Posted by **Katie** on Sunday, May 22, 2011 at 8:07pm.

How long will it take a charged 80-ìF capacitor to lose 20 percent of its initial energy when it is allowed to discharge through a 45 ohm resistor?

- Physics -
**Katie**, Sunday, May 22, 2011 at 8:08pm
i mean 80 microfarads

- Physics -
**drwls**, Sunday, May 22, 2011 at 8:39pm
The time constant for charge decay is

RC = 3.6*10^-3 s, if your ì symbol means "micro"

For 80% of the initial energy, the charge must decrease to sqrt(0.8) = 89.4% of the initial value.

exp(-t/RC) = 0.894

t/RC = .1121

t = 4*10^-4 seconds

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