# Faraday's Law vs Kirchhoff's Law !!

#### mik3

Joined Feb 4, 2008
4,843
Come on guys, none is interested?

#### mik3

Joined Feb 4, 2008
4,843
KL7AJ,

Did you watch the videos?

#### vvkannan

Joined Aug 9, 2008
138
Actually how will the voltmeter read 0.1 v if i just flip it from right side to left side where it reads 0.9v?
we are not going to change any connections and we only change the voltmeter to the other side.
I understand the voltage across the 100 ohm resistor is 0.1v and the voltage across 900 ohm resistor is 0.9v but its difficult to imagine how flipping the voltmeter from right to left will affect its reading

#### studiot

Joined Nov 9, 2007
4,998
Should there be any sound?

#### thingmaker3

Joined May 16, 2005
5,084
There was sound back in November...

#### Wendy

Joined Mar 24, 2008
21,929

#### vvkannan

Joined Aug 9, 2008
138
Sorry i just now saw the second video as i didnt have enough time before as i was in college .

Well ,nothing to say .I have to go and read about non-conservative fields.

#### Ratch

Joined Mar 20, 2007
1,070
mik3,

I played both links and had no problems with sound or picture. Interesting concept, but he went through it faster than I could comprehend it at first glance. Especially with the toilet paper tube camera coverage. If I were in his class, I would be able to refer to all the boards where he set up his equations and diagrams. It helps to refer to previous parts of the presentation instantaneously. As a side note he refers to the resistance formula as "Ohm's law", which it is not, but a misnomer does not detract from what he is presenting.

I hope to study this further, and have found some links which might be interesting.

http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel5/5289/30487/01405925.pdf?arnumber=1405925 Anything from the IEEE you usually have to buy, but the abstract seems to be on topic with what the prof is talking about.

http://www.wepapers.com/Papers/5607/6_-_Non-conservative_Fields_-_Do_Not_Trust_Your_Intuition This appears to be the notes to which the professor is refering.

So to pursue this topic further, one has to search for information on "non-conservative" fields and systems, and pick the reference that matches your level of presentation comprehension and understanding.

Ratch

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#### masa6614

Joined Nov 9, 2008
48
magic!!!!!!!!!!!!!!!

#### thatoneguy

Joined Feb 19, 2009
6,359
Doesn't Kirchoff's law only work when there is a source that one an put nodes/"wires" on? A simplified version of earlier theories?

i.e. KVL/KCL will give you voltages relative one node of the source. The demonstration gives you voltages relative to the component.

#### studiot

Joined Nov 9, 2007
4,998
Got the sound working.

Fascinating demonstration.

Thanks a bundle, Mik.

I does go to show what I am always banging on about - Make sure the conditions of validity apply before you use an equation. This is all too often overlooked.

#### thatoneguy

Joined Feb 19, 2009
6,359
Anybody here have a giant coil and a DSO to repeat it?

#### steveb

Joined Jul 3, 2008
2,436
I does go to show what I am always banging on about - Make sure the conditions of validity apply before you use an equation. This is all too often overlooked.
Agreed. When dealing with a new situation, or when trying to answer a mystery, start with first principles (in this case Maxwell's Equations), and carefully derive the simplifications that are allowable based on the assumptions you are convinced you can make.

This "Faraday Law" effect is sometimes observable in modern switching converters, because of the high flux change. The measured coupled switching noise changes based on the loop you make with the O-scope or spectrum analyzer connections. I'll bet many of us here have noticed that, but maybe didn't care or think about the reason.

Of course, the effect is not that surprising if you know Faraday's Law. What is really surprising is Prof. Lewin's comment that he made that lecture to other electrical engineering and physics PROFESSORS and some of them did not believe the results! It's one thing to not have experienced or thought about a measurement like that, but for them to not understand once it is explained is really surprising.

Another surprising thing was that a large lecture hall full of MIT students didn't stop him and correct the mistake he made with Ohms law. Or, maybe they just figured the mistake was so simple and obvious that it didn't need correcting. I hope it's the latter. You know, it's really discouraging to teachers when they make a simple mistake like that, and nobody notices. If students don't see simple mistakes, what chance is there that they are comprehending the new and difficult concepts. Yeah, I know - it's 8:00 AM in the morning and they were out partying the night before. But still, this is MIT, not the local community college. These kids are supposed to eat Maxwell's equations for dinner, and crap out the Lorentz transformation before breakfast.

Anyway, thanks mik3 for posting this, somehow I missed it when you first made it available. Walter Lewin is a good teacher and I've started watching the whole lecture series which is an enjoyable review of electromagnetics. I was lucky to have a good electromagnetics professor at my University, but he never did all those fun demonstrations. Now I can spend a few minutes a day pretending I'm a stupid kid again.

#### t_n_k

Joined Mar 6, 2009
5,447
It was great!

I wish some of my teachers had such passion.

