Question 1

Which of the following statements about electric potential energy $$U_e$$ for two point charges $$q_1$$ and $$q_2$$ separated by a distance $$r is $$correct?

Accepted Answer

$$U_e = \frac{k q_1$ q_2$}{r}$$ and depends only on the location of the charges, not the path.

Question 2

A proton and an electron are brought closer together from an initial separation $$r_i to a $$final separation $$r_f$$ ($$r_f \u003c r_i). $$What happens to the electric potential energy of the system?

Accepted Answer

It decreases because opposite charges attract and $$\Delta U_e \u003c 0.$$

Question 3

If two particles both have positive charge, what must be true about the work required to bring them closer together?

Accepted Answer

Work must be done by an external agent because $$\Delta U_e \u003e 0.$$

Question 4

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

Accepted Answer

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

Question 5

A charge $$q$$ moves through a potential difference $$\Delta V. $$What is the change in its potential energy?

Accepted Answer

$$\Delta U_e = q \Delta V$$

Question 6

A $$+2e$$ charge moves from point P ($$V_P = 500.0 kV) to $$point S ($$V_S = 200.0 kV). $$What is the change in kinetic energy?

Accepted Answer

$$\Delta K = +600$$ keV

Question 7

An electron is accelerated from rest through a potential difference and reaches a speed $$v_f. $$Which equation correctly relates the potential difference $$\Delta V to$$ $$v_f$$?

Accepted Answer

$$\Delta V = \frac{m v_f^2$}{2q}$$

Question 8

Which integral gives the electric potential difference between two points $$a$$ and $$b in a $$known electric field $$\vec{E}$$?

Accepted Answer

$$V_b - V_a = -\int_a^b \vec{E} \cdot d\vec{l}$$

Question 9

For a uniform electric field $$E$$ along the $$y-$$direction, what is the potential at position $$y if$$ $$V=0 at$$ $$y=0$$?

Accepted Answer

$$V = -E y$$

Question 10

Given three point charges $$q_1$$, $$q_2$$, $$q_3 at $$positions forming a right triangle, what is the total electric potential energy of the system?

Accepted Answer

$$U_e = \frac{k q_1$ q_2$}{r_{12}} + \frac{k q_1$ q_3$}{r_{13}} + \frac{k q_2$ q_3$}{r_{23}}$$

Question 11

What is the value of the electric field $$\vec{E}$$ inside a conductor in electrostatic equilibrium?

Accepted Answer

$$\vec{E} is $$zero everywhere inside the conductor.

Question 12

Which property is true for the surface of a conductor in electrostatic equilibrium?

Accepted Answer

The surface is an equipotential.

Question 13

What is the electric flux $$\Phi_E$$ through a closed surface enclosing a net charge $$Q$$ according to Gauss' Law?

Accepted Answer

$$\Phi_E = Q / \$$epsilon_0$$

Question 14

Which Gaussian surface is most appropriate for finding the electric field at a distance $$r$$ from a long, straight, uniformly charged rod?

Accepted Answer

A cylinder of radius $$r$$ and length $$L$$ coaxial with the rod.

Question 15

Why does Gauss' Law not simplify the calculation of the electric field near the ends of a finite rod?

Accepted Answer

Because the field lines are not radial and the flux through the end caps is not zero.

Question 16

If excess charge is placed on an isolated conductor, where does it reside?

Accepted Answer

It resides entirely on the surface.

Question 17

What is the relationship between the electric field $$\vec{E}$$ and the electric potential $$V$$?

Accepted Answer

$$\vec{E} = -
abla V$$

Question 18

A conducting spherical shell has a charge $$+q$$ placed at its center and no excess charge on the conductor. What is the induced charge per unit area on the inner and outer surfaces?

Accepted Answer

Inner surface: $$-q$$, Outer surface: $$+q$$

Question 19

Which of the following is true about equipotential surfaces and electric field lines?

Accepted Answer

Electric field lines are always perpendicular to equipotential surfaces.

Electric Potential and Gauss’ Law – Study Notes for PHY 131

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