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atyy
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stevenb said:You really think MIT people can miss a factor of ten in this way?
I do. (It's only MIT, not Caltech.) But they didn't in this case.
stevenb said:You really think MIT people can miss a factor of ten in this way?
atyy said:I do. (It's only MIT, not Caltech.) But they didn't in this case.
sarumonkee said:@ stevenb: look at the video part 2 at around 5:23. Freeze the video and look at his setup. The probes are NOT "effectively" connected to the same point. There is about 4-6" of wire on the secondary loop with the resistors on it. That means inductance to me. Your claim of moving the scope to the other side befuddles me, because the ENTIRE point of prof. Lewins lecture is not about the how to measure this system, but that across each resistor, a different voltage occurs, which I agree with.
Think of it this way: If 1mA amp is induced in the system, OF COURSE (as he puts it) there will be a -0.1V drop on the 100 Ω res, and 0.9V drop on the 900 Ω res. Now, if we simplify the system by saying "node A" is actually "effectively" the same point, we know have 1 V drop across "node D", a supposedly 0 Ω line (by Lewin's model). That means Lewin also denies that Ohm's Law applies in this situation, because that would mean infinite energy.
Ohm's law is hard to dispute, because this is more of a definition than a theory. Therefore, I contend we have a breakdown of his model.
Conversely, since you (stevenb) seem to be very sure of your answer (I am not 100% sure of mine, that's why I asked the question, and would like to learn), can you tell me how the energy got into the system from the main coil to his resistor circuit? Does it not involve inductance? I don't know of any case where you can couple energy like this without some kind of coupled inductor setup.
stevenb said:I'm not sure why you would say that I and the Professor have missed the "transformer effect". This effect is the source of the EMF that drives current in the loop. The main difference between this situation and a real transformer is that one does not usually put two large resistors in the loop winding of a transformer. Certainly the transformers you took measurements on were not built like this. But, this is a side issue.
The professor goes through a process of setting up the problem. First he describes the case with a 1 V battery in the loop, and then he erases the battery cell and uses changing flux in the center of the loop to replace the 1 V EMF. How can you say "he just miss the voltage source of the transformer"? He didn't miss it at all.
Faraday's Law in integral form just tells you that the 1V EMF exists somewhere in the loop. It doesn't specify where it is in the loop. Typically, a transformer is tapped to change the number of loops is the circuit, not to somehow tap a section of one loop. The details of what happens when tapping one loop need to be considered more carefully, as has been done in the provided analysis. In this case we know where the potential drops are (we can measure them with a meter that does not encircle the flux change) and we see that it adds up to 1V around the main loop. There is very little potential drop across the wires themselves. The transformer EMF in the main loop is 1V, so all is well with Faraday's Law. Also, all is well with the classical definition of KVL (given by Maxwell). Obviously, the version KVL that says the sum of potential drops is zero is violated, which makes the professor jump up and down and denounce his physics books for spouting bad physics.
In doing the proper measurement for potential on each resistor, you trace a path (through the meter and the resistor) that does not encircle the flux change and this tells you which resistor potential you are actually measuring. FL and KVL (both versions of KVL, mind you) work here. The Professor also reveals that if you consider the path through the other resistor, you see an apparent contradiction. You end up tracing a loop through the other resistor that encircles the flux change and you are not really measuring the potential on that resistor. Faraday's Law still works through that other path, and the classical definition of KVL also works, but the other definition of KVL fails yet again.
Since the professor is not actually acknowledging the classical definition of KVL, we can just ignore that aspect, and conclude that everything he is saying is correct. The results do not depend (other than small parasitic changes) on where you tap the node along the wire, but they do depend on how you route the leads of the meter. If we had the experiment in front of us, there would be a very simple way to prove this. Simply move the exact point where you tie into the nodes and see if the measurements change. It is my contention that they will not change very significantly. Anyone who doubts this should just do the experiment and convince themselves. Do you really think the people who set up this experiment would go through all this trouble, and then not verify this straightforward thing? You really think MIT people can miss a factor of ten in this way? That's hardly proof, but experiments are proof. I've done similar experiments in the past as part of my work. I can't ask anyone to accept my word, and if others don't want to accept the professor's capability, then just do the experiment.
stevenb said:Good one!
