Re: Power = P = VI = I2R, V2/R
- From: Chuck <nospam@xxxxxxxxxxxxx>
- Date: Mon, 28 Jan 2008 17:56:20 -0500
On Mon, 28 Jan 2008 13:57:12 -0800 (PST), whit3rd <whit3rd@xxxxxxxxx>
wrote:
On Jan 28, 1:36 am, Selva <kobu.se...@xxxxxxxxx> wrote:
The power equations P = VI = I^2 R = V^2 / R work for dc signals and
ac sine signals if we use RMS values (peak / root2).
Well, RMS values are NOT 'peak/root2'. Those are two
entirely different schemes for computing an average,
and while the numbers are EQUAL for sine waves,
the schemes are not the same.
P=VI requires not only RMS averages for V and I, and
simple average for P, but also a third condition,
that of 'unity power factor', to be correct for AC sinewave
signals. That 'unity power factor' is correct enough for electric
heaters and incandescent light bulbs, which are said
to be 'resistive' loads, but not for motors or lights on dimmer
switches.
It may not be helpful to view rms values as averages. But by using rms
values of V or I, you can calculate the average (or equivalent )
power.
Also, if you take the product of the instantaneous voltage and the
instantaneous current, you get the power at that instant. Not the
average power or the rms power, but the instantaneous power. Always.
Regardless of the power factor. Regardless of whether the voltage or
current are sine waves or even DC.
Chuck
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