Anyway, I'm kind of confused about transformers.

http://www.allaboutcircuits.com/vol_2/chpt_9/2.html

I know that they step up or step down the voltage depending on the ratio of the primary turns to secondary, and that they do the opposite to the current. And it conserves power (ignoring small losses).

But I always thought that a voltage across a resistance drives a current, which is Ohms Law. I thought that if you had 100V across a 20 Ohm resistor, there would be 5 Amps. Simple.

But this http://www.allaboutcircuits.com/vol_2/chpt_9/2.html says that on one side of the transformer, there is "High voltage, low current", and on the other side, there's "Low Voltage, High current". How is this possible? Doesn't this directly contradict Ohm's Law? What is the voltage across, the two poles of the transformer on one of the sides?

Thanks a lot!