Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Problem 18.41(a)
Two ideal emf sources are connected to a 24 Ω resistor, and a 1.5 Ω light bulb, as shown at right. The equivalent resistance of this circuit is: (A) 1.4 Ω.
(B) 10 Ω.
(C) 13 Ω.
(D) 26 Ω.
Correct answer (highlight to unhide): (A)
(The two ideal emf sources are in series with each other, but with opposite polarities, such that they can be replaced with an equivalent emf source of 12 V – 3.0 V = 9 V, in the same location and orientation as the 12 V emf source.)
The two resistors are in parallel with each other, such that their equivalent resistance is given by:
Req = (R1–1 + R2–1)–1,
Req = ((24 Ω)–1 + (1.5 Ω)–1)–1 = (0.70833333333 Ω–1)–1 = 1.4117647059 Ω,
or to two significant figures, the equivalent resistance of this circuit is 1.4 Ω.
(Response (B) is 1.4 Ω + 9.0 V; response (C) is the average of 24 Ω and 1.5 Ω; response (D) is 24 Ω + 1.5 Ω.)
Section 30882
Exam code: quiz04j0uL
(A) : 15 students
(B) : 1 student
(C) : 0 students
(D) : 10 students
Success level: 58%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.70
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