# AC or DC, heat dissipating from resistor

Discussion in 'Homework Help' started by EL7819, Dec 4, 2011.

1. ### EL7819 Thread Starter New Member

Apr 15, 2011
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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
VI=W

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

Dec 26, 2010
2,147
300
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.

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3. ### bountyhunter Well-Known Member

Sep 7, 2009
2,498
507
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.

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4. ### T.Jackson New Member

Nov 22, 2011
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That was the reason why RMS was derived in the first place. This is usually like in the first chapter of most books on electronics.

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