20110503

Physics quiz question: loading LR circuit

Physics 205B Quiz 6, Spring Semester 2011
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 20.55.

Consider an ideal emf source connected to an ideal resistor and inductor. The switch is closed at t = 0. After the switch is closed, the current through the inductor __________, while the current through 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: (D)

The junction rule states that the amounts of current passing through the resistor and inductor must all be identical. From inspection, the current I in all parts of this circuit starts from an initial value of 0, and increases to its limiting value of I = emf/R after a long time, when the inductor is no longer reacting to the change in current, such that the amounts of current through both the resistor and inductor increases to a finite value.

Student responses
Section 30822
Exam code: quiz06eMf0
(A) : 0 student
(B) : 5 students
(C) : 1 student
(D) : 2 students

Success level: 25%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.67

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