Latest Dr. Lewin's Video

I can TOTALLY believe it. We have Flat Earthers, conspiracy theorists and generally a bunch of non conformists.

Also Electro Motive Force. Depends on the usage. I know - confusing to have two things with the same acronym in the same field.

I cant believe people attacked other academics about this. I listened to both sides of the story and I didn't get an urge to go spam insults at some one else. My urge was to go and investigate myself from both approaches to see what I could work out on my own and that's exactly the beauty of two differing opinions. It prompted me to investigate and not from written text or quotes from professional papers. Its the genesis for becoming aware and understanding on your own then developing your own theory or eventually agreeing with one party whilst potentially giving you insight into other probabilities.

emf is different then voltage and EMF is.. electro magnetic field

Yes, you can work around non-conservative electric fields. One way to do that is to define voltage only in terms of the conservative E field.

I would like o ask, that can not we say, ok external field is not conservative, but the main systems field yes, and just add the effects of the external field with plus elements into the main model? e.g. a transformer in our case. Than everybody is true. (Sorry for my english, I speak other 2 languages better)

Original Kircchoff defined his loop law based on the Ohm-laws, KVL is a generalization of them. So the original aspect is the usage of elementar containors of physical phenomens e.g. resistors, inductors etc. So we have to make an accurate model, and KVL will hold for it. It can contains unending sums, we can handle it. It is true, that Kirchhoff also said, that this law can be determined from the Maxwell equations, but he used the elementar concept. So if you want to solve a problem you can use an accurate model with KVL, or the more theoretic way of Faradays law, they don't contradict to each other, they just use an other concept, an other point of view.

Hi! Thanks for responding. I wouldn't say the theory leads to two different quantities at the same point in time. What I would say is that it leads to a different quantity for two different paths at the same point in time. There's nothing physically inconsistent about that. Other than that, what you said is fine.

Hi Mehdi, thanks for responding! I did type many comments (which might have cluttered your email), but I deleted them since Patreon was being a bit weird and hiding them and stuff. So I typed up a pretty looking pdf in LaTeX instead. Hopefully you can have a read and point out any typos/mistakes I might have made . Feel free to discuss more if I didn't make something clear. I'm a Australian student who is currently studying EE at uni and I've done some physics courses also, as well as studying electrodynamics in my own time. Hopefully the little knowledge I've gained from that can help clear this up! You can find the pdf here <a href="https://drive.google.com/open?id=1s36Y3_NoiAqNc1K05iisaTzRAg96NJwY" rel="nofollow noopener" target="_blank">https://drive.google.com/open?id=1s36Y3_NoiAqNc1K05iisaTzRAg96NJwY</a>

Hi Henry Harvey and Malthe Høj-Sunesen! Thanks a lot for your detailed explanations! For me, it is not so important if a theory or definition is old, new, original or changed. It can happen, that theories are wrong, or that we need different theories for different situations. And I don't want to argue about names and conventions, that's useless. For me it is important that we don't put the theory above the physics! So if a therory leads to two differnt quantities at the same point at the same time, this can be explained, like you did above. But it should not be turned around by saying that this is physically real, like Dr. Lewis did. However, I think I can put what you said into the following way: "Yes, voltage is defined as a physical quantity (which can only have one value at one point of time), but we cannot use this quantity directly to move any charge. To use it, we must find a conductive path between the two points, and the way of how this is done changes the voltage at an ohmic load or the voltmeter that we want to connect. So even if we know, that the voltage between two points has only one quantity, it does not help because we can use only a modified quantity and that depends on the path." Now here comes the hard part: "So if we can see only the effect of the modified voltage, we can just define this as the voltage between the two points. And this fit's to the traditional way of calculating voltage by integrating the electric field." Is this correct? I don't like this way of saying because it ignores the physical situation at the two points, e.g. if we must calculate the required isolation strength between two points in a real system. At least it always needs some extra explanation how to look at this.

You could argue that you never actually had to change how voltage was defined. Your average physics textbook defines the voltage difference in a way that makes Lewin correct. Depending on how you define it, the potential between two points could either be path-dependent or just undefined when there are changing magnetic fields. (I suppose you could define it to ignore induced fields, in which case KVL does hold.)

Dr. Lewin's conclusion was that the voltage in a loop is not defined, and that's what I find untrue.

ElectroBOOM [Mehdi Sadaghdar]

Hi Henry, isn't the definition of KVL "due to law of conservation of energy, the sum of voltages in a loop must be zero"? I never heard or used KVL as the loop integral of E.dl, although that can be used in purely electric circuit.

ElectroBOOM [Mehdi Sadaghdar]

Emf and voltage is not exactly the same thing. Moreover, Romers experimental setup is meant to disturb the readings. The two voltmeters do measure different values, but that is only because they form different circuits.

