That's a pretty good explanation Adam. I'm going to exercise my pedantry and clarify a few points though.

The RMS of an AC sine wave is the DC equivalent voltage.

The RMS voltage of ANY AC WAVEFORM (regardless of its shape) is the voltage that, if it were DC, would produce the same MEAN POWER DISSIPATION, or the same HEATING EFFECT (which is the same thing), in the same RESISTIVE (OHMIC) load, when measured over a one cycle period (for a repetitive waveform). So for a given resistive load, 110V AC RMS will produce the same amount of heat as 110V DC.

A sine wave is not DC so it can't produce the same power as the DC with the same peak voltage, because it is constantly changing.

More to the point, because the instantaneous voltage of the sinewave, measured at any instant during the cycle, is not always equal to the peak voltage.

Another way of saying this is that with any AC waveform, apart from a perfect rectangular wave with equal positive and negative voltages, if you draw a rectangle only just enclosing each half cycle, the rectangle is not fully filled with continuous voltage, so by definition, the mean voltage, or the mean power, in that half cycle cannot be as high as the peak voltage.

So if you had a sine wave with a peak value of 1.414 Volts then the DC equivalent voltage is going to be the RMS of this which is 1.414 Volts * 0.707

Which is 1.0V DC.

You work backwards on this and do the SQUARE and then the MEAN and then the ROOT. So what does the SQAURE bit do. All it does is removes the negative part of the sine wave and puts it along side the positive part of the sine wave.

That's one part of it; the other part is that it takes into account the fact that for a resistive load, power is proportional to the SQUARE of the voltage. This is because current is proportional to voltage, and power is proportional to voltage × current. So power is proportional to voltage squared.

This is also shown by the formula P = V

^{2} / R, which is the result of combining Ohm's Law (I = V / R) with the Power Law (P = V × I). Substituting V/R for I in the second formula, you get P = V × (V / R), which is equivalent to P = (V × V) / R, which is equivalent to P = V

^{2} / R. Therefore for a constant load resistance, power is proportional to the square of the voltage.

Here's another way to imagine it. For simplicity, let's use a 1Ω resistive load. Take any random point on any repetitive AC waveform that is being fed into that load. How much power will the load dissipate at that instant in time? If P = V

^{2} / R, and R = 1Ω, then P = V

^{2}. So to calculate the power dissipation at that instant, you need to square the voltage at that point on the wave.

If you do this for every point in a cycle of the AC waveform, then take the mean of the results, you will get the mean power dissipated in the load during one cycle of the waveform. Then, if you take the square root of that mean power figure, you convert power back into voltage, and you get the equivalent DC (continuous) voltage that is required to produce that amount of power in the load.

For a sinewave, that voltage is equal to the peak voltage multiplied by 0.707. (0.707 is the square root of 0.5, or the reciprocal of the square root of 2.) For other waveforms, the relationship between the RMS and peak voltages is usually different.

As I mentioned, for a perfect symmetrical rectangular wave, the RMS voltage is equal to the peak voltage. For a triangle or sawtooth waveform, the RMS voltage is equal to the peak voltage multiplied by 0.577 (which is the square root of 1/3). It's possible to calculate these constants, and explain them, but my maths skills aren't up to it, and it might get boring. Have a look at

https://en.wikipedia.org/wiki/Root_mean_square if you want a dry technical explanation.

This makes it easier to work out the mean because the mean DC voltage of a sine wave with equal + and - voltages is equal to zero. So now the MEAN of this new waveform is going to be half, the same two waveforms / 2. Then the ROOT removes the original SQUARE and so you have Sqrt of 1/2 which is 0.707.

I can't follow that explanation, but I'm pretty sure that bit is wrong. Steve or Harald can probably explain, if they have time.

I think I got that right?

All up to the last paragraph, AFAIK

Another reasonably readable explanation is at

http://www.learnabout-electronics.org/ac_theory/ac_waves02.php