## 20140425

### Physics quiz question: brighter light bulb?

Physics 205B Quiz 5, spring semester 2014
Cuesta College, San Luis Obispo, CA

Cf. Giambattista/Richardson/Richardson, Physics, 2/e, Conceptual Questions 18.21-18.23

An ideal voltmeter is connected to a switch in a circuit with an ideal 12.0 V emf source and two light bulbs, as shown at right. While the switch remains open, the __________ light bulb is brighter.
(A) 3.0 Ω.
(B) 5.0 Ω.
(C) (There is a tie.)
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (B)

No current will flow through the ideal voltmeter and its connecting wires (shown in blue), only through the lower part of the circuit (indicated in green). Since the 3.0 Ω and the 5.0 Ω light bulbs are in series, from applying Kirchhoff's junction rule for circuits both light bulbs must have the same amount of current passing through them. As brightness is related to power dissipated, which depends on current, voltage drop, and resistance:

P = I·∆V = I·(I·R) = I2·R,

then with the same amount of current passing through each light bulb, the greater resistance 5.0 Ω light bulb will dissipate more power, and be brighter than the 3.0 Ω light bulb.

Sections 30882, 30883
Exam code: quiz05b4L8
(A) : 12 students
(B) : 17 students
(C) : 9 students
(D) : 0 students

Success level: 45%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.38