## 20160313

### Physics quiz question: switching emf polarity

Physics 205B Quiz 4, spring semester 2015
Cuesta College, San Luis Obispo, CA An ideal emf source is connected to a light bulb and a resistor, as shown at right. If the direction of the emf source is switched, the light bulb would then have __________ current passing through it.
(A) less.
(B) the same amount of.
(C) more.
(D) (Not enough information is given.)

Correct answer (highlight to unhide): (B)

The equivalent resistance of this circuit (resistor and light bulb are in series with the emf) is the sum of these resistances. Thus the current flowing through all parts of this series circuit is given by:

I = ∆V/Req = (1.5 V)/(4.0 Ω + 0.20 Ω) = (1.5 V)/(4.2 Ω) = 0.3571428571 A,

or to two significant figures, 0.36 A. This is regardless of whether the ± polarity of the emf is switched, which would change the direction of current leaving the + terminal of the emf to go through the light bulb first instead of the resistor first--it would still be the same 0.36 A of current flowing through all parts of each of these two circuits. (This is an application of Kirchhoff's junction rule, in the case here where there is no junction in either these series circuit.)

Sections 30882, 30883
Exam code: quiz04sm7H
(A) : 4 students
(B) : 30 students
(C) : 10 students
(D) : 0 students

Success level: 68%
Discrimination index (Aubrecht & Aubrecht, 1983): 0.85