A question - don't we deal with the concept in transformer theory? We model a system such as this based on mutual inductances using sources that don't really exist.

The curiosity here is that the load resistors comprise part of the inductive secondary loop.

Interesting to consider a simple wire loop with no resistors included - does one measure a potential difference anywhere around such a loop. How does one avoid getting the measuring device involved in the changing magnetic field and thereby introducing its own induced potentials into the measurement?

It would be nice to see the experimental apparatus close up.

#### studiot

Joined Nov 9, 2007
4,998
I wonder if perhaps Proff Lewin was being a tad disingenuous with this demonstration.

Old fashioned versions of Kirchoff's law state

The sum of the EMF's = The sum of the voltage drops (or potential differences)

They went on to state that this sum is known as the total EMF in a circuit.

Used in this form it appears to me that the law is satisfied.

The total EMF in the circuit is 1 volt (the EMF induced by the coil) and the total voltage drop is 0.9 + 0.1 volt.

I believe it used to be phrased in this way to allow for just such a situation.

It also brings out the difference between EMF (which is distributed around the circuit in this case) and Potential Difference.

This shows there are cases where you can't just work your way around a loop, counting batteries as positive and resistors as negative and sum to zero.

#### t_n_k

Joined Mar 6, 2009
5,447
I like the way you think studiot!

It's just that the Prof. makes it so tantalizingly annoying - and he obviously drew in a lot of challenges from his peers.

It's great to have fellows like him challenge my pre-conceptions. Regrettably, I suspect I would have been one of those guys getting upset with him.

It isn't easy admitting I could be wrong. Fortunately, I'm human and prone to failure - which hopefully keeps me sane.

#### steveb

Joined Jul 3, 2008
2,436
I wonder if perhaps Proff Lewin was being a tad disingenuous with this demonstration.

Old fashioned versions of Kirchoff's law state

The sum of the EMF's = The sum of the voltage drops (or potential differences)

They went on to state that this sum is known as the total EMF in a circuit.

Used in this form it appears to me that the law is satisfied.

The total EMF in the circuit is 1 volt (the EMF induced by the coil) and the total voltage drop is 0.9 + 0.1 volt.

I believe it used to be phrased in this way to allow for just such a situation.

It also brings out the difference between EMF (which is distributed around the circuit in this case) and Potential Difference.

This shows there are cases where you can't just work your way around a loop, counting batteries as positive and resistors as negative and sum to zero.
Personally I don't think he was being disingenuous at all. Perhaps he is a bit melodramatic, but I like his theatrical approach since it is a very effective teaching method.

It's clear that Kirckoff's Law is intended for circuit equations and we can only apply them to lumped circuit elements. In the experimental setup he is showing, there are two possible voltages one could assign to the upper node (point B). That is, 0V or 1 V. You just can't solve circuit equations like this - it's a field problem. There is definitely a 1V EMF generated, but where do you put it in an equivalent circuit? Any equivalent circuit you make for one physical arrangement, will no longer work if the physical arrangement changes, even if the electrical connections are identical. (for example moving the position of the voltmeter).

He is basically playing a magic trick on us. He sets up a field problem and expects us to be surprised that circuit equations do not work. If you only know circuit equations, you are surprised and can't explain the result. If you know field theory but don't use it often, you are surprised at first, but will understand once he explains. If you do field theory all the time, you just say, "yeah, so what?".

The situation really looks magical (or frustrating, depending on your nature) if you are not aware of a time varying magnetic field cutting through your circuit. Your voltage measurments change based on where you move the meter. Kirchoff's law doesn't tell you how to deal with that. If it did, there would be no difference between Faraday's Law and Kirchoff's Law.

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#### studiot

Joined Nov 9, 2007
4,998
It's clear that Kirckoff's Law is intended for circuit equations and we can only apply them to lumped circuit elements. In the experimental setup he is showing, there are two possible voltages one could assign to the upper node (point B). That is, 0V or 1 V. You just can't solve circuit equations like this - it's a field problem. There is definitely a 1V EMF generated, but where do you put it in an equivalent circuit? Any equivalent circuit you make for one physical arrangement, will no longer work if the physical arrangement changes, even if the electrical connections are identical. (for example moving the position of the voltmeter).
Why?

All Prof Lewin has demonstrated is that an EMF and a Potential Difference are not two names for the same thing. They are in fact different animals.

The give away clue is in his statement about conservative and non conservative fields.
For PD the line intergral $$\oint$$E.dl is zero around the loop.
For EMF it is not.

Another way to look at it is that an EMF is capable of introducing energy into the system, but PD is not.

A third way to look at it is to note that PD's result from the solution of Laplaces equation, EMF's result from the solution of Poissons equation, where there is a forcing function.

In all this I am in total agreement with Prof Lewin.

However, if correctly stated, Kirchoffs Law can be successfully applied as I showed. Here I must differ from the good Prof.

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