## 20160325

### Physics quiz question: pushing capacitor plates together

Physics 205B Quiz 4, spring semester 2015
Cuesta College, San Luis Obispo, CA

"What's In A Candle Flame?"
Veritasium (Derek Muller)
youtu.be/a7_8Gc_Llr8

A capacitor is constructed using two circular metal plates (each with an area of 0.13 m2) separated by 0.15 m, and connected to a 20,000 V emf source[*]. As the plates of this capacitor are brought closer together (while still connected to the same 20,000 V emf source), its __________ will increase.
(A) capacitance.
(B) charge.
(C) (Both of the above choices.)
(D) (Neither of the above choices.)

[*] youtu.be/a7_8Gc_Llr8.

Correct answer (highlight to unhide): (C)

The capacitance is given by:

C = A/(4·π·k·d),

so decreasing the separation distance d ("the plates of this capacitor are brought closer together") will increase the capacitance.

The relation between the capacitance, the charge stored, and the potential difference applied is:

C = Q/∆V,

so decreasing the separation distance d, while maintaining the potential difference ("still connected to the same 20,000 emf source") would increase the capacitance C, and thus increase the amount of charge stored.

Sections 30882, 30883
Exam code: quiz04eQu7
(A) : 19 students
(B) : 7 students
(C) : 16 students
(D) : 1 student

Success level: 36%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.62