If a voltage difference given by
is applied across a capacitor C, then the resulting current is given by
We now try to make this situation similar to the resistor circuit by insisting
that the magnitude of the current be given by a formula that looks like Ohm's law:
The quantity 
is called the
capacitive reactance, has units of ohms,
and is used just like a resistance to find the magnitude of the current that
flows due to an applied voltage.
Note that the applied voltage and the resulting current are not given by the same function of time, meaning that they are out of phase with each other. If we start watching this circuit at time t=0, then we see the current at its maximum value and the applied voltage at zero. A quarter of a cycle later the current has dropped to zero and the voltage has risen to its maximum value. We describe this situation by saying that the current is ahead of the voltage, or that it leads the voltage. Sometimes we also say that the voltage is behind, or lags, the current. It may be helpful to remember the word ICE for this circuit; the C stands for capacitor, and in ``ICE'' the current, I, is ahead of the voltage, or applied emf, E.