Q: A neutral lithium atom contains 3 protons, 4 neutrons, and 3 electrons. If it loses one electron, what is the net charge of the resulting ion?

$$+1.602 \times 10$$^{-19}$$\ \mathrm{C}$$

Q: What is the net force and net torque on an electric dipole placed in a uniform electric field $$\vec{E}$$?

Net force is zero; net torque is $$\vec{p} \times \vec{E}$$

Question 1

Which of the following statements best describes the property of electric charge?

Accepted Answer

Electric charge is a property of matter that is conserved and quantized.

Question 2

A neutral lithium atom contains 3 protons, 4 neutrons, and 3 electrons. If it loses one electron, what is the net charge of the resulting ion?

Accepted Answer

$$+1.602 	imes 10$$^{-19}$$\ \mathrm{C}$$

Question 3

Which of the following best explains why your hair stands on end after removing a wool hat on a dry day (as seen in the first image)?

Accepted Answer

Friction transfers electrons, causing each hair to acquire the same type of charge and repel each other.

Question 4

Which of the following is true about conductors and insulators?

Accepted Answer

Conductors have free electrons that can move easily, while insulators do not.

Question 5

A negatively charged rod is brought near a neutral metal ball suspended by a nonconducting thread. What happens to the charges in the ball?

Accepted Answer

Electrons in the ball move away from the rod, causing the side near the rod to become positively charged.

Question 6

Two point charges, $$q_1 = +2$$\ \mu\mathrm{C}$$ and q_2$ = -2\ \mu\mathrm{C}$$, are placed 0.5 m apart. What is the magnitude of the force between them? (Use $$k = 8.99 	imes 10^9$\ \mathrm{N\cdot m^2$/C^2$$}$)

Accepted Answer

$$0.144\ \mathrm{N}$$

Question 7

A test charge $$q is $$placed in an electric field $$\vec{E}. $$What is the force $$\vec{F}$$ acting on the charge?

Accepted Answer

$$\vec{F} = q\vec{E}$$

Question 8

Which of the following best describes the direction of the electric field produced by a positive point charge?

Accepted Answer

The electric field points radially outward from the charge.

Question 9

A particle of mass $$m$$ and charge $$q$$ enters a region with a uniform electric field $$E$$ between two parallel plates with initial velocity $$v_0$$ parallel to the plates. If the distance between the plates is $$d$$ and the length of the plates is $$L$$, what is the condition for the particle to pass through without hitting the plates?

Accepted Answer

$$\frac{qEL}{mv_0$^2} \u003c d$$

Question 10

What is the net force and net torque on an electric dipole placed in a uniform electric field $$\vec{E}$$?

Accepted Answer

Net force is zero; net torque is $$\vec{p} 	imes \vec{E}$$

Question 11

A ring of total charge $$Q$$ and radius $$R$$ lies in the $$xy-$$plane. What is the electric field at a point on the $$z-$$axis a distance $$z$$ above the center of the ring?

Accepted Answer

$$E_z = \frac{Qz}{4\pi\$$epsilon_0$$ (R^2$ + z^2$)^{3/2}$$}$

Question 12

Which of the following is true about the electric field lines for two equal but opposite charges (an electric dipole)?

Accepted Answer

Field lines begin on the positive charge and end on the negative charge.

Question 13

A comb is rubbed through dry hair and then brought near small pieces of paper. The paper is attracted to the comb. Which phenomenon best explains this observation?

Accepted Answer

The comb induces opposite charges on the paper, resulting in attraction.

Question 14

What is the potential energy $$U of an $$electric dipole with dipole moment $$\vec{p} in a $$uniform electric field $$\vec{E}$$?

Accepted Answer

$$U = -\vec{p} \cdot \vec{E}$$

Question 15

A point charge $$Q is $$located at the origin. What is the electric field at a distance $$r$$ from the charge?

Accepted Answer

$$\vec{E} = \frac{1}{4\pi\$$epsilon_0$$} \frac{Q}{r^2$} \hat{r}$$

Question 16

If the electric field at a point is zero, which of the following must be true?

Accepted Answer

The vector sum of all electric fields from all charges at that point is zero.

Question 17

A charged rod is brought near a metal ball suspended by a nonconducting thread. The ball is attracted to the rod, even though the ball is initially uncharged. What is the best explanation for this attraction?

Accepted Answer

The rod induces a separation of charge in the ball, resulting in attraction.

Question 18

Which of the following is a correct statement about the quantization of electric charge?

Accepted Answer

The charge of any object is always an integer multiple of the elementary charge $$e.$$

Electric Charge and Electric Field: Study Notes for Physics with Calculus

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