Zenarthra
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I know how to find it, but what exactly is it?
Why is it useful?
How does it apply to heating effect?

Thanks!
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mintsponge
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From my (basic) understanding, it is effectively the average voltage value when you've got an alternating voltage. An alternating voltage is one which goes from positive to negative again and again. Google "sine wave" if you don't know what I'm talking about. You can't just calculate the typical "average" value of this voltage wave, because that would be zero, so rms is a clever mathematical way of getting around that so you get a true measure of the average voltage.
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Tech
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(Original post by mintsponge)
From my (basic) understanding, it is effectively the average voltage value when you've got an alternating voltage. An alternating voltage is one which goes from positive to negative again and again. Google "sine wave" if you don't know what I'm talking about. You can't just calculate the typical "average" value of this voltage wave, because that would be zero, so rms is a clever mathematical way of getting around that so you get a true measure of the average voltage.
All good advice.

Just to elaborate a little more - what you're interested in is power (which tells you how much energy you're converting from electrical to heat form in a particular component over time). This energy will be dissipated regardless of which direction your current is moving in!.

In most circumstances, the voltage sine wave is useful because it tells us two things - the amplitude of our voltage, and its direction (backwards or forwards aka positive or negative) for every moment in time. But as discussed above, we don't care whether it's going backwards or forwards, only what amplitude it is. If you've done any statistics, you've probably learned how to calculate the standard deviation - we had the same problem there in that we only want to know how far a particular value is from the mean, and NOT whether it is above or below the mean. We calculate this by taking the SQUARE, and then taking the ROOT of the square. Sounds familiar now, right? It's the same idea :cool:

Hope that helps and good luck.
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Stonebridge
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(Original post by Zenarthra)
I know how to find it, but what exactly is it?
Why is it useful?
How does it apply to heating effect?

Thanks!
The other posters have explained it well.

It's worth adding that rms voltage and current (both calculated the same way) are essentially a way of identifying the AC equivalent of a DC source that produces the same rate of conversion of electrical energy in a circuit.

So if you have a lamp in a circuit running on DC with a current, say, 3A and you swap this over to an AC supply resulting in the lamp having the same brightness (same power) then the AC supply was 3A rms.

An AC supply of Vrms = 5V and Irms = 2A supplies power at the same rate (10W) as a DC supply of V = 5V and I = 2A

In practice it's this aspect of rms values that is often important.
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lerjj
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RMS of any function is basically the square root of the sum of all the values squared. i.e. sqrt( value1^2+vlue2^2+value3^2...) for a sine wave, this will always work out at the maximum height (peak amplitude) of the wave divided by root 2.

Wikipedia explains it quite well, albeit with Wiki's typical treatment of any maths subject by making it seem more complicated than it really is...

http://en.wikipedia.org/wiki/Root_mean_square
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Zenarthra
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Thanks guys!
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