# Homework Help: Time required to fully discharge the capacitor where did i go wrong?

1. Jul 22, 2011

### yjk91

1. The problem statement, all variables and given/known data

A cube of gold (resistivity ρR = 2.44 10-8 Ω m, density ρD = 1.93 104 kg / m3, and specific heat c = 129 J / kg °C) that is 3.5 mm on a side is connected across the terminals of a 40-µF capacitor that initially has a potential difference of 340.0 V between its plates.

http://www.webassign.net/bauerphys1/26-p-052.gif

What time is required to fully discharge the capacitor? (Assume the capacitor is fully discharged when the charge is less than 0.01%.)

3. The attempt at a solution

R = pL / A = (2.44 * 10^-8) * (3.5 * 10^-3m) / (3.5*10^-3m)^2 = 6.97 * 10^-6 m^2

C = 40 * 10^-6 F

Q = Q0 e (-t/(RC))

so

t = 2.56 e -9

which is not the right answer for question a

2. Jul 22, 2011

### Staff: Mentor

Your units for the resistance of the gold block should be Ohms, not square meters

I can't find a fault with your calculation (although the result when I calculated it rounded to 2.57 x 10-9 seconds).

Does the marker require any particular units to be used for the result (picoseconds? nanoseconds?)