I teach AP Physics C and one of my better students is always asking good
WHY questions. Usually I can answer them to his satisfaction (Isn't that
the goal of physics -- to give us satisfying answers to our questions? And
if a model or theory no longer gives satisfying answers (doesn't agree with
experiment), we (try to ) modify it or toss it and invent a new one. But I
digress).
I made the statement in class (as I had done for years) that the
electrostatic electric field established at the terminals of the seat of
emf will follow the path of the conducting wires when an external circuit
is completed and that the electric current will move in the wires. This
student asked WHY? as in why does the field bend, twist, and turn with the
wires?
The answer in the 9th edition of University Physics (Young, Freedman) gave
the following non-satisfying (to him) answer: "Notice that where the wire
bends, equal amounts of positive and negative charge persist on the
'inside' and 'outside' of the bend. These charges exert the forces that
cause the current to follow the bends in the wire."
We can see how the vector sum of the E field due to these + and - charges
and the original E field give an E field which turns with the wire. But he
wanted to know why the charges accumulate in the first place.
I mumbled a few things such as:
(1) Well, if an electron escaped from the wire, the positive ion (positive
image ?) it left behind at the surface would attract it right back into the
wire where, since it can't escape, it would be "pushed" from behind around
the bend by the electrons moving along the straight section of the wire.
Semi-satisfied.
(2) Then I drew a square well labeling the vertical side the amount of work
to remove the electron from the wire and said that this the experimentally
determined the work function to remove electrons from the metal. And that
the electrons do not acquire enough kinetic energy from the E field to
escape. Semi-satisfied.
(3) Then I mumbled something about since, we know the current is confined
to the wire and that E is proportional to J, it follows that E is confined
to the wire. Unsatisfied -=> too circular.
Can you help me out with an explanation that might be satisfactory to a
future double nickel (5, 5) on the AP Physics C test? Then maybe I'll be
able to understand it.
No need to start a new thread here, so just reply to me privately.
Thanks a lot. I learn so much from you guys. ("guys" is gender neutral to me)
Bye, DiRT
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David R. Thiessen FUN + PHYSICS = PHUN
Science Department PHYSICS IS PHUN !!
Deerfield High School
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Apples instead of driving them bananas ??
dthiessn@nslsilus.org (Quote sources long forgotten)
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