Physics midterm question: bringing together opposite charges

Physics 205B Midterm 1, spring semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Questions 17.1, 17.4, 17.6

Two point charges (+3.0 µC and –1.0 µC) are held a certain distance apart. As the –1.0 µC charge is moved to x = +4.0 m, their electric potential energy:
(A) decreases (becomes a smaller positive number).
(B) decreases (becomes a larger negative number).
(C) remains constant.
(D) increases (becomes a larger positive number).
(E) increases (becomes a smaller negative number).
(F) (Not enough information is given.)

Correct answer (highlight to unhide): (B)

The initial electrical potential energy of the two charges is given by:

EPE_i = k·(+3.0×10^–9 C)·(–1.0×10^–9 C)/r_i,

which is by inspection a negative value, so decreasing the distance to r_f = 4.0 m would make the electric potential energy a larger negative number (which means it decreases, as EPE always decreases in the direction that charges "want" to go).

Student responses
Section 31988
(A) : 2 students
(B) : 6 students
(C) : 0 students
(D) : 4 students
(E) : 1 student
(F) : 0 students

Success level: 46%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50