Some time ago, I wrote: >> That is done in working with AC (alternating current) electricity. The >> power(in watts) is the voltage (in volts) multiplied by the current >> (in amperes)ONLY if the voltage and current are constant. (Let's face >> it: if they are NOT constant, you would not know what value to use?)
and John (kilopascal) replied: > You lost me here! What do you mean by constant?. I mean "unchanging" as in DC. John went on to say: > The power formula for DC > (P=EI) is modified when used with AC. Yes, that's what I meant. The power is equal "simply" to voltage times current *only* if the voltage and current are constant. If they are not constant (as in AC) the equation must be modified (as John says), such as by multiplying by additional factor of the cosine of the phase angle as well as deciding which values of a constantly varying voltage and current should be used. Of course, the cosine of the phase angle is used only for sinusoidal waveforms. For other non-constant wave forms (square wave, saw tooth, half or full rectified sine wave, etc.) the equation would be modified in different ways. (The relationship John referred to here requires the use of the so-called effective values of the current and voltage. This is defined as the root-mean-square averages of the fluctuating AC current and voltage. These numbers are "constant" but they are just particular type of average values for the current and voltage, which are not constant.) In any case, the only time that the formula is simply current times voltage is for constant (DC) current and voltage. For any other case it is different. Finding the result for any non-constant wave form is done by taking infinitesimally small time segments (during which even fluctuating values may be assumed to be approximately constant since the time span is so short), calculating the power for that small time interval (by multiplying the momentary current times voltage), and then averaging over the infinite number of infinitesimal results in one cycle to get the average power. (That procedure involves the mathematical process we all know as integration.) For sinusoidally fluctuating voltage and current, the general result is the voltage (effective value) times current (effective value) times the cosine of the phase angle. Regards, Bill Hooper retired physics professor, Florida, USA P.S. John's original qustion was submitted quite a while ago. Sorry I was not more timely with my reponse. -------------------------------------- "Simplification" begins with "SI" --------------------------------------
