Physics quiz question: separating capacitor plates

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

A parallel plate capacitor is charged by connecting it to a 9.0 V battery, and is then disconnected. Afterwards, the parallel plates are separated a little more, without any charge on the plates being lost. As the parallel plates are separating, the potential difference of the capacitor:
(A) decreases.
(B) remains constant.
(C) increases.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (C)

Separating the parallel plates of the capacitor will decrease the capacitance, as increasing d will decrease C in:

C = A/(4·π·k·d).

Since the battery is disconnected from the capacitor, the charge Q on the capacitor will remain constant, as the plates are electrically isolated as they are separated from each other, resulting in an increase in ∆V (while decreasing C), as seen from:

C = Q/∆V.

Student responses
Sections 30882, 30883
Exam code: quiz04Br7w
(A) : 7 students
(B) : 7 students
(C) : 9 students
(D) : 0 students

Success level: 39%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.46