Conceptual Questions:

1. Give a physical explanation for why the potential energy of a pair of like charges Q,Q is positive, whereas that of a pair of opposite charges Q,-Q is negative.

Numerical Problems: answers link to a help page.

1. If the distance between two negative point charges is increased by a factor of three, the resultant potential energy is what factor times the initial potential energy?

ans: 1/3

2. A proton (+1.6 x10^-19 C) moves 10 cm on a path parallel to the direction of a uniform electric field of strength 3.0 N/C. How much work is done on the proton by the electrical field?

ans: 4.8 x10^-20 J

3. If an electron is accelerated from rest through a potential difference of 1200 V, find its approximate velocity at the end (q_e = -1.6 x10^-19 C; m_e = 9.1 x10^-31 kg)

ans: 2.1 x10⁷ m/s

4. The quantity of electrical potential, the Volt, is dimensionally equivalent to

a. force / charge
b. force * charge
c. electric field * distance
d. electric field / distance

5. In which case does an electric field do positive work on a charged particle?

a. A negative charge moves opposite to the direction of the electric field.
b. A positive charge is moved to a point of higher potential energy.
c. A positive charge completes one circular path around a stationary positive charge.
d. A positive charge completes one elliptical path around a stationary positive charge.