Re: Explaining

[Leigh Palmer <palmer@sfu.ca>]

> In a message dated 98-02-19 15:16:31 EST, you write:
>
> << >Well, I've had many undergraduates who will respond to a question testing
> > this concept, saying that they believe that resistors heat up because the
> > electrons lose kinetic energy when passing through the resistor, which is
> > the same as saying they go in faster than they come out. I would have
> > thought they'd not respond this way if they really grasped the continuity
> > equation, or Kirchoff's laws. >>
>
> What is the present model for this?  The model I learned delt with average
> drift velocity.  The electric field accelerated electrons locally, and when
> these electrons interacted with the lattice, their kinetic energy was reduced,
> increasing the lattice energy.  Of course, on average, the drift velocity
> remained constant unless the electric field was changed, or the change in
> lattice energy caused a change in its resistance to electron flow.  I don't
> see how localized acceleration, then loss of kinetic energy in interactions
> with the lattice,  means that on the average electrons entered the resistor at
> a faster speed than leaving it.

Well that is because you have a more nearly physically correct
description. Unless you imagine that the electrons can acquire
their kinetic energy during their transit of the resistor,
however, you have to acknowledge that they pack the energy in
with them as electric potential energy when they enetr the
resistor, and they fritter it away in a series of short falls
and inelastic bounces, a frantic glissade.

Such descriptions as these I find do stimulate students'
thinking, but they are too vivid for some. The latter group is
really turned on by the revelation that it can all be put down
to Pynting flux, by the way.

Different strokes...

Leigh