Question

The three light bulbs in the circuit above are identical, and the battery has zero internal resistance. When switch S is closed to cause bulb 1 to light, which of the other two bulbs increase(s) in brightness?

(A) Neither bulb

(B) Bulb 2 only

(C) Bulb 3 only

(D) Both bulbs

(E) It cannot be determined without knowing the emf of the battery.

Answer 1

The correct option is: (C) Bulb 3 only

Explanation:

Closing the switch adds another parallel branch, increasing the total current delivered by the battery. Bulb 3 will get brighter. Bulb 2, in its own loop with bulb 3 and the battery will then lose some of its share of the potential difference from the battery and will get dimmer.