# How to find capacitance

1. Feb 8, 2004

### nautica

A capacitor, in a series with a 720 ohm resistor, is being charged. At the end of 10 ms its charge is half the final value. The capacitance is about:

a)9.6 uF
b)14 uF
c)20 uF
d)7.2 uF
e)10F

Now how in the heck and I suppose to know this one. I know ohms law and I know how to find capacitance. But what does either of these has to do with time?

2. Feb 8, 2004

### gnome

The rate at which a capacitor is charged in an RC circuit is:

$$q(t) = Q_{max}(1-e^\frac{-t}{RC})$$

You know t and R.
You don't know q(t) or Qmax but you do know the ratio of each to the other, and that's all you need to solve for C.