Physics midterm question: separating like charges

Physics 205B Midterm 1, spring semester 2012
Cuesta College, San Luis Obispo, CA

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

Two point charges (–1.0 nC and –1.5 nC) are held a certain distance apart. As the separation distance between the two charges increases, 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): (A)

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

EPE = k·(–1.0×10^–9 C)·(–1.5×10^–9 C)/r,

so increasing r would make the electric potential energy a smaller positive number (decreasing its positive value).

Student responses
Section 30882
Exam code: quiz04j0uL
(A) : 25 students
(B) : 1 student
(C) : 0 students
(D) : 1 student
(E) : 0 students
(F) : 0 students

Success level: 93%
Discrimination index (Aubrecht & Aubrecht, 1983): 0