Indeed, I remember learning this... "distinction" in my early years at my engineering school. However, let's face it, such distinction is actually utterly unnecessary. Units of measurements are NOT vectorial entities. There is an entire science devoted to this called dimensionals. In other words, [VA] = [W].
OBS.: For those of you unfamiliar with dimensional notation the symbols '[]' mean exactly that. Actually, a more rigorous use of them would be: [P] = VA = W, P meaning power. Dimensionals simply follow the rules of mathematics as if they were all scalars, BTW. Marcus On Tue, 26 Mar 2002 15:28:52 Barbara and/or Bill Hooper wrote: >on 3/26/2002 12:41 AM, Bruce Hebbard at [EMAIL PROTECTED] wrote: > >> ... could someone please explain why serious >> power equipment often uses VA (kVA, MVA, etc.) as distinct from W ? > >That is done in working with AC (alternating current) electricity. The power >(in watts) is the voltage (in volts) multiplied by the current (an 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?) > >AC power is found by calculating the product of the instantaneous current >multiplied by the instantaneous voltage, moment by moment over a whole cycle >of oscillations. If the current and voltage are in step (rising and falling, >and reaching peaks and valleys at the same time), then the average power is >found by multiplying the average* voltage by the average* current. > >BUT, often (usually?) the current and voltage don't rise an fall together in >step. Then, the average power is NOT equal to the average* voltage times the >average* current. (If they are exactly one quarter cycle out of phase, the >power will be ZERO!) Yet, sometimes, the product of the average voltage and >the average current is of importance to electrical engineers (I don't know >the details of this). So they multiply the two together and get a value in >volt-amperes WHICH IS NOT EQUAL TO THE POWER, so they do not express it in >watts. Instead they call it volt-amperes. > >Regards, Bill Hooper >retired physics professor, Florida, USA > > *The "average" I refer to above is the root-mean-square (RMS) average, >sometimes called the '"effective value". > -------------------------------------- > "Simplification" begins with "SI" > -------------------------------------- > > > > See Dave Matthews Band live or win a signed guitar http://r.lycos.com/r/bmgfly_mail_dmb/http://win.ipromotions.com/lycos_020201/splash.asp
