AC or DC, heat dissipating from resistor

Thread Starter


Joined Apr 15, 2011
I was looking at a job practice test and found this problem. What dissipates more heat from a resistor, AC or DC voltage?

Im thinking AC because the voltage is always changing but I don't know for sure.

Both practically the same formula for power
cos theta VI=P=Watt

Can someone help me understand this question or is this a trick question?


Joined Dec 26, 2010
It depends how the voltages are stated. RMS AC voltages have equal heating power (in a resistor) to the same DC voltages.

That is true at least under ordinary conditions at power frequencies. At higher frequencies special effects like parasitic inductance and skin effect might be significant, but I seriously doubt that this is relevant to the sort of question you are referring to.

The current phase angle and its cosine the power factor are relevant to circuits where current and voltage are out of phase, often because of inductance. The question you mention is about voltage applied to a resistor, which has zero phase angle so that its power factor is 1. Again, this may not always be strictly true especially at higher frequencies, but I doubt that matters at the level this question is intended for.


Joined Sep 7, 2009
It depends how the voltages are stated. RMS AC voltages have equal heating power
In fact, the best "true RMS" AC meters had extremely precise thermo elements to measure the heat created by the waveform.

These days, most "true RMS" AC meters are not TRMS at all for many signals. They have circuits that do some integration and can't give true RMS on complex waveforms.