On 05/12/2011 01:01, Lieutenant Scott wrote:
Or is it lagging by 270 degrees? That makes sense. Voltage had to
happen before current surely?

No, it's leading. The voltage has to "happen" at source (the power
station) but the current leads the voltage measured at the load (your
appliance).

Imagine this circuit (you'll need a fixed width font):

A o-------------/\/\/\/\/\/\/\-------------
10 Ohm |
Resistor |
--------- C
Capacitor
--------- D
|
|
B o----------------------------------------

Consider the voltage measured across the capacitor between points C and
D when 10 volts are applied to the input terminals A and B. Initially
both plates of the capacitor are at 0V so there's a potential difference
of 10V across the resistor and a current of 1A will flow through the
resistor into the capacitor. As the current flows it will charge up the
capacitor increasing the voltage across C-D. As the voltage of C rises
the voltage across the resistor falls and the current flowing into the
capacitor falls until we reach the stage when the capacitor is fully
charged with 10V across C-D and no current is flowing.

So the current flowing into the capacitor leads the voltage measured
across its terminals.

With an AC supply a similar thing happens in reverse for the negative
half cycle and you see a current waveform which leads the voltage at the
capacitor by 90 degrees.

You might find it easier to imagine the capacitative load delaying the
voltage waveform seen at your end of the wire.

--
Mike Clarke