Malthe Høj-Sunesen

Hi Henry Harvey and Malthe Høj-Sunesen! I think the discrepance lies deeper. Of course every discrepancy could be dissolved just by changing definitions, but changing the definition of voltage is just not acceptable. Dr. Lewis should not tell his students, that between two points there can be different voltages at the same time. This is simply wrong. Voltage (Dr. Lewis calls it electromagnetic force, which is very visual) can be compared to a force in the mechanics, e.g. the force in a loaded spring. It is the force that can drive a current through a resistor. This is nothing very abstract. How can he say that there can be different voltages at one point of time? A definition of voltage, that does allow this, is against every general understanding of what voltage is and should not be used. In fact Dr. Lewis also has no reason to use such a definition: The rising magnetic field creates an electric field. This moves electrons in the wire which compensates the field inside the conductor. But it has created a voltage that cannot be ignored.

Here's another way to understand this. Voltage is commonly defined as V = W/q - i.e., the work required to move a charge from point A to point B. Normally, this does not depend on the path taken, so V is path-independent. A result of path independence is that V around any closed loop is 0 (i.e., what KVL claims). However, if you have a time varying magnetic field, it will induce a rotational electric field around a loop. If you follow the loop around one way, in the direction of the electric field, it will less work than to follow it the other way, against the electric field. This is means that in the presence of a changing magnetic field, V depends on the path taken. This means that KVL is not true since you can go from A to B via one path, and then go back from B to A via another path, and they won't cancel out to zero. This is how a physicist understands KVL.

That's the whole point of Lewin's experiment though! Lewin was trying to show the path dependence of the electric field in the presence of time-varying fields. The leads are *supposed to be* part of the experiment, because that's how he measures the electric field around a path. If KVL holds (depending on how you interpret it), the electric field should always sum to zero around a closed loop. However this isn't true in general, as Lewin showed. This is why Lewin is so arrogant, since he's technically correct given the strict physicists' interpretation of KVL. What is sad is that he can't try to explain it and dissolve the misunderstanding and instead has to berate people for not being smart enough when really he's not smart enough to see *why* smart people are disagreeing with him.

Hi Mehdi, as others have said: you seem to misunderstand the experiment. I agree with you on what you show; but the experiment is really all about how the voltmeters changes everything. As engineers we are well-versed in how our probing changes the circuit so we are always on the lookout for the pitfall of affecting the measurements, substituting voltmeters in our heads with non-interfering devices when looking at eg. Romers experimental description. But Romer described how the presence of voltmeters (both of them! At the same time!) changes the circuit - as we all would expect, masters and Masters alike, physics and engineers. It just goes against our training to do that experiment as we know it would be "bad probing". I can't fault you for this though. Being a third party to a discussion makes it easier to understand the misunderstandings.

Malthe Høj-Sunesen

Hi Mehdi, Lewin wasn't doing any bad probing. The magnetic interference his probes picked up is exactly what he was trying to show - the path dependence of the electric field caused by time varying magnetic fields. Whether or not KVL holds depends on how you define it. If you define KVL as "the path integral of the electric field around any closed loop is zero" which is the "physicists' definition", then KVL does not hold when there are any time varying electromagnetic fields - a direct consequence of Maxwell's equations themselves. This is what Lewin means and is exactly what he showed. The way you, and engineers, approach KVL is with the lumped element model representation of circuits as discrete idealized components. In this case, you can sometimes make the model accurate by adding in parasitic components to model the electromagnetic coupling caused by Maxwell's equations. However, this is not always the case. Sometimes even adding discrete parasitic elements is not enough, like in a long transmission line, where you can no longer use a finite number of discrete components to accurately model their behaviour. So I guess you can try to "make" KVL hold by putting in imaginary components, and this works fine for engineers 99% of the time. Whether KVL holds depends on whether you interpret voltage as the integral of the electric field around a physical loop in space (in which case it doesn't hold), or some algebraic sum around an abstract circuit model. I think this is the result of different perspectives. If a nuclear bomb detonated nearby, throwing off EM waves and inducing a voltage in your circuit, the physicist would explain it by invoking the Faraday's law of induction, whereas the engineer might explain it by saying that the atomic bomb is the primary coil of a transformer! I think the reason for the misunderstanding is that engineers always try to get rid of magnetic interference by habit. But in this case, the emfs caused by the magnetic fields was the whole point of the experiment! Thanks. Appreciate the videos.

I'm so tired of people being blamed for what their fans do. ElectroBOOM never called for any "army" to attack Prof. Lewin. If 200 emails were sent from some number of people cursing/cussing, they are jerks and should be blocked. How can any voltmeter make different readings unless the leads are part of the circuit? Two identical voltmeters is indistinguishable from one voltmeter in two positions. So how can one voltmeter read two different voltages under identical conditions except for the position of the leads, unless the difference is caused by unwanted interaction of the leads? That makes it an error in conditions of the test. By dressing the leads to run along the wires of the loop, Prof. Lewin made the test leads part of the circuit. The voltages measured will then depend on direction and length.

Steven J Greenfield

Think of it as a shorted turn in a transformer. There is no voltage in the wire, the electric fields causing current flow are external. Also, you are considering resistance as if it were a thing separate from the driving source.

