🚀 go-pugleaf

In article <11DEC199214524483@lims02.lerc.nasa.gov> edwlt12@lims02.lerc.nasa.gov (MIKE JAMISON) writes:
>Paul Houle suggests finding the direction of the B field, the E field and 
>then determining the Poynting vector from this information.  There may be a 
>problem with finding the B field (where is your reference point, 
>specifically).
>
>I suspect the "easiest" answer is to measure the voltage between the 
>superconductiing wires, noting the sign.  Use the compass to determine the 
>B field at one of the superconducting wires (remember that the B field 
>generated by a wire looks like a bunch of concentric circles with the wire 
>at the center, in a plane perpendicular to the direction of current flow in 
>the wire).
>
>The compass will align with the B field (its B field will point opposite to 
>that of the wire).  Using the right hand rule, you can find the direction 
>of current flow in either of the wires, assuming you know which direction 
>the B field points on a compass (N to S or S to N, sorry, I can't remember 
>- Tom, can I take my EM book with me as a reference :))
>
>Coupling the direction of current flow with the E-field information, you 
>can find out where the battery is.  Q.E.D.

	Yes, I actually got a little confused about this later and
thought it out from your perspective and I got the same answer.  I was
a little bit bothered about what the actual physical difference
between the source and sink of energy are, but then I thought how the
current flows from - to + on a battery and + to - on a sink. The right
hand rule works for my first answer too -- S = E X B (where X is the cross
product), so you can, say, take your right hand and point your middle
finger along the E field, and point your thumb along the B field and
then your pointing finger is the poynting vector,  and that is the way that
energy is flowing.  (Try it!)  Using your right hand lets you instruct a
small child (but training a dog or most physics students might be harder)
to do it without any thinking about currents.

	Frames of reference create no problems,  since the equations for
electromagnetism are Lorenz invariant and the energy flux will come out
right for whatever frame you are in.

	It might actually be interesting to set something like this up for
physics students (very smart high school students or average college
students) with ordinary wire (Just make it thick enough and the voltmeter
cheap enough (The leads short enough!) that one could not detect voltage
drop.  Then let five or six of them into the room (omit a blackboard if
you are cruel) and let them argue about the answer.




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