Cuesta College, San Luis Obispo, CA
Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Comprehensive Problem 18.65(a).
An ideal 16.0 V emf source is connected to a 15 Ω resistor and two 2.2 Ω light bulbs, as shown at right. The equivalent resistance of this circuit is: (A) 1.0 Ω.
(B) 2.0 Ω.
(C) 4.1 Ω.
(D) 6.5 Ω.
Correct answer: (C)
Light bulb 1 and the 15 Ω resistor are in parallel with each other, and so their equivalent resistance is the reciprocal of the sum of their reciprocals:
R_(bulb_1,_15 Ω) = ((2.2 Ω)^-1 + (15 Ω)^-1)^-1 = 1.9 Ω,
And this interim equivalent resistor is in series with light bulb 2, such that the equivalent resistance of the circuit is the summation:
R_(circuit) = 1.9 Ω + 2.2 Ω = 4.1 Ω.
Response (A) is the result of all three circuit elements in parallel with each other; response (B) is where light bulb 1 and the 15 Ω resistor are in series with each other, then combined in parallel with light bulb 2; response (D) is the average resistance of all three circuit elements.
Student responses
Section 30822
Exam code: quiz05t4Uo
(A) : 2 students
(B) : 2 students
(C) : 4 students
(D) : 0 students
Success level: 50%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.50
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