Calculating power

there are these 2 questions that i can't find the answers for:
a) In a dc circuit, the power is the product of voltage and current. this relationship does not always apply to effective values in an ac circuit. explain why.

b) in a dc circuit, ir the current and resistance are known, power (I²R) can be determined. this also applies in ac circuit. explain why.

thanks loads

Manugirl,

a) In a dc circuit, the power is the product of voltage and current. this relationship does not always apply to effective values in an ac circuit. explain why.

Click to expand...

In a DC circuit, the voltage and current are always in phase. In a AC circuit, not so.

in a dc circuit, ir the current and resistance are known, power (I²R) can be determined. this also applies in ac circuit. explain why.

Click to expand...

The formulas I²R and E²/R have no phase relationship to worry about.

Ratch

thanks dude