Does Kirchhoff's Law Hold? Disagreeing with a Master

hi making this video makes me nervous this is a serious and complex video about an electronic topic and i have to do it very carefully see it's like you disagree with stephen hawking who's likely wrong you are and now i'm about to disagree with dr walter lewin if you don't know dr lewin he has a phd in physics he's a former professor of physics at the massachusetts institute of technology or mit he has made many discoveries has written tons of books has received many awards for his scientific achievements has educated thousands in schools and millions over the web through his online lectures just check his wikipedia page i on the other hand have a master's degree in electronics have worked for a while as an engineer and then turned youtuber where i constantly get shocked my mom thinks i'm mostly okay dr lewin teaches in one of his courses that the curse of voltage law or kvl doesn't hold true in some cases and i disagree with him directive fire i tried to contact dr lewin to discuss this but understandably he doesn't want to talk about it anymore because so many people online have annoyed him with their stupid reasonings so i humbly make this video hoping that he or some other great professor sees it and tells me what they think about my results if he's right more power to him i couldn't be happier to be wrong and learn something new now what's kirchoff voltage law or kvl i read the definition from wikipedia as my alibi which is the principle of conservation of energy implies that the directed sum of the electrical potential differences or voltage around any closed network is zero this is also my understanding and how i learned it if you find any closed loop in your circuit the sum of the voltages across its components at a given time is zero adding them up in one direction this has never failed me but dr levin says kvl doesn't hold when there is a changing magnetic field involved in a circuit the potential difference between two points in a circuit is no longer defined when we have an induced emf two voltmeters connected to exactly the same two points can measure different values i'll leave links to his relative lectures in the description and i suggest you watch them but let me quickly summarize his reasoning imagine we have a circuit like this a one volt supply in series with 100 ohm and 900 ohms so one volt divided by one kilo ohm total means there is a one milliamp running through the loop or 0.1 volt here and 0.9 volt there if we calculate kvl in the loop in this direction entering positive of r1 we have plus 0.1 volt and entering positive of r2 we have plus 0.9 volts and entering negative of the supply we have -1 volt and the sum of all these is 0. kirchhoff's law holds fine here but let's remove the supply and short it and instead apply a changing magnetic field through the loop coming out of the page we know that a changing magnetic field creates current in a loop and according to the lens law the direction of the current is this way let's say at a moment in time the current reaches 1 milliamp which means we will have 0.1 volt across r1 and 0.9 volts across r2 the sum of the voltages in the loop is not zero so dr lewin concludes so kirchhoff is for the birds and fire day is not then dr levin does an interesting experiment to prove this he measures across the exact same two points in the circuit with two different scopes at the same time and shows that each scope shows a different number three two one zero look on the left there's v one notice it's negative look on the right there's v2 it's about nine times larger than v1 he then concludes that in this special case depending on which way you look at the circuit you read a different voltage so this half of the circuit has a different voltage than the other half which contradicts the kirchoff voltage law and it doesn't hold he then says that it's the faraday's law that always holds not the kirchhoff's law which is just a special case of faraday's law dr lewin says that the two points across the wires are electrically identical i want you to understand and appreciate that a1 and a2 from an electrical point of view are identical they are the same because the resistance of the wire is extremely small compared to the resistors which is true and also the inductance of the wire is extremely small which is also true so the changing current through the wire creates very negligible voltage across the wire if anything i repeated the same test and i agree with his measurements but not his conclusions let me repeat the test for you like dr lewin i have a coil or solenoid and i'll switch a battery through a one ohm resistor over my coil the resistor limits the current to 10 to 12 amps the coil creates a rise of changing magnetic fields and settles and then i have my loop with one kilo ohm resistor on one side and 10 kilo ohm on the other side we can agree the wire resistance and inductance between them is negligible compared to the resistor values so when i apply voltage in my coil the magnetic field in it changes like this and settles the electromotive force or voltage induced in my loop is the rate of change of magnetic flux and so the induced voltage changes like this now i'll measure across these two points in the loop using my sense wire that connects to the scope notice that i've twisted the sense wire having twisted wire is important because say we have magnetic fields coming out of the board the induced current in the first twisted loop is in this direction and in the next loop is also the same direction this means that say in the black wire the current wants to go in this direction in loop one and in this direction in loop two and they cancel each other so using twisted pairs makes sure that the voltage and current in the wires are not affected by external fields connecting the scope doesn't affect the main loop behavior at all because it has a hundred mega ohm input impedance so we can safely say there is no loading effect from connecting the scope to the loop where we measure now the sense wires are on the 10 kilo ohm side ready click see i have a voltage pulse with a peak of around 25 millivolts now all i do is i flip the sense wires to the one kilo ohm side of the loop ready click there we have a negative pulse around one tenth of the first pulse because although the current in the loop is the same the resistance is one tenth what does it mean we are