Of course appeal to authority is not a good argument, but it's important to look at qualifications and evidence of due dilligence, as part of a debate like this. There seems to be enough horse power behind the experiment, and analysis to suggest that claims of fraud should be backed by a detailed analysis and/or a documented experiment, rather than vague hand-waving type arguments about parasitic inductance and transformer tapping.
yungman said:I think we are talking in circles.
yungman said:You mean you don't accept the idea of generating voltage of wire of less than 1" or 2" long forming a one turn loop? And you don't accept a 4" wire with two resistors connect at two ends is a loop? You think the 4" wire connecting the resistors to form the loop is insignificant? You really want me to go through the trouble to type out a simple transformer equation here?
If you ever design transformer for switching power supply, you will have no difficulty understanding what I have been talking. As switching frequency goes up, efficiency goes up, less turn is needed. That is the reason why the switching power supply is so so much smaller because less turn is needed, size of the core can be drastically reduced because core is lot more efficient at higher frequency. For a working engineer, I don't think it is hard to even get the idea of this. This is really simple!
I pretty much tell you how to reproduce the experiment in the other post, just wind the wire onto a Big ball point pen and you can do the experiment. Just be careful and wear rubber groves to avoid shock because when you open the switch, the coil can momentary generate very high voltage...Like ignition coil.
I can tell you, I designed high speed pulsing circuits with transformer driving MOSFETS. Because the design is 5KV switching, I had to stack 8 MOSFET in series to take the voltage. The driving circuit of each MOSFET has to be able to float. The transformer is the best approach. I actually design the transformer onto the PC board as trace. I only used 3 turns on the secondary to generate 15V to drive the gate of the MOSFET. 3 turns for 15V! The whole length of the secondary is less than 3". I hope you stop and think a little on this, this is real products been produced in the 90s. I am not a switching supply engineer, I was the manager and I came up with all the ideas on low turns, fast switching DC to DC converters that made our products exceptional at the time. I had my engineer did the detail calculation to get the turn number but the idea absolutely sound and was implemented on successful products.
sarumonkee said:@Claude: I totally agree with your example. Thank you for agreeing with Yungman and myself. A standard model of the secondary of a transformer with sufficient frequencies being put through it (not too high, not too low) is a voltage source (or current source depending on your take on things). I am contending that KVL still applies, because the secondary of the transformer still exists, and is a voltage source that Prof Lewin left out, thus making his example suspect.
Also, @stevenb. If orientation of your scope probes matters that much, what happens if the oscilloscope is directly above the table? Is there some voltage between 0.9 and -0.1V? Or does it just suddenly change between the two when you cross some threshold degree? I just don't see it yet. I am not being facetious, I really do want to understand what you are saying. If physical orientation of a voltage probe matters, why isn't it taught in school? Physical placement is important, and also taught in school.
Also, everyone who keeps harping on the fact that the Prof got the factor about 10 (actually exactly 9) correct, I never said he wouldn't. A resistor that is 1/9 the size of another resistor with the same current WILL have 1/9 the voltage drop on it. That is not impressive.
What IS impressive is calling a node a node even if it has supposedly two different voltages on it. I thought that the definition of a node is that it has one voltage on it.
sarumonkee said:Also, @stevenb. If orientation of your scope probes matters that much, what happens if the oscilloscope is directly above the table? Is there some voltage between 0.9 and -0.1V? Or does it just suddenly change between the two when you cross some threshold degree? I just don't see it yet. I am not being facetious, I really do want to understand what you are saying. If physical orientation of a voltage probe matters, why isn't it taught in school? Physical placement is important, and also taught in school.
sarumonkee said:What IS impressive is calling a node a node even if it has supposedly two different voltages on it. I thought that the definition of a node is that it has one voltage on it.
stevenb said:I'm not taking your questions as facetious at all. I can see you are trying to come to grips with this difficult concept. The problem is that it hard for people to learn when they start off with lack of trust. You don't trust that the Prof is trying to teach you and is qualified to teach you. So, the learning process is going to be slower than it needs to be. Although your question to me is genuine, it also implies your lack of trust that I might know what I'm talking about too. Can't say I blame you since you don't know me at all, but it places me at a serious disadvantage in trying to help you. On top of that, this format is not terrible conducive to getting ones thought across clearly. As an example, I never said the orientation of the probes matter that much. What I said was that the path formed by the leads is the critical thing. So, I'm not hopeful that I can be of great help, and I think I've passed the frustration threshold for this thread in general.
