

Accepted Answer

$$V = \frac{Q}{4\pi\$$epsilon_0$$ r}$$

Question 7

A parallel plate capacitor has an electric field strength of $$1600\ 	ext{V/m}$$ and plate separation $$0.0075\ 	ext{m}. $$What is the potential difference across the plates?

Accepted Answer

$$\Delta V = Ed = 1600 	imes 0.0075 = 12\ 	ext{V}$$

Question 8

A proton moves from a point at potential $$+125\ 	ext{V} to a $$point at $$-45\ 	ext{V}. $$How much work does the electric field do on the proton?

Accepted Answer

$$W = q \Delta V = (1.6 	imes 10$$^{-19}$$\ 	ext{C}) 	imes (-170\ 	ext{V}) = -2.72 	imes 10$$^{-17}$$\ 	ext{J}$$

Question 9

What is the capacitance $$C of a $$parallel plate capacitor with plate area $$A = 0.0280\ 	ext{m}^2$$ and separation $$d = 0.550\ 	ext{mm}$$, using $$\$$epsilon_0$$ = 8.85 	imes 10$$^{-12}$$\ 	ext{F/m}$$?

Accepted Answer

$$C = \frac{\$$epsilon_0$$ A}{d} = \frac{8.85 	imes 10$$^{-12}$$ 	imes 0.0280}{0.00055} \approx 4.5 	imes 10$$^{-10}$$\ 	ext{F}$$

Question 10

A capacitor is connected to a battery and then disconnected. What happens to the potential difference across the capacitor?

Accepted Answer

It remains equal to the battery voltage.

Question 11

A defibrillator stores $$360\ 	ext{J} of $$energy and releases it in $$2\ 	ext{ms}. $$What is the average power delivered?

Accepted Answer

$$P = \frac{E}{t} = \frac{360}{0.002} = 180,000\ 	ext{W}$$

Question 12

What is the current in a wire if $$120\ 	ext{C} of $$charge flows through it every minute?

Accepted Answer

$$I = \frac{Q}{t} = \frac{120}{60} = 2\ 	ext{A}$$

Question 13

A potential difference of $$24\ 	ext{V} is $$applied to a $$150\ \Omega$$ resistor. What is the current?

Accepted Answer

$$I = \frac{V}{R} = \frac{24}{150} = 0.16\ 	ext{A}$$

Question 14

What is the resistance of a 40-W automobile headlight designed for 12 V?

Accepted Answer

$$R = \frac{V^2$}{P} = \frac{12^2$}{40} = 3.6\ \Omega$$

Question 15

In a circuit consisting of a battery and a single resistor, what happens to the energy supplied by the battery?

Accepted Answer

It is converted to thermal energy in the resistor.

Question 16

Which bulb glows more brightly in a series circuit with two identical bulbs?

Accepted Answer

Both bulbs are equally bright because they have the same current and resistance.

Question 17

In a parallel circuit with two identical bulbs, how does the brightness of bulb A compare to bulb B?

Accepted Answer

Both bulbs are equally bright because they are connected to the same potential difference.

Question 18

A circuit consists of three resistors connected in series: $$R_1 = 250$$\ \Omega$$, R_2$ = 150\ \Omega$$, $$R_3 = 100$$\ \Omega$$. What is the equivalent resistance?

Accepted Answer

$$R_{eq} = R_1$ + R_2$ + R_3$ = 250 + 150 + 100 = 500\ \Omega$$

Electric Potential, Capacitance, and DC Circuits: Study Notes

Study Guide - Practice Questions

Study Guide - Flashcards