Question 1

Is Ohm's law valid for AC circuits?

Accepted Answer

For pure resistors, yes, using RMS values. With inductors and capacitors you replace R with the impedance Z and phase matters.

Question 2

Why does a light bulb draw more current when it is first switched on?

Accepted Answer

A cold tungsten filament has much lower resistance than a hot one, so the inrush current is several times the running current.

Question 3

How do I find resistance from power and voltage?

Accepted Answer

Rearrange P = V²/R to R = V²/P. A 230 V, 1,000 W heater is about 52.9 Ω when hot.

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