Re: electrical power

[David Bowman <dbowman@TIGER.GEORGETOWNCOLLEGE.EDU>]

Regarding Michael E.s comment:

> I am less comfortable with John's statement that IV is the joule heating.

Me too.

> 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.

Some other examples (of many) are the radiation power from an antenna
(definitely *not* Joule heating), and even the rate at which energy
is being stored in the magnetic field of an inductance, or the rate
at which energy is being stored in the electric field between the
conductors of a capacitor.

BTW, if the light bulb example that Michael is thinking of is an
ordinary incandescent bulb, then I would agree (with J. D.) that such
a case *would* have P = I*V represent the Joule heating of the
filament.  If after the electrical energy is dissipated the
temperature rises to such an extent that it radiates thermal
radiation it does so as a means of cooling the filament that was
previously heated by dissipating the electrical work done on it by
the flowing current.  But if the "bulb" is a laser pointer, then I
would not consider the light energy emitted as coming from
previously dissipated Joule heat (sorry Jim G.).

David Bowman
David_Bowman@georgetowncollege.edu