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Re: All that glisters is not gold



Paul,
We agree that's there is energy stored in an electric field.
However there is energy also being dissipated in the wire
and so we have a continuous, steady-state situation where we
need to bring energy from the battery to the load (as well
as having a fixed amount of energy stored in the field).
The common misconception is that the current is transporting
the energy and that's what I disagree with. I claim that
the electromagnetic field will get the energy there to where
the electric field is.
Brian McInnes
----------
From: paul o johnson <pojhome@FLASH.NET>
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: All that glisters is not gold
Date: Sat, 13 Feb 1999 1:25 AM


Brian: I think I see your point, i.e., your need to invoke the
poynting
vector, but this seems to imply that a simple E field does not
convey
energy. We all know that energy is stored in a simple E
field, that
the
energy density is proportional to the square of the field's
magnitude,
why is that not sufficient to explain the (almost) immediate
transport
of energy from source to load? Of course, it doesn't explicitly
give the
rate of energy transfer.

poj

Brian McInnes wrote:

Paul, As I see it the problem with the plain old electric
field is that, although it will explain the local
acceleration of electrons along the wire, it doesn't really
explain the transport of energy from the battery to the
load. That's why I like to invoke an electromagnetic field
and the poynting vector (as apparently William Beatty does,
judging by his contribution where he describes this model in
excellent detail).
Brian McInnes
----------
From: paul o johnson <pojhome@FLASH.NET>
To: PHYS-L@LISTS.NAU.EDU
Subject: Re: All that glisters is not gold
Date: Fri, 12 Feb 1999 3:14 PM


Brian: I agree that conduction electrons continually lose it in
collisions with the ionic lattice
and they can't get the energy to where it is being
dissipated in
a
time
interval anywhere near that which is observed. But it's not any
specific
electron(s) that deliver the energy at the other end of the
conductor;
it's the electron(s) that were already near the other end when
the
potential difference was applied. The near-instantaneous
electric
field
causes those far-end electrons to drift out of the wire and into
the
load at the same time the near-end electrons start their drift.

I don't see why an electromagnetifc field is needed to explain
the
energy transfer. It seems to me that a plain ole electric field
will do
nicely.

poj

Brian McInnes wrote:

Paul Johnson called attention to my statement that "the
energy does not go
down the wire but through the space not the wire". Paul
wrote that he had always thought that
the energy is in the field and the field is in the wire and
that is what gives the electrons their drift velocity.

Electrons are very inefficient in transporting energy; they
continually lose it in collisions with the ionic lattice.
Also because their drift speed is so slow, they can't get
the energy to where it is being dissipated in a time
interval anywhere near that which is observed.

It's the electromagnetic field that is transporting the
energy quickly and efficiently.

The energy is in the electromagnetic field (the combination
of electric and magnetic field). It is the Poynting vector
(E cross B) that describes the magnitude and direction of
the energy propagation. This vector is non-zero in the
space about the wire.

The electric field arises from the charge gradient on the
surface of the wire (as explained so well by Sherwood and
Chabey); the magnetic field from the current flowing in the
wire.

Brian McInnes