reading two different voltages across the same spot maybe there is voltage across these wires unlike what we thought i moved the sense wires across the loop wire let's measure click huh the scope just triggers a noise there is no voltage across the wires so does it mean that two different voltages from two resistors are applied across the same spot and the kirchhoff flow doesn't hold i'm sorry but this is where dr lewin and i part ways please don't hit me wait we are not back in old iran you can't hit me can you let me show you dr levin's theory is not dependent on the location of the resistors so let's rearrange them here i move the resistors side by side and very close if i measure across these two points i still read two different voltages depending on which side i do the measurement same as before here i have both my resistors on one side and i'm measuring across these two points on the 10 kilo ohm side there we go kelly see we see a pulse around the same size i flip the wire to the one kilo ohm side and kill click see i see the same one-tenth pulse like before so nothing's changed there but instead let's measure right across r1 and r2 and both r1 and r2 with minimal localized sensing loops here's the voltage across the 10k bam see it's around the same 25 millivolts and the voltage across the 1k the same two and a half millivolts and the voltage across both resistors bam see it's actually higher than 25 millivolts pretty close to 27 and a half so you see that the voltage across both r1 and r2 is equal to the voltage across r1 plus the voltage across r2 which is the same voltage across the loop this shows that the voltage across the loop is not zero unlike what we thought but is equal to vr1 plus vr2 the loop is the secondary of a transformer with the primary being my coil dr levin's model misses a transformer in the loop with the voltage across the winding equal to vr1 plus vr2 kvl holds but then dr lewin would say you stupid kid well he wouldn't say that he's a nice guy but he would say if instead of measuring locally like this we measure across the loop on the other side we would read zero volt across the loop which is correct so which one is right well let me show you an extreme case let's assume our resistors are infinite or open and so there is no current running through the loop now if we measure the voltage across this tiny gap on this side we see a voltage pulse here's the loop with no resistors measuring right across the gap and we have a voltage pulse and when we move the sense wires around like this we see zero jump and here i have the same loop with the sense lines running right on top of the loop and we have no voltage jump just some noise of course if i make the sense lines a little bit smaller or larger than the main loop i'll have voltage jumps so if in fact we have a voltage v across this gap for reference say zero volt here and plus v on this side why do we read zero volts across the sense lines let's take halfway through the loop if the voltage here is plus v the voltage here would be half a v so we have half a v across this half of the loop and another half of v across the other half there is no current running through the sense lines so it doesn't matter that they are going to the scope both the sense lines and the loop have the same shape and are in the same field so they behave the same which means we have half a v on this sense line and another half a v on the other sense line so if the voltage of the loop on this terminal is v it is reduced on the sense line by half a v here and on the zero terminal it is increased by half a v here so you see that the voltage difference between the sense lines is zero at this point and the scope shows zero so the entire reason dr lewin was reading two different voltages was due to bad probing no matter where you cut your circuit the voltage across these two points is always a unique number the voltage on this half of the circuit is the same as the other half well this looks a bit complicated but if you look at this half of the circuit the voltage across this half is v1 which is the voltage across this piece of loop wire plus the resistor voltage which is vr1 plus v2 of this piece of wire now if you sense it like this the same voltages of the loop wires are induced on the sense lines which subtract from the voltage across these points and all you read at the scope is vr1 and if you sense on the other side the induced voltages on the sense lines change to match that side and you read vr2 now you might say this is different than my extreme case because there is current running through the loop but according to the lens law this current actually reduces the original field so the induced voltage on the sense lines is less and they end up matching there were never two different voltages across these points this emphasizes the importance of proper probing now i'm not hundred percent sure but if my quick analysis is correct to measure the voltage across say these two points properly the sense lines must come straight together and twist all the way to the scope or meter and keep your scope probe away too because even if this loop is close to your fields voltage is induced in it and will affect your readings my conclusion krish of laws hold and have held nicely for me so far if you see any discrepancy it's because you're either missing a component in your model or probing wrong have i been wrong before well yeah just look at my previous videos am i wrong now i don't think so because science and analysis and experiments support my claim as well as a bunch of textbooks written by smarter people the sad part is that almost all college physics book halliday and resnick john coley ohanian jung and freeman they all when they deal with selfie doctors with solenoids they all say let us now use kiss of lupu that statement is a crime well either way i'm sure the science will win think about this give this some thought this is not easy and have a good weekend h

Channel: ElectroBOOM

Views: 1,766,163

Keywords: educational, electrical, ElectroBOOM, science, electronics, engineering, entertainment, equipment, measurement, experiment, mehdi, mehdi sadaghdar, arc, mishap, physics, Sadaghdar, test, tools, circuit, funny, learn, shock, spark, discharge, lewin, walter lewin, Kirchhoff, Kirchhoff’s law, probing, KVL, KCL, lectures, MIT, faraday’s law

Id: 0TTEFF0D8SA

Length: 13min 21sec (801 seconds)

Published: Mon Nov 05 2018