There are two separate issues we should consider here.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.
....
However, if correctly stated, Kirchoffs Law can be successfully applied as I showed. Here I must differ from the good Prof.
I already answered that indirectly above. If the inductor is a lumped inductor and there is no magnetic flux change cutting the circuit, then I use your modified version of KVL. This is acutally Faraday's Law (applied to a circuit) and not KVL, but I just think of it as KVL when doing circuit equations.So whose equation would you use if you had both the battery and the inductor in circuit?
Yes, one resistor would demonstrate the effect too. Without a battery, one meter would read zero volts and the other would not. But, this does not deny ohms law. The meter that reads zero volts on the resistor makes a loop around the time varying magnetic field; hence, the EMF from the changing field exactly cancels the voltage on the resistor. The other meter does not make a loop around the changing field, and so it reads the resistor voltage only. You can also think about it another way making a path through the wire. The meter that reads zero volts does not make a loop around the field but goes through the wire, which drops no voltage. The other meter is reading a voltage across the wire (which is strange) but this is due to the EMF from the fact that it makes a loop around the field change.You don't even need two resistors to get apparent anomalies. What happens if you repeat the experiment with only one resistor. First with the battery in circuit then without?
What is then the 'voltage' across the ends of the resistor? In one direction it is zero the other it is given by ohms law IR. Are we also denying Ohms law?.
I'm not sure what I need to address about your comment about the differential representatons of the Laplace or Poisson equatons. Those equations are consistent with electrostatics only, and are not really relavent here.You haven't addressed my comments about either the differential or integral representation of this situation, or the energy viewpoint. Nor is there an answer to Recca.s question.
Is it not true that when a wire moves relative to a magnetic field (or the field changes relative to the wire) an electric field, E appears in the wire such that.I'm not sure what I need to address about your comment about the differential representatons of the Laplace or Poisson equatons. Those equations are consistent with electrostatics only, and are not really relavent here.
I don't know what to say to a comment like that. I'm not obsessed with voltmeters. But the whole point of that lecture by Prof Lewin is the nonintuitiveness of the experimental results, which are revealed by the voltmeter measurements. The voltmeters are not being tricked. Those are the actual results that demonstrate that Faraday's Law works, and Kirchoff's Law doesn't. A voltmeter measures current through a high resistance placed in parallel. It is a competely valid circuit to consider.Steve I don't understand your obsession with voltmeters.
In what way incorrect?By the way your expression of Faraday's Law is incorrect. You are supposed to take the line integral of the electric field. See the following link.
OK, I see what you are saying, but most people don't use this obscure form, as it gets into issues of frames of reference. Anyway this is taking us off course. Getting back to the issue at hand.In what way incorrect?
The first line of your reference is exactly the same as my second term.
Steve, thank you for an interesting discussion so far, you have made me stop and think before I answer. So please don't take it that I am getting at you in any way, nothing could be further from the truth and I apologise if anything I have said has looked that way.#15 03-17-2009, 02:03 AM
steveb
Senior Member Join Date: Jul 2008
Posts: 328
--------------------------------------------------------------------------------
Quote:
Originally Posted by studiot
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.
I've had a quick look to see if I can get hold of the original paper he published as a student, "Ueber den Durchgang eines elektrischen Stromes durch eine Ebene, insbesonere durch eine kreisförmige", however I cannot get an original copy that I can translate.Now I don't have access to Kirchoff's original statements. Perhaps Dave would oblige?
I look forward to what further discussion comes out of this later.This brings us to Faraday's law. If we regard your circuit as being three turns, with one turn shorted, then we can work. Otherwise F too has problems.
This amount of work deserves some response, surely someone else can also contribute?I'm not sure if anyone will find this interesting,
So using thisKirchoff doesn't care where the EMFs come from (chemical, magnetic, elctromechanical etc) he just equates their total to the pds.
Studiot, thanks for your response. We seem to get the same answers.I suggest Faraday's law won't help here, but Kirchoff will as he simply adds the EMFs , without regard to their origin.
Also, I must stress that I attempted to answer Recca's question, although you may not agree with the answer. Faraday's Law is clearly shown in the PDF document and in the previous web link I provided, and also I described it in words. I would maintain that strickly speaking KVL amounts to the following.It will take some time but I will try to present a fuller analysis and answer Recca's question, since no one else has actually done this.
OK so my terminology is a bit lax and I really meant not directly applicable. Of course Faraday's Law works where it is applicable.Faraday's Law not working
I think I found a good way to shed some light on this. After checking all of my books, each which provided a different interpretation of Kirchoff's Voltage Law (KVL), my thought was that we needed to get a translation of Kirchoff's original work to get his exact statement, but I was unable to do so. I found comments similar to Dave's statements that Kirchoff was talking about potential differences in resistor networks. This may in fact be true, but we need that paper to be sure about what he stated as a useful law/rule/condition.Firstly there are a whole bunch of theorems, laws and equations honouring ( in no particular order) Maxwell, Gauss, Faraday, Kirchoff, Lenz, Lorenz, Laplace, Poisson, Coulomb, Ampere etc.
Many of these are equivalent statements in an alternative system because we can approach the math via differential calculus or integral calculus, vectors /scalars. In some cases some are similar but apply to different circumstances such as conductor / non conductor or conservative /non conservative fields etc.
Further we also have to consider both electric and magnetic effects and their inter-relation.
I was planning to try to rationalise these to show the relationships, including that between Faraday and Kirchoff.
My understanding of Faraday's law is that it is in differential form as we have both stated here. It is more far reaching than Kirchoff's Law as it connects electric and magnetic effects. Kirchoff's Law relates purely to electric effects. However the downside of this is that there must actually be magnetic flux to vary to yield the EMF.
If you make the experiment with a wire loop then you will measure half of the EMF if you place the voltmeter across an arc which is half the circumference of the loop in length.Finally consider Prof Lewins's experiment with no resistors, just a wire loop.
We now have an EMF of 1 volt but zero potential difference in the system.
A pretty dramatic demonstration that EMF and PD are not the same thing, although both are measured in volts.
Thread starter | Similar threads | Forum | Replies | Date |
---|---|---|---|---|
Could a Faraday's flashlight be improved? | General Electronics Chat | 19 | ||
Faraday's law and Lenz's law | Homework Help | 9 | ||
Faraday's Law and Lenz's Law | Homework Help | 4 | ||
M | Faraday's Law is False | General Science, Physics & Math | 17 | |
M | Maxwell's third equation (Faraday's law of induction) | General Electronics Chat | 6 |
by Kate Smith
by Steve Arar