Capacitor Discharge: Calculating Time to 0.8μC

[Kalus]

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

 

[gabbagabbahey]

Your calculations look fine to me, so I can only assume that there is a typo in either the answer key or the question sheet.

 

[Kalus]

Thankyou for taking a look. I'd hoped that was the case and i wasn't going mad ;)

Andy

 

[waht]

I punched those number real quick, and they are fine.