# Can someone please explain the following terms?

This is not for homework or anything, just curious:

-difference between volt-amperes and watts
-relative power vs. actual power
-root-mean-square

Thanks!
baum

VA vs.W is an AC vs DC thing.
W can only simply be applied to DC systems

relative power corrupts relatively, actual power corrupts actually.

RMS is just a method of measuring AC voltage, like peak-to-peak.

relative power corrupts relatively, actual power corrupts actually.

what?

but thx for everything else!

Sorry, poor attempt at a joke
"All power corrupts, absolute power corrupts absolutely"

I don't recall the terms "relative power", but guessing that it refers to the type of load - "apparent power" vs. "real power"
"apparent power" is the product of current and voltage, but current and voltage may be out of phase, depending on the characteristics of the load.

Google "Power correction factor"

i get it now!
thanks!
baum

Root-mean-square is important because it is related to power independent of AC waveform (square, sine, anything).

Instantaneous power is I-squared-R (or equivalently V-squared-over-R). Thus the average power during a single cycle of the AC waveform is the sum (more properly 'integral') of the instantaneous power and thus is proportional to the mean-square value of current or voltage.

Taking the square-root of the mean-square gives a value that's back in voltage or current terms. If we use r.m.s. values then we can use the equation power = voltage x current. If we use amplitudes then that equation only works for square/rectangular waves.

For instance 240V AC mains (r.m.s.) has an amplitude (peak) of 340V (since its sine, not aquare), but we always talk about the r.m.s. value because we can then do sensible calculations about power. Also a (tungsten) lightbulb designed for 240V AC r.m.s. will also work the same with 240V DC (same power dissipated on average), ditto for heater elements.

I get it, but:

For instance 240V AC mains (r.m.s.) has an amplitude (peak) of 340V (since its sine, not square)

???

The RMS value of a sine wave of amplitude A is A/sqrt(2). 340/sqrt(2) ~= 240

baum:
I get it, but:

For instance 240V AC mains (r.m.s.) has an amplitude (peak) of 340V (since its sine, not square)

???

There are several different voltage values that can be assigned to a AC sine wave, RMS, Peak, Peak to Peak, average. They are different measurement values of the same waveform. RMS is the most common as it reflects the exact same power as a DC voltage of the same value. If however one is trying to determine a maximum voltage specification for an insulator or component to be used, s/he needs to be aware of the peak value.

Lefty