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Re: AC electricity



At 12:52 PM 1/18/01 -0500, Ludwik Kowalski wrote:
>
1) Poyting vector is not discussed in many
introductory physics courses. Is this a serious
omission and a source of misconceptions?

I would not call it a "serious" omission nor a "serious" misconception.

In circuits where the Kirchhoff laws are valid approximations, the rule
power = V times I
is valid and equivalent to
power density = E cross B

When we teach any approximate law, we run some risk of causing
misconceptions. But that's what we do for a living. Remember, F=ma is
only an approximation, which we teach without feeling guilty.

We need to explain the _limits of validity_ of the laws we teach.
--) It is relatively easy to say that F=ma breaks down when the
velocities are not small compared to c.
--) It is requires somewhat more effort to explain the limits of validity
of Kirchhoff's laws.


2) Consider a purely ohmic load which is heated.
It is clear that energy must enter somehow into the
load. It arrives through the space surrounding wires
and goes into the load. How does this happen?

It happens just like Ludwik said. It arrives through the space surrounding
the wires and goes into the load.

But remember this is a sophisticated view, more sophisticated than is
appropriate for analysis of ordinary practical circuits. It's appropriate
if you are designing antennas, but virtually all practical circuits are
designed so that the Rs, Ls, Cs, transistors, and wires don't act like
parasitic antennas.

Suppose the load is a high R wire perpendicular to
the energy-guiding wires (see below). Near that
load the Poyting vector is directed everywhere into
the load. The energy of the field "enters the metal"
and is dissipated in it. Is this a correct explanation?

Sounds fine to me, if you want to take the ultra-sophisticated view.