Steven J Greenfield

I don't really care that much, get back to making videos of you shocking yourself :P

Hmm... "Lectures by Walter Lewin. They will make you ♥ Physics. 20 hours ago Thanks Meike for your kind words. Many months ago I responded to ElctroBOOM's msgs many times and I pointed out to him that in the case of an induced emf the potentials in a circuit is not defined. I sent him websites which confirmed that. Keep in mind that this is first year college stuff and I told him that I was not going to debate this with him much further. Already in 2004 when my famous demo of Lect 16 (8.02) was on OCW did I decide to not debate this with people who had a preconceived high school believe that 2 identical voltmeters connected to the same 2 points in a circuit can NOT read VERY DIFFERENT values. He insisted on my email address to continue the debate. I told him to contact Prof John Belcher at MIT and try to debate it with him. He never did. Instead he made an anti Lewin video and turned his entire army of 1.9 million subscribers (most of whom are unconditionally loyal to him) against me. I posted 2 carefully thought through videos in which I taught correct physics in a way that most high school students would be able to follow. In response his army sent me more 200 msg with four letter words (ofcoz I blocked those msgs). This morning I decided to apologize for my blunt videos. Of course none of what I discussed in the videos was wrong, but they were blunt. My apology video has more or less paralyzed and calmed down his army and that is progress."

Let's just look at Romer's experiment for a sec. It seems that there is a miscommunication here. An electrical engineer will always try to make their probe wires (the wires from the Volt meters) NOT influence the circuit they are trying to measure. But in Romer's experiment, the wires going to and from the Volt meters are considered part of the circuit no? Lewin talks about it as if they. If a theoretical volt meter without wires was used, this would not occur. And so if you look at just the inner loop, and your goal is to "probe" the voltage on it between 2 points, then inductance on your probe leads, or considerable current going through them would be an undesirable effect and be considered "bad probing", The ideal probe would have no leads at all. But if you consider the entire circuit, as Romer drew it, than the explanation is clear as day, according to faraday's law. Is this correct?

Roel Binnendijk

What was the reason you did not use two sets of probes? Once from a 2channel oscilloscope and once from two separate ones.

Rav

no where. I don't think we are quite settled yet in our argument!

ElectroBOOM [Mehdi Sadaghdar]

oh right! maybe daunting is a better word!

ElectroBOOM [Mehdi Sadaghdar]

Seems like a new path... Now which throat are you going for next ?

How many people watched that videos, and didn't question this? If this were the stock market, you'd be rich.

When you think about it Dr. Lewin does make a point. If you just take a single loop of wire and induce a current in it there will be a voltage on any section of that wire and if you add the voltage of all those sections together (using wire's resistance and using Ohm's law). Now all those voltages are in the same direction of the loop and if you add them all up you won't get 0 which means that either KVL doesn't always stand or Ohm's law is flawed.

One of the lessons learned from years of electronics troubleshooting and repair is that where you place your test leads can make a HUGE difference in the readings and what they mean. Ever chased a strange spike around, only to find out it was a poor choice of 'scope probe ground? There is a reason that 'scope probes come with a little coil spring - it is to make a really short ground connection for measurements in high frequency circuits. And you don't have to go very high for this to become important.

Steven J Greenfield

What I don't get: When I learned KVL, AC impedance and AC sources, and therefore instantaneous voltages, were included. When did this change?

Steven J Greenfield

I was going to correct you and say the word you want is "daunting", not "taunting", but then... the internet =is= taunting!

Steven J Greenfield

It's great that a professor explain something as fundamental as this, but he should have chosen a setup that was a little less disrespectful to the decades of evolution that went in to practical EE, where kvl alone has dealt with much more complicated setups, and works like a charm, and gave us all the nice gadgets of modern life.

Dr. Lewin defines Kirchhoff's law in a way that does not include the voltages that are induced by inductive coupling. This is clear already at the begining of the first of his new videos and of course he knows about this point. He neglegts the resistant of the coil (that's OK) and then says, that with no R there is no field and thus no voltage, if the regarded path is going through the metal of the wire. He ignores that the magnetism (when changing) is moving electrons and creates a field even inside an ideal wire. However, he does this only when he uses Kirchhoff's law, but not if he discusses absolute correctly the effect that the coil has in this circuit. The same happens again with the probe wires in the second video. That's a bit weird, I don't understand why he regards Kirchhoff's law in this way. But at the end it's more a discussion of how the law should be defined, and that's useless. Except of this point the understanding of physics is exactly the same on both sides, as far as I can see.

No need to shield them you just need to use the 3rd dimension. Put the voltmeters in front of and behind the solenoid. If they're ideal voltmeters, their location in space shouldn't matter. But the whole thing depends on the fact that it DOES matter, which is nonsensical.

Felix

Mehdi, is it any way to shield probe wires to not be affected by EMF and still works in Dr. Lewin's configuration?

You win Mehdi

Happy you got a much deserved apology. Have a good day :)

There are two voltage sources in parallel. That alone is a no-go regardless what everything else is. Period.

Holger P Kleinert

Faith um humanity restored 😊

Steven da Silva