Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 18.89.
Consider this circuit with an initially uncharged ideal capacitor, and an ideal emf and ideal resistor. At t = 0 the switch
is closed. After the switch is closed, while the capacitor is still charging, the voltage across the capacitor __________, while the voltage across the resistor __________. (A) decreases to zero; decreases to zero.
(B) decreases to zero; increases to a finite value.
(C) increases to a finite value; decreases to zero.
(D) increases to a finite value; increases to a finite value.
Correct answer: (C)
From the loop rule:
Summation(delta(V))_loop = 0 = +emf - delta(V_C) - delta(V_R),
0 = +emf - Q/C - I*R.
As the capacitor charges, Q increases, such delta(V_C) increases. Also as the capacitor charges, current will decrease over time, such that as I decreases, delta(V_R) decreases to zero, making delta(V_C) increase up to its limiting value of +emf.
Student responses
Section 30822
Exam code: quiz05t4Uo
(A) : 1 student
(B) : 0 students
(C) : 5 students
(D) : 2 students
Success level: 63%
Discrimination index (Aubrecht & Aubrecht, 1983): 1
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