Lewin is brilliant with his delivery of this lecture. He has all the grace and sly of a good magician or illusionist. Most were captivated. But not all were fooled.
At the very beginning, Lewin plants a seed that what you are about to see is "a very non-intuitive result". In reality, this of course is not the case, but you have already been set up to believe this is so. Call it "preconditioning".
Lewin begins by illustrating and discussing a simple DC circuit that contains an EMF source, and two potential drops across resistors. Take careful note that BOTH the EMF and the potential drops are illustrated. Also note that the potential differences calculated across the resistors is based on the circuit current. Ostensibly, his focus is on the potential difference between points D and A (V
D-V
A). See "DC_cct01.png" below.
Next, Lewin erases the battery and replaces the EMF source with an increasing magnetic field inside the loop. Here, Lewin notes that the resulting induced loop emf (peak) is 1V.
Lewin goes on to explain that the circuit current is still 1mA because the induced emf is 1V. Then he asks again; "what is V
D - V
A?" At this point, most viewers do not notice the slight of hand so brilliantly performed by the good professor. Did you?
Lewin is again calculating V
D-V
A using 1V emf and 1mA current, yet he has NOT illustrated the 1V emf source on his diagram. Why did he not illustrate the induced circuit
emf?
This is in fact the faux pas that invalidates the rest of his lecture. Why? Because professor Lewin boisterously exclaims that Kirchhoff's rule (or law) does not hold in this case. He is absolutely incorrect with his assertion, and had he illustrated the induced circuit emf, it would be obvious why.
You can NOT have any potential difference across a circuit load, unless you ALSO have a circuit emf (or EMF in cases of DC) present. How can you have an emf induced on a piece of wire? I would ask, how can you not? This is Faraday's law.
The "induct_cct01.png" diagram illustrates what Lewin SHOULD have drawn on the board before he went off leading his audience down the garden path. Take careful note that the polarity of each emf source (red lines and polarity markers) is in opposition to the potential differences (PD) across the loads (green polarity markers). This is very important to understand. This is ALWAYS the case in every circuit, regardless if it is a DC or an induction circuit such as this. The total induced emf is equal to the total potential drops across the circuit non-reactive loads (i.e. the two resistors); the sum of the two being equal to zero. In other words, the closed loop integral equals 0V. And at ANY instant of time.
What this means, is that Kirchhoff's Law DOES hold in this case, contrary to what Lewin erroneously espouses in his lecture.
The simple matter of HOW to prove this and properly measure the potential drops AND emf's in the loop have been illustrated in several posts prior to this. It's important to realize that Lewin never made one emf nor PD measurement in his experiment. He only measured the electric field, and that's all he
could measure considering the way his probes were situated. When you see this, you realize professor Lewin was confused about what he was measuring. It is clear he was mixing apples and oranges.
This should answer the question as to what the purpose of all this testing and discussion has been about.
btw, what is the closed loop integral of the induced
electric field in this experiment? It is 1V of course!
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"Some scientists claim that hydrogen, because it is so plentiful, is the basic building block of the universe. I dispute that. I say there is more stupidity than hydrogen, and that is the basic building block of the universe." Frank Zappa
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Topic: Professor Walter Lewin's Non-conservative Fields Experiment (Read 329910 times)