The simplest answer I can give is that full understanding of Faraday's Law removes all mysteries here. Study FL thoroughly and when you feel that FL is telling you something that you just don't want to believe, then figure out how to do an experiment (yourself, since you don't trust others, no matter what their qualifications - not always a bad thing, by the way) to convince yourself of its truth. Your question about what happens if the meter is placed above the circuit is outstanding, and convinces me that you will understand this soon. Please explore the answer using FL. It's not difficult to answer, but the answer will help you come to grips with the concepts here.
sarumonkee said:So after all that lead in, you don't answer my question about the scope above the table :)? If you know the answer, please tell me... Or at least hint at the direction I should look (but I think just hinting will make it look like you don't know the answer).
I still don't get what you mean by path. If a node is a node, the path to it, and through it are transparent. The two probes have the same path if connected to the same nodes. I just don't think these are actually nodes in this case.
Thanks.
sarumonkee said:So after all that lead in, you don't answer my question about the scope above the table :)? If you know the answer, please tell me... Or at least hint at the direction I should look (but I think just hinting will make it look like you don't know the answer).
I still don't get what you mean by path. If a node is a node, the path to it, and through it are transparent. The two probes have the same path if connected to the same nodes. I just don't think these are actually nodes in this case.
Thanks.
atyy said:Any time a scalar potential is used to derive all the physics, it is being assumed that the electric field is conservative.
In situations where the electric field is not static, a scalar potential may still be useful if only approximate. This is the quasistatic approximation.
KVL uses a scalar potential, and is closely related to conservative fields.
atyy said:Hint:
integral(E.dl)~d(B.A)/dt
A is a vector perpendicular to the area in question.
There is a dot product, so there will be a cosine of some angle.
That angle is related to the angle in your question.
sarumonkee said:So you are saying pointing the "loop" of the voltage probe at a certain angle, I will get 0 Volts?
I actually rigged up this experiment, and sufficiently convinced myself that KVL is holding... I held the probes above, below, across, etc, and got the same numbers time and time again. I am seeing the inductance that yungman has been talking about, and a voltage drop across the wire, which definitely should not be a node. I think Lewin should have had at least a coupled inductor in his model (or two the way his experiment was setup).
atyy said:How did you ensure that the changing B field is confined to the central loop?
Also, what are the parameters of your setup?
atyy said:Cool! So in the part where you got different readings of 9:1, did you connect the voltmeters to exactly the same two points?
sarumonkee said:I made my setup similar to what I think he did, except I made one connection from the 100Ω to 900Ω resistors very short. I then put ground of both probes between the two resistors on the short connection. The other sides of the resistors were connected with a 6" wire. I probed the resistors close to the actual resistor on the opposite side from the short "node".
The primary coil was about 40 turns of something like 14-16AWG wire, around a huge core I had laying around, with an effective core cross sectional area of probably just under 2 square inches. I then introduced a 10 V step onto the primary, while having the secondary (the resistors and wire) around the core, like Lewin's setup is presumably from 5:33 on part 2.
I observed a factor of about 1:9 as expected in the two voltages, since this was the ratio of the resistances. Now, the fun part. I connected the grounds of the probes to half way between the long wire, about 3" from both resistors, leaving the probe ends in the same location. Since this is a "node" in Lewin's analysis, I should not see any voltage across it if I make another step function on the primary.
Well, I introduced my step, and both probes read about the same magnitude (one was negative from the other, since it points the other way), and the sum of the two magnitudes (had to invert one because I wasn't using differential probes) equaled the sum of the previous points in standard KVL style, all adding to 0 if you do the loop. I was measuring a voltage across the 6" wire in two 3" segments.
I also held the probes above, across, and in many different orientations, and it still produced the same results. I plan on taking some pictures and maybe making a video this weekend if I have time.
