Physics quiz question: separating capacitor plates

Physics 205B Quiz 4, spring semester 2010
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problems 17.57, 17.58

A parallel plate capacitor is connected to a battery that supplies a constant potential difference. While the battery is still attached, the parallel plates are separated a little more. As the parallel plates are separated, the charge will:
(A) decrease.
(B) remain constant.
(C) increase.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (A)

Since the battery is still connected to the capacitor, the potential difference ∆V remains constant. Separating the parallel plates of the capacitor will decrease the capacitance, as increasing d will decrease C in:

C = A/(4·π·k·d).

Thus the charge Q on the capacitor will decrease, as decreasing C (while ∆V remains constant) will decrease Q, as seen from:

C = Q/∆V.

Student responses
Section 31988
Exam code: quiz04v0L7
(A) : 7 students
(B) : 3 students
(C) : 1 student
(D) : 0 students

Success level: 63%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67