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Re: electrical power



A couple more thoughts...

* * * *
I think what Fred Lemmerhirt wrote is pretty good...

The product of the voltage across any segment of a circuit and the current
in the same segment represents the rate at which the energy of the charge
carriers is being increased or decreased as they pass through that
segment.

It is indeed pretty generic, and therefore doesn't have the detail of
language that includes active/passive or resistive/inductive/capacitive,
etc. But failing to appreciate the very simple picture can make it very
hard to understand the more complicated picture. Fred's statement is a good
way to phrase the starting point.

* * * *

Considering watts versus volt-amps... help me out here to see if I am using
the words the standard way.

When we describe AC voltage and AC current the common understanding would be
we are using rms volts and rms amps. If we calculate (rms voltage) times
(rms current) then we are sort-of calculating power, but this won't be quite
right if the current and voltage are not in phase.

If we calculate an rms power as the rms value of (instantaneous voltage)
times (instantaneous current), and the load is not purely resistive, then
this value will be less than the rms voltage times the rms current.

Therefore volt-amps is always rms-voltage times rms-current, but power in
watts is the same as this only when the load is resistive. If the load is
not resistive then power in watts is less than the volt-amps.

Volt-amps is easy to measure because it is easy to measure rms volts and rms
amps. Power is more difficult to measure, i.e. requires more than a simple
multimeter.


Michael D. Edmiston, Ph.D. Phone/voice-mail: 419-358-3270
Professor of Chemistry & Physics FAX: 419-358-3323
Chairman, Science Department E-Mail edmiston@bluffton.edu
Bluffton College
280 West College Avenue
Bluffton, OH 45817