A typical flashlight bulb at full brilliance draws a current of approximately 0.

Question

A typical flashlight bulb at full brilliance draws a current of approximately 0.5A when connected to a 3V voltage source. For this problem, assume that the changing resistance causes the current to be 0.5A for any voltage between 2V and 3V . 1)) suppose this flashlight bulb is attached to a capacitor as shown in the circuit from the problem introduction. If the capacitor has a capacitance of 3 (an unusually large but not unrealistic value) and is initially charged to 3 , how long will it take for the voltage across the flashlight bulb to drop to 2 (where the bulb will be orange and dim)? Call this time in seconds.

Explanation / Answer

The charge on a capacitor Q is equal to C * V. At 3 volts: Q = CV Q = 3 * 3 Q = 9 And at 2 volts: Q = CV Q = 3 * 2 Q = 6 The difference is 3 coulombs and at a current of 0.5 A (0.5 coulombs per second), it will take about 6 seconds for the voltage to drop. 6 s

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