Q: A neutral lithium atom has 3 protons, 3 neutrons, and 3 electrons. What is the net charge of a lithium ion that has lost one electron?

$$+1.60 \times 10$$^{-19}$$\ \mathrm{C}$$

Q: Which equation from Maxwell’s equations relates the electric field to the charge density?

$$\nabla \cdot \mathbf{E} = \frac{\rho}{\$$varepsilon_0$$}$$

Question 1

Which of the following statements best describes the principle of charge conservation?

Accepted Answer

The total electric charge in any closed system remains constant over time.

Question 2

A neutral lithium atom has 3 protons, 3 neutrons, and 3 electrons. What is the net charge of a lithium ion that has lost one electron?

Accepted Answer

$$+1.60 	imes 10$$^{-19}$$\ \mathrm{C}$$

Question 3

Which of the following materials is a good conductor of electricity?

Accepted Answer

Copper

Question 4

When a negatively charged plastic rod is brought near a neutral metal ball, but does not touch it, what happens to the electrons in the metal ball?

Accepted Answer

They are repelled away from the rod, causing a positive charge near the rod.

Question 5

Which equation from Maxwell’s equations relates the electric field to the charge density?

Accepted Answer

$$
abla \cdot \mathbf{E} = \frac{ho}{\$$varepsilon_0$$}$$

Question 6

A charged comb can pick up small pieces of paper even though the paper is neutral. What physical effect explains this phenomenon?

Accepted Answer

Polarization of the paper molecules by the electric field of the comb

Question 7

Two point charges, $$q_1 = +2.0$$\ \mu\mathrm{C}$$ and q_2$ = -3.0\ \mu\mathrm{C}$$, are placed $$0.50\ \mathrm{m}$$ apart in vacuum. What is the magnitude of the force between them?

Accepted Answer

$$0.216\ \mathrm{N}$$

Question 8

Which statement about the quantization of electric charge is correct?

Accepted Answer

All observable charge is an integer multiple of the elementary charge $$e$$

Question 9

A negatively charged plastic rod attracts a positively charged glass rod. What does this observation demonstrate about electric charge?

Accepted Answer

Opposite charges attract each other

Question 10

If you rub a balloon on your hair and stick it to a wall, which of the following best explains why the balloon sticks?

Accepted Answer

The balloon polarizes the molecules in the wall, resulting in an attractive force

Question 11

Which of the following is the correct expression for Coulomb’s law for the magnitude of the force between two point charges $$q_1$$ and $$q_2$$ separated by distance $$r$$?

Accepted Answer

$$F = k_e \frac{|q_1$ q_2$|}{r^2$}$$

Question 12

A metal ball is charged by induction using a negatively charged rod. After grounding and removing the rod, what is the net charge left on the ball?

Accepted Answer

Positive

Question 13

Which of the following best describes the difference between conductors and insulators?

Accepted Answer

Conductors allow free movement of electric charge, while insulators do not.

Question 14

A positively charged comb is brought near a neutral insulator. What force does the comb exert on the insulator?

Accepted Answer

An attractive force due to polarization

Question 15

Which of the following is the value of the elementary charge $$e$$?

Accepted Answer

$$1.60 	imes 10$$^{-19}$$\ \mathrm{C}$$

Question 16

If three point charges are placed on a straight line, how do you determine the net force on one of the charges?

Accepted Answer

By vector addition of the individual forces from the other two charges

Question 17

Which of the following statements about the interaction between a charged object and a neutral insulator is correct?

Accepted Answer

A charged object of either sign exerts an attractive force on a neutral insulator.

Question 18

Electrostatic painting uses charged paint droplets. Why do the droplets stick to the metal object being painted?

Accepted Answer

The induced positive charge on the metal object attracts the negatively charged paint droplets.

Question 19

Compare the magnitude of the electric force and gravitational force between two protons separated by $$1.0 	imes 10$$^{-10}$$\ \mathrm{m}. $$Which force is stronger?

Accepted Answer

The electric force is much stronger than the gravitational force.

Electric Charge and Electric Fields (University Physics Ch 21.1–21.3)

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