Re: Explaining

["Bob Sciamanda" <trebor@velocity.net>]

An instructive parallel is marbles falling through syrup in the earth's
gravitational field.  They travel at a terminal velocity, serving in
effect as a mechanism for transforming into heat the work done by the
earth's gravitational field.  In the electrical current carrying wire
electrons are falling through an electrostatic field (due to surface, etc
charges) at a terminal velocity (effectively) and "immediately" giving
that energy up to the lattice as heat.

I teach it the other way around:  because there is a p.d across the wire
there must be an E field doing work on the carriers.  Since they do not
come out with increased KE (I is the same, etc) there must be an energy
sinking mechanism in the wire (or indeed in any two terminal device which
exits the an unchanged current after a p.d. drop).  Touch the wire and
you will discover where the "lost" KE is going.  Then make the marble
analogy.

In the same way a potential rise (and constant current) implies an energy
source, since the carriers climb a potential hill, against a field,
without losing KE.

-Bob

Bob Sciamanda   sciamanda@edinboro.edu
Dept of Physics   trebor@velocity.net
Edinboro Univ of PA  http://www.edinboro.edu/~sciamanda/home.html
Edinboro, PA   (814)838-7185

-----Original Message-----
From: Raacc@aol.com <Raacc@aol.com>
To: phys-l@atlantis.uwf.edu <phys-l@atlantis.uwf.edu>
Date: Thursday, February 19, 1998 4:35 PM
Subject: Re: Explaining


> 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.
>
> Bob Carlson