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How many time constants will elapse befo...

How many time constants will elapse before the charge on a capacitor falls to `0.1%`of its maximum value in a discharging RC circuit.

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`q = Q^e-t/rc`
` q = 0.1% Q = 1 xx (10^-3)Q `
` So, 1 xx (10^-3)Q = Q xx (e^(-t/rc))`
` rArr e^(-t/rc)= 10^-3`
` rArr t/rC = - (- 6.9) = 6.9.`
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