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Old 31st Aug 2017, 2:05 pm   #1
ms660
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Default Kirchoff etc ??

I'm not qualified enough to comment on what he says or the experiment shown towards the end of the video, best watched from 27:16 in to save time or start at the beginning if you have time:

https://www.youtube.com/watch?v=nGQbA2jwkWI

Anyone?

Lawrence.
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Old 31st Aug 2017, 3:42 pm   #2
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Default Re: Kirchoff etc ??

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Originally Posted by ms660 View Post
I'm not qualified enough to comment on what he says or the experiment shown towards the end of the video,
Neither am I but it was interesting.
Frank
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Old 31st Aug 2017, 7:52 pm   #3
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Default Re: Kirchoff etc ??

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Neither am I
Ditto.

The voltage measurements make sense for the loop current. As to the apparent difference, we are looking at a transient rather than the steady state. Certainly a surprising experiment though.

Peter
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Old 1st Sep 2017, 12:26 am   #4
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Default Re: Kirchoff etc ??

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Originally Posted by peter_scott View Post
As to the apparent difference, we are looking at a transient rather than the steady state.
I think this is exactly the point. We can't expect to see transient voltages induced in a loop by induction, in either amplitude or polarity, match that of a steady state condition, as his experiment elegantly points out. The same applies to steady state conditions that occur say after the oscillations in a resonant circuit have decayed to nothing, versus the conditions when the circuit was previously oscillating and energy was exchanging between the reactive components. But it is a really good example of the trap one could fall into measuring voltages around a loop.
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Old 1st Sep 2017, 9:19 am   #5
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Default Re: Kirchoff etc ??

These notes by Prof. J. Belcher (slightly amended by W. Lewin) might be of some use in trying to explain things:

http://web.mit.edu/8.02/www/Spring02.../lecsup4-1.pdf

Lewin's supplement notes pertaining to the lecture he gave in the video:

http://web.mit.edu/8.02/www/Spring02...lecsup3-15.pdf

Lawrence.
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Old 1st Sep 2017, 11:14 am   #6
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Default Re: Kirchoff etc ??

Those two articles are very helpful and explain how two voltmeters connected to the same place in a circuit can show an opposite polarity readings (and sometimes different magnitudes) when they are part of an induction loop where the emf is generated by a changing magnetic field in the loop.

This effect, as noted, is quite different from a battery generating the emf in the loop where its all so intuitive, where both volt meters would give exactly the same reading.

It relates to the fact that the voltmeter wiring and circuit for each voltmeter becomes part of the induction loop. Explaining why they can be connected to the same place, but show two different readings, which seems counter-intuitive and it is not the kind of thing we would expect to see with the typical daily use and application of a volt meter.
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