This might help you understand Ohm's Law. Taken from a post somewhere on the net in a forum, might even have been this one.
[i]"The best description of ohm's law I read talked of it as two hose pipes. If you are trying to squeeze water through a 1cm pipe it can only flow as quickly as that pipe can take it, the resistance. If you then attach another 1cm pipe along side it you have twice the flow rate and half the resistance.
Therefore, an amp that is rated 500w at 8 ohms can only push about 300w of power through to an 8 ohm speaker. If you connect a second 8 ohm speaker you reduce the resistance to 4 ohms and are able to use the full 500w".[/i]