Physics quiz question: replacing light bulb with ideal wire in parallel branch

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 6.0 V emf source is connected to an ideal ammeter, a light bulb, and two resistors, as shown at right. The ammeter reading if the light bulb were instead replaced with an ideal wire would be __________ the ammeter reading while the light bulb was lit.
(A) less than.
(B) equal to.
(C) greater than.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (B)

From applying Kirchhoff's loop rule for circuits to the outermost loop (shown in green), the amount of current flowing through the ammeter will be unaffected by changes in the other parallel (uncolored) branch. Thus the ammeter reading will be the same whether the light bulb is replaced with wire, or is left intact.

Sections 30882, 30883
Exam code: quiz05b4L8
(A) : 15 students
(B) : 14 students
(C) : 9 students
(D) : 0 students

Success level: 37%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.52