Physics quiz question: unknown test charge attracted to negative source charge

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

A small test charge of unknown sign at 
x = +4.0 cm is released from rest, and as a result moves closer to 
a –5.0 µC charge that is fixed at the origin. As a result, the test charge moves through a region of __________ electric potential.
(A) decreasing (becoming a smaller positive number).
(B) decreasing (becoming a larger negative number).
(C) constant (unchanging).
(D) increasing (becoming a larger positive number).
(E) increasing (becoming a smaller negative number).
(F) (Not enough information is given.)
Correct answer (highlight to unhide): (B)

Regardless of the ± sign of the test charge q (which turns out to be positive, as it is attracted to the Q = –5.0 µC source charge), the –5.0 µC source charge has a negative sign, such that its potential V is given by:

V = k·Q/r = k·(–5.0×10–9 C)/r,

which would be negative for all possible values of the distance r away from it. Thus mathematically decreasing r (moving the test charge closer to the –5.0 µC source charge) would cause the already negative value of the potential to become a larger negative number. Conceptually the potential of this –5.0 µC source charge could be visualized as a "well," such that moving closer to it (inwards towards it) could be thought of as falling further "downwards.")

Student responses
Sections 30882, 30882
Exam code: quiz04sm7H
(A) : 10 students
(B) : 19 students
(C) : 4 students
(D) : 4 students
(E) : 6 students
(F) : 1 student

Success level: 43%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.54

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