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

Cyberkatru wrote:
MIT physicist/astronomer is quite fun to what lecture. In lectures 20
and 16 of his online basic E&M lectures he makes some interesting
points. I will tell you about both of them and ask your opinions but
notice that the second one I mention (which actually comes first in
lecture 18) is the most interesting and maybe a bit puzzling so be sure
to get through this whole post.

First from lecture 20:
At some point a little past the 8 minute mark in Walter Lewin's basic
E&M video lecture #20 he makes the following statement:
"Almost every college physics book does this wrong!"
He is referring to the use of Kirchoff's voltage rule for loops in
cicuits with self inductors.
I think he is right about this.
In a way he is denying the assumptions of the so-called "lumped matter
discipline" in electrical engineering--with regard to inductors
anyway.

It is all quite entertaining. You can see the video here starting at
about the 8 minute mark of lecture number 20.
.http://ocw.mit.edu/OcwWeb/Physics/8-02Electricity-and-MagnetismSpring2002/VideoLectures/index.htm

He also has a write up of this issue here:

http://ocw.mit.edu/NR/rdonlyres/Physics/8-02Electricity-and-MagnetismSpring2002/C29636DB-AB80-49E0-BFAB-BCE00159F00E/0/lecsup41.pdf

So do we agree with him on this bit of basic physics 101?

Now on to the more interesting case of "nonconservative" fields and
wacky voltmeters:
This starts at about minute 38 in video lecture 16 when he commands to
"hold onto your hats!". A few minutes later gives a demonstation of
this where he says professors visiting in the audience refused to
believe what they were seeing.

Do you agree with Walter's interpretation of what is happening?

I think I basically agree but frankly I am not sure I like his
insistance that this is a case of what should be called
"nonconservative" fields (except in some unusual sense of
nonconservative). Here is why:

Mathematically, the field depends on time and so is now properly a
field on spacetime. Now a closed loop in spacetime should return not
only to the same place but also the same time. So he has not really
demonstrated that the field is nonconservative in the usual
mathematical sense (cohomology). He has just used a path that didn't
really return to the same point in the manifold (spacetime).

Electrical engineers would note that "inductive" reactance isn't the
same as "capacitive" reactance. When an antenna is resonant, that is,
its length equals the fomula electrical length, only ohmic and
radiation resistance are present. Only when the antenna is NOT
resonant is reactance present. If an antenna is too long for a given
frequency, it is daid to have "inductive" reactance... if it's too
short the reactance is "capacitive". Power lost in reactance isn't
lost in the same way it's lost in ohmic resistance.

Re the "coupling" matter... technically a radiating antenna is
"coupled" to its surrounding media when it's working correctly (i.e. is
resonant).

.

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