1. We will be in Read Only mode (no new threads, replies, registration) for several hours as we migrate the forums to upgraded software.

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
    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?
  2. Adjuster

    Late Member

    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.
    EL7819 likes this.
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    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.
    EL7819 likes this.
  4. T.Jackson

    New Member

    Nov 22, 2011
    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.
    EL7819 likes this.