# RC circuit final charge

1. Feb 10, 2009

### itryphysics

1. The problem statement, all variables and given/known data
Consider an RC circuit in which a capacitor is being charged by a battery connected in a circuit. Initially the capacitor is uncharged. Then the switch is closed.
After a time equal to 4.5 time constants, what percent of the final charge is present on the capacitor?

2. Relevant equations
Q = Qo [1 - e^ (-t/RC)]

3. The attempt at a solution

I picked a value of 100 for Qo
so
Q= 100 [1 - e^ (-t/4.5)]
my approach was going to be to find Q and then see wht percent that is of the initial charge Qo
but I dont know wht to put in for the time "t"

2. Feb 10, 2009

### LowlyPion

I think you would take t = 4.5RC

3. Feb 10, 2009

### itryphysics

so for the expression e^(-t/RC) I do e^(-4.5) ?

4. Feb 10, 2009

### Delphi51

4.5 time constants means a time of 4.5*R*C.
R*C comes out in units of time.

5. Feb 10, 2009

### LowlyPion

Yes that should do it.

6. Feb 10, 2009

### itryphysics

Awsome! Thank you so much!