# Homework Help: Capacitor discharge

1. Jul 11, 2011

### Rupturez

1. The problem statement, all variables and given/known data
Determine how long it takes the capacitor to discharge to a value of 3v
C=1.5E-6F
R=1E3ohms
Vi=25v
Vf=3v

2. Relevant equations
T=RC
v=Vi*e^(t/T)

3. The attempt at a solution
T=1000*1.5E-6
T=1.5E-3s

Vf(3v) is 12% of the initial 25v

Im having trouble solving for time with this equation v=Vi*e^(t/T)

2. Jul 11, 2011

### Rupturez

Should I be solving for t or using a normalised universal timeconstant(RC) curve to estimate time?

3. Jul 11, 2011

### jambaugh

Firstly $V = V_i e^{-t/T}$ there's a negative sign reflecting the decay of the voltage... it doesn't grow!

Secondly solving this for t is basic algebra, apply inverse operations until you isolate t.
$A = B e^{t/C}$
$A/B = e^{t/C}$
$\ln(A/B) = t/C$
$C\ln(A/B) = t$

4. Jul 11, 2011

### Rupturez

Thanking you jambaugh