RC circuits: A light bulb is connected in the circuit shown in the figure with the switch S open and the capacitor uncharged. The battery has no appreciable internal resistance. Which one of the following graphs best describes the brightness B of the bulb as a function of time t after closing the switch?
A) 1
B) 2
C) 3
D) 4
E) 5

Question

RC circuits: A light bulb is connected in the circuit shown in the figure with the switch S open and the capacitor uncharged. The battery has no appreciable internal resistance. Which one of the following graphs best describes the brightness B of the bulb as a function of time t after closing the switch?  
A) 1 
B) 2 
C) 3 
D) 4 
E) 5

Quizplus · Accepted Answer

When the switch is closed, the capacitor will start charging, and the voltage across the capacitor will increase while the current through the light bulb decreases. Therefore, the brightness of the bulb will decrease with time. 
Graph 2 shows a decrease in brightness with time, which is the correct behavior for the RC circuit. Graphs 1, 3, and 5 show an increase in brightness, which is not possible. Graph 4 shows a constant brightness, which is also not possible as the current decreases over time.

RC Circuits: a Light Bulb Is Connected in the Circuit