I agree with John Denker's comparison of P=VI and P=I^2R.
Also note why John's statement about P=IV being exact is true... V
(potential difference) is the energy change per coulomb. I is current in
coulombs per second. Therefore IV is joules per second (energy change over
elapsed time) which is watts (power).
I am less comfortable with John's statement that IV is the joule heating. I
would say P=VI is the joule heating if joule heating is the only thing the
device is doing. If the device is a motor, or a light bulb, or some device
that is "emitting power" other than via thermal energy, then P=IV is the
total power consumption, and the joule heating would be some number less
than this.
I suspect this might be the type of thing the original question is trying to
address. If a device is consuming power and some is dissipated as thermal
energy and some appears as other forms, how can we calculate just the
thermal part? This is essentially a question about efficiency. I am not
sure there are good ways to know this aside from engineering studies of the
devices.
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