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Why are we using RMS value over average value for a periodic wave?

Debarati Mukherjee
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    Suppose there's no DC bias, what's the average value of the wave? What does it tell you about the wave that's any use to you? – The Photon Sep 02 '19 at 05:44
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    See also my answer to https://electronics.stackexchange.com/questions/328185/average-value-of-current-or-voltage/328198#328198. – Transistor Sep 02 '19 at 06:21

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A resistive heating element gets hot when connected across 220V AC mains, even though the mean (average) voltage of the peak voltages (+311V and -311V) is 0 volts. Many loads don't care that the voltage reverses polarity with each half cycle. The 220VRMS value gives a more practical way to estimate the real power delivered to the load. For non-ohmic (inductive or capacitive) loads, there is a power factor correction, but as long as voltage and current are in phase, VRMS = IRMS * Resistance.

MarkU
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Assume you are powering on a bulb with a AC RMS voltage of 10 V. It will create same brightness as that of 10 V DC.

RMS value is used to represent the strength of the AC signal or equivalent of DC signal.

A periodic Sinewave for example, with peak to peak of 28 V will also create same brightness but the average value of the signal is zero (no information on the strength is conveyed with the average value).

User323693
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  • What is the "strength" of a signal? In what way is it equivalent to a dc signal? – Elliot Alderson Sep 02 '19 at 12:07
  • strength here I refer to the ability to deliver certain amount of Power to the load (independent of the shape of the signal) which is equal to the amount of power delivered by the same DC value – User323693 Sep 02 '19 at 12:12
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RMS should be used to represent signal having zero average (due to symmetry of signal to the positive side and the negative side ). Periodic waves like sine waves usually have zero average. Average value gives the DC content of the signal.

Amalnath Satyan
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  • **Any** periodic signal has an average value. You have missed the significance of rms. – Elliot Alderson Sep 02 '19 at 12:06
  • @ElliotAlderson Yes. Any periodic signal have an average value, and it can be zero too. That's what I meant. – Amalnath Satyan Sep 02 '19 at 12:18
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    I think your use of the word "represent" is vague and confusing. There are many ways to represent a signal and they are all useful in the appropriate circumstances. Why shouldn't I use peak-to-peak for a sine with zero mean? – Elliot Alderson Sep 02 '19 at 12:25
  • @ElliotAlderson I understand that It has an ambiguity in that. – Amalnath Satyan Sep 02 '19 at 12:28
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    @AmalnathSatyan: *"RMS should be used to represent signal having zero average ..."* This isn't correct. RMS measurement is required with **any time varying signal* (even if it's all positive or all negative) when power calculations or measurement is required. See my answer linked below the OP's question. It deals with an AC signal but it is exactly the same if you reflect the negative part of my sample waveform due to the squaring operation. – Transistor Sep 02 '19 at 18:04