Q: What is the charge of an atom with 16 protons and 7 electrons? (Elementary charge $$e = 1.6 \times 10$$^{-19}$$ C)

$$Q = (16 - 7) \times 1.6 \times 10$$^{-19}$$ = 1.44 \times 10$$^{-18}$$ C

Q: A 1.5 $$\muC $$charge with a mass of 50 g is suspended in an electric field that balances its weight. What is the magnitude of the electric field?

$$E = \dfrac{mg}{q} = \dfrac{0.05 \times 9.8}{1.5 \times 10$$^{-6}$$} = 3.27 \times 10^5$ N/C

Q: A 5 $$\muC $$charge is placed at a point where the electric potential is 200 V. What is the electric potential energy of the charge?

$$U = qV = 5 \times 10$$^{-6}$$ \times 200 = 1 \times 10$$^{-3}$$ J

Question 1

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

Accepted Answer

Electric charge can only be transferred from one object to another; it cannot be created or destroyed.

Question 2

What is the charge of an atom with 16 protons and 7 electrons? (Elementary charge $$e = 1.6 	imes 10$$^{-19}$$ C)

Accepted Answer

$$Q = (16 - 7) 	imes 1.6 	imes 10$$^{-19}$$ = 1.44 	imes 10$$^{-18}$$ C

Question 3

If two identical charges are separated by a distance $$d$$ and experience a force $$F$$, what will the force be if the distance is doubled?

Accepted Answer

$$F/4$$

Question 4

Which equation correctly gives the electric force between two point charges $$q_1$$ and $$q_2$$ separated by a distance $$r$$?

Accepted Answer

$$F = k \dfrac{q_1$ q_2$}{r^2$}$$

Question 5

A 1.5 $$\muC $$charge with a mass of 50 g is suspended in an electric field that balances its weight. What is the magnitude of the electric field?

Accepted Answer

$$E = \dfrac{mg}{q} = \dfrac{0.05 	imes 9.8}{1.5 	imes 10$$^{-6}$$} = 3.27 	imes 10^5$ N/C

Question 6

Which of the following best describes the direction of electric field lines?

Accepted Answer

From positive to negative charges

Question 7

A capacitor consists of two parallel plates with area $$A$$ and charge $$Q on $$each plate. What is the electric field between the plates?

Accepted Answer

$$E = \dfrac{Q}{\$$epsilon_0$$ A}$$

Question 8

What is the potential energy of two point charges $$q_1$$ and $$q_2$$ separated by a distance $$r$$?

Accepted Answer

$$U = k \dfrac{q_1$ q_2$}{r}$$

Question 9

A 5 $$\muC $$charge is placed at a point where the electric potential is 200 V. What is the electric potential energy of the charge?

Accepted Answer

$$U = qV = 5 	imes 10$$^{-6}$$ 	imes 200 = 1 	imes 10$$^{-3}$$ J

Question 10

Which of the following statements about conductors in electrostatic equilibrium is correct?

Accepted Answer

The electric field inside the conductor is zero.

Question 11

A 2 nC charge is initially 5 mm away from a 10 nC charge. It is then moved 2 mm closer. What is the work done by the electric force?

Accepted Answer

$$W = k q_1$ q_2$ \left( \dfrac{1}{r_i} - \dfrac{1}{r_f} \right )$$

Question 12

Which of the following is true about the movement of charges in an electric potential field?

Accepted Answer

Positive charges move to lower potential, negative charges move to higher potential.

Question 13

What is the capacitance of a capacitor if it stores a charge of 18 $$\muC at a $$potential difference of 9 V?

Accepted Answer

$$C = \dfrac{Q}{V} = \dfrac{18 	imes 10$$^{-6}$$}{9} = 2 	imes 10$$^{-6}$$ F

Question 14

A 3 $$\muC $$and a –2 $$\muC $$charge are separated so that their potential energy is –100 mJ. What is their separation?

Accepted Answer

$$r = k \dfrac{q_1$ q_2$}{U} = \dfrac{9 	imes 10^9$ 	imes 3 	imes 10$$^{-6}$$ 	imes (-2 	imes 10$$^{-6}$$)}{-0.1} = 0.54 m$$

Question 15

Which of the following best describes the effect of doubling the charge and halving the area of the plates in a parallel-plate capacitor on the electric field between the plates?

Accepted Answer

The electric field increases by a factor of 4.

Question 16

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

Accepted Answer

$$V = k \dfrac{Q}{r}$$

Question 17

A 5 g, 3 $$\muC $$point charge is moving with an initial speed of 20 m/s away from a –5 $$\muC $$charge. If they are initially 5 cm apart, how far can the 3 $$\muC $$travel before stopping?

Accepted Answer

Use energy conservation: $$\dfrac{1}{2}mv^2$ = k \dfrac{q_1$ q_2$}{r_f} - k \dfrac{q_1$ q_2$}{r_i}$$

Electrostatics: Concepts, Laws, and Applications

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