Physics quiz question: bringing together opposite charges

Physics 205B Quiz 4, spring semester 2017
Cuesta College, San Luis Obispo, CA

An electron is located 0.48 nm from a proton, then moves to a closer distance of 0.21 nm from the proton. As a result, its 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·q₁·q₂/r_i,

which by inspection is a negative value (as the electron is negatively charged, while the proton is positively charged), so decreasing the distance from r_i = 0.48 nm to r_f = 0.21 nm 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
Sections 30882, 30883
Exam code: quiz04Br7w
(A) : 8 students
(B) : 7 students
(C) : 1 student
(D) : 7 students
(E) : 0 students
(F) : 0 students

Success level: 30%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.36