# RC Circuit find capacitor charge

1. Feb 10, 2010

### matt72lsu

1. The problem statement, all variables and given/known data
The capacitor in an RC circuit (R = 130 \Omega, C = 40uF) is initially uncharged.
1) Find the charge on the capacitor in the circuit one time constant (tau = RC) after the circuit is connected to a 9.0- V battery.
2) Find the current in the circuit one time constant (\tau = RC) after the circuit is connected to a 9.0-V battery.
2. Relevant equations
R = V/I
Q = qo[1-e^-t(RC)] ???

3. The attempt at a solution
Im actually not sure I even have the correct equations and I'm not sure what to use for qo

2. Feb 11, 2010

### ehild

Q=qo[1-e^(-t/(RC))]

ehild

3. Feb 11, 2010

### Lok

Qo=C*V

4. Feb 11, 2010

### matt72lsu

I'm so confused

5. Feb 11, 2010

### rl.bhat

In the problem C and V is given. When the capacity is fully charged, the maximum charge Qo = C*V. Find this one.
In the relevant equation, put t = RC and find Q.

6. Feb 11, 2010

### matt72lsu

i got the first part but i got the second part wrong. i did V/R = 6.9e-2. then i multiplied that by 1-e^-1 and got 4.4e-2 then multiplied that by 1000 to go from A to mA and got 44. where did i go wrong?

7. Feb 11, 2010

### Lok

There is a similar formula to the Q=qo[1-e^(-t/(RC))] for I(ntensity). It is a must have.

8. Feb 11, 2010

### matt72lsu

yeah I used that and substituted qo for V/R and worked it out the same way i did the first one

9. Feb 11, 2010

### rl.bhat

The current I = dQ/dt = (qo/RC)[1-e^(-t/(RC))]
Now find the current.

10. Feb 12, 2010

### matt72lsu

i cant wrap my mind around this !!!

11. Feb 12, 2010

### rl.bhat

The current I = dQ/dt = (qo/RC)[e^(-t/(RC))]
Now find the current.