Let me know if you have any measurements you would like me to make, or if you think my setup or analysis is flawed somehow... I'm here to learn.
sarumonkee said:No, I just realized I need to show that one as well... I will next time I'm back in that lab. The time I got 1:9 ratio is when I had the probe leads as close as possible to the resistors, and the grounds on the short side (1 cm wire connection). The probe leads were separated by the 6" wire on one side direction around the loop, and the two resistors on the other.
I never actually connected the probes to the exact same point (probe on probe, and ground on ground). I will try to remember to do that next time, and spread the probes to opposite sides of the loop like I bet you will ask for.
I hope that paints the picture for you.
Studiot said:Hikaru, thank you for being the only respondant to have the courage to post and answer to my question.
So did Farady state his law in 'Integral Form'
I think that what is happening here is that a non original form of Faraday's Law is being compared with a non original form of KVL. By non original I mean extended in the light of more modern knowledge.
sarumonkee said:Are you referring to a current measurement? The point of Prof. Lewin's experiment was not about voltmeter measuring errors, it was to supposedly show measuring across different points of two "nodes" can give you different voltages. I don't think they were actually "nodes" as inductance was not included.
yungman said:Claude, I am glad you use your example and we can build on this. If you remember the professor original drawing of replacing the 1V battery with a short circuit and then sent a magnetic field to get 0.1V on the 100ohm and 0.9V on the 900ohm.
Use your example. We use 110ohm in series with 9ohm across the secondary. Then we pulse the primary to get 9V on the 9ohm and 110V on the 110ohm. As in the professor case, let A be the junction between two resistors which in this case has 0 length. Where is point D?
He obvious know that the setup he had, by replacing the 1V battery with a short wire result in a loop formed by the wire and the resistors. Then he said he measure 0.1V across 100ohm and 0.9V across 900 ohm. Then he claimed he measure say clockwise from one point is 0.1V and counter clockwise as 0.9ohm! So where is the transformer that gave him the induced voltage in the picture?
Back to your example, the wire is the secondary of your transformer, point D is not a point, it consist of the winding (wire) of your secondary winding.
Is there any way for me to post a simple drawing of just two resistors and the secondary of the transformer without have to using a PDF and then attach, then have to wait for a day for other people to see it? Let me try this.
Let us redo the presentation again:
1) Let's arrange the components in counter clockwise.
2) Start with the 900ohm, then 100ohm and call the point between the two resistor is point A. I call the open end of the 900ohm resistor point C.
3) Then we connect one end of 4" wire to the 100ohm resistor and call this junction B. Remember I am still going counter clockwise now.
4) Then the other side of the 4" wire connect back to the open end of the 900ohm resistor which I call point C in step 2.
With this, we form a closed loop starting from point A between the two resistor, travel counter clockwise through 100 ohm resistor, to point B to the wire, through the wire to point C that connect to the other side of the 900ohm resistor.
Then you inject a pulse of magnetic field, you measure 0.1V across the 100 ohm resistor, and 0.9V across the resistor. Just like the good professor did.
Lets call point C is +ve and call point B as -ve. Let's travel from point B counter clockwise through point C to point A, you get 1V-0.9V=0.1V. If you travel from point B clockwise this time to point A, you get 0.1V! Why are they the same now? Because the transformer effect, this time I include the transformer in the picture and voltage come together! IN this case, KVL works beautifully.
cabraham said:Please provide a diagram. My xfmr example demonstrated that KVL does not always work. We are not in agreement here.
Claude
“In any complete circuit formed by the conductors the sum of the electromotive forces taken around the circuit is equal to the sum of the currents in each conductor multiplied by the resistance of that conductor.”
“Whenever a conductor cuts magnetic lines of force an EMF is generated. This EMF is proportional to the time rate at which the lines are cut.”
Studiot said:Further differences are that Faraday does not require a closed loop, although he does not prohibit one either.
sarumonkee said:In my studies, the secondary resistance is modeled as a series resistance with the secondary inductance. It is the inductance that allows the energy transfer in a transformer, and is modeled as a voltage source when fed from the primary. I don't see a KVL error here.
Source: Fundamentals of Power Electronics 2nd ed., Erickson/Maksimovic
Studiot said:You can apply Faraday's law to an (infinite) straight conductor moving in an infinite parallel magnetic field.