In the circuit shown in the figure, all the lightbulbs are identical. Which of the following is the correct ranking of the brightness of the bulbs?
A) B and C have equal brightness, and A is the dimmest.
B) A and B have equal brightness, and C is the dimmest.
C) A is brightest, C is dimmest, and B is in between.
D) A is the brightest, and B and C have equal brightness but less than A.
E) All three bulbs have the same brightness.

Question

In the circuit shown in the figure, all the lightbulbs are identical. Which of the following is the correct ranking of the brightness of the bulbs? 
A) B and C have equal brightness, and A is the dimmest. 
B) A and B have equal brightness, and C is the dimmest. 
C) A is brightest, C is dimmest, and B is in between. 
D) A is the brightest, and B and C have equal brightness but less than A. 
E) All three bulbs have the same brightness.

Quizplus · Accepted Answer

Since the bulbs are connected in parallel, they all have the same potential difference across them. Therefore, the brightness depends on the resistance of each bulb. The bulb with the least resistance will draw more current and thus be brighter. The two bulbs on the left are in parallel, so they have an equivalent resistance that is half of the right bulb's resistance. Therefore, the right bulb will draw less current and be dimmer. Since the two left bulbs have the same resistance, they will draw the same current and have the same brightness. Therefore, A is brighter than both B and C, and B and C have equal brightness but are less bright than A. So D is the correct choice.

In the Circuit Shown in the Figure, All the Lightbulbs