Re: MIT's Walter Lewin's twice surprises the EE professors! (fun)


Chris H. Fleming wrote:


I don't get the last paragraph of the first page.

"There is no electric field in this loop if the resistance of the wire
making up the loop is zero."

Then he says.

"(this may bother you - if so, see the next section)"

Well I read on and it didn't stop bothering me.

Remember that ideal capacitors have zero resistance in their plates and
terminals, just as ideal inductors have zero resistance in their
windings. If you apply a voltage across the terminals of an ideal
inductor, the value of current will start changing continuously, and it
would go to infinity if you wanted to wait for infinite time to pass.
A perfectly conductive loop is not a short circuit, instead it is a
small-value inductor.



When he is explaining
this on page 4, in paragraph 3 he sets all the currents at every point
in the loop to be the same, but in the very next paragraph he
contradicts himself with charge buildups at certain points on the loop.

He's just simplifying things by ignoring the charge-buildup process.
The circuit will "adjust itself" over a very brief time, creating
surface charges and patterns of potential. After this "transient" has
occurred, things will work as he describes. In other words, he might
be describing what happens over a period of many milliseconds, while
ignoring the changes which occur in a range of nanoseconds. A more
complete explanation would have to include ALL changes at ALL time
scales, including the currents which led to the charge buildups.

Here's another cool article which goes into similar problems, similar
in that most books ignore it (but it's about flashlight circuits, not
about induced currents in conductor loops:)

Chabay/Sherwood, A unified treatment of electrostatics and circuits
http://www4.ncsu.edu/%7Erwchabay/mi/circuit.pdf


((((((((((((((((((((((( ( ( (o) ) ) )))))))))))))))))))))))
William J. Beaty Research Engineer
beaty a chem washington edu UW Chem Dept, Bagley Hall RM74
billb a eskimo com Box 351700, Seattle, WA 98195-1700
ph425-222-5066 http://staff.washington.edu/wbeaty/

.

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