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Let me add one more comment that will hopefully help in understanding this.
I think I might have identified another misconception that might be contributing to the confusion. That misconception is that the RMS value of a waveform is the "equivalent" or "heating" value. This leads to the mistaken notion that the RMS value of the power must be the "equivalent" or "heating" value. Voltage and current, by themselves, don't do any heating. Heating takes power. And the amount of heating is the determined by the average power. If two different power waveforms have the same average value, they'll create the same amount of heat and otherwise do the same amount of work, even if their RMS values are different. So what's the whole thing about RMS? The only importance of RMS is that when you multiply the RMS values of two waveforms together (such as V * V, I * I, or V * I), you get the average of the product of the two. That is, RMS(i) * RMS(v) = Avg(i * v), where i and v are the instantaneous values of the current and voltage, and i * v is the instantaneous power. Likewise, RMS(v) * RMS(v) / R = Avg(v*v/R) and RMS(i) * RMS(i) * R = Avg(i*i*R). In fact, RMS(x) * RMS(y) = Avg(x * y) where x and y can be any periodic waveforms or quantities. (I derived these mathematically in an earlier posting.) So we calculate the RMS values of voltage and current only so we can use them to calculate the average power -- not because the RMS value of any waveform is its "equivalent" or "heating" value -- which it isn't. Roy Lewallen, W7EL |
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