- #1
Kalus
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Homework Statement
A 10 microfarad capacitor is initally charged to 15 volts. If the capacitor then discharges through a 12kohm resistor how long before the charge stored has falled to 0.8 x 10^-4 C?
Homework Equations
Q=CV
Q= Q_0 x e ^(-t/RC)
The Attempt at a Solution
Calculate initial charge using Q=CV
Q=CV
Q= 15x 10x10^-6
Qinital = 1.5x10^-4
Then using Q= Q_0 * e^(-t/RC)
Q= Q_0 x e ^(-t/RC)
0.8 x10^-4 = 1.5x10^-4 * e^(-t/12000*10x10^-6)
ln(0.8/1.5) = -t/12000*10x10^-6
ln(0.8/1.5)*12000*10x10^-6 = t
which when calculated comes out as 0.0754, or 75 ms.
However, the answer in the practise test is 120ms... anyone see what i am doing wrong?
Many thanks, Andy