# Homework Help: Decay in an RC Circuit

1. Oct 11, 2007

### Ryo124

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

A capacitor is charged to 1 coulomb; the capacitance is 9.00×10-5 farads.
Then a switch is closed which puts the capacitor in a closed circuit with a resistor; the resistance is 9.0x10^1 ohms.

Calculate the charge on the capacitor after 4.00 milliseconds. (1 ms = 0.001 s).

I know some equations relating capacitance, charge, and voltage, but I don't understand how to approach this problem

2. Relevant equations

Q(t) = Q0[e^-t/RC]

3. The attempt at a solution

No clue. I have no idea how to approach this problem and no idea what the value for V is.

Last edited: Oct 11, 2007
2. Oct 11, 2007

### learningphysics

Just use: Q(t) = Q0[e^-t/RC]

where are you getting stuck?

3. Oct 11, 2007

### Ryo124

lol... ugh I was using 9 ohms as a resistance instead of 90! Thanks.

4. Dec 17, 2007

### Matty R

I'd like to ask for help in this thread as my question is directly related.

Here is my problem.

Q = 37.1

Q0 = 37.44

e = 2.718

t = 0.25

C = 15.6x10-9

R = ?

How do I rearrange the equation to find R?

Q = Q0[e^-t/RC]

I don't have any idea how to go about it and would appreciate any help.

5. Dec 17, 2007

### rock.freak667

Q=Q0[e^-t/RC]

divide by Q0

Q/Q0=e^-t/RC

take ln
ln(Q/Q0)=-t/RC

RC*ln(Q/Q0)=-t and so forth

6. Dec 18, 2007

### Matty R

Thanks for the reply. Its a great help.