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Energy in charging a capacitor. In a simple RC circuit, the time constant is T =

ID: 1280953 • Letter: E

Question

Energy in charging a capacitor. In a simple RC circuit, the time constant is T = RC which means it takes the voltage (current and charge) of the capacitor a time equals to T to drop to a value 1 /e of its original value in a discharge process. How long will it take its energy to drop down to 1 /e of its initial energy in a discharge process? e here is the base of the natural logarithm and equals to approximately 2 . 71828.

(b) Completely discharged capacitor.

The equation that describes the decay of charge on capacitor plates suggests that it takes infinite time for then charge to reach zero. Since the smallest charge possible to exist is e = 1.6

Explanation / Answer

time constant T = RC

T = 680000* 0.920 e-6

T = 0.625 secs
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to dischagre fully e^-t/RC   = 0.01

-t/RC = ln(0.01)

t = 4.6* 0.625

t = 2.87 secs

this is = 4.6 time costants