Kirchhoff’s Voltage Law versus Faraday’s Law: the Conclusion

hi good news dr. Lewin responded to my previous video with three new videos science time but before getting to the science let's address the drama a little so I made a video disagree with dr. Liu in a scientifically well decorated ex professor at MIT the disagreement was on that dr. no-win said reach off voltage law or KVL doesn't hold when magnetic fields are involved the first two video responses dr. Lewin made were explanations as a response to the flood of my viewers commenting under his videos maybe he made those videos without watching my video the rumor has it it's only a rumor the person who says he has a master's degree makes the following statement the entire reason dr. Rubin was willing to give up voltages was due to bad protein such a ridiculous statement I have not been able to find that video and the needle even exists I feel like he's telling me he actually watched my video but to save me the embarrassment I should remove my video anyway dr. Levine didn't find my video I'm using he assumed I don't agree with Faraday's law and I don't agree with his measurements and in the comments he said that my video is dedicated to discredit his lectures and his insulting and an embarrassment considering I have a master's degree it is equivalent to say the earth is flat no not flat earther call me an idiot but but a flat earther it is fine people I don't have any problem with dr. Lewin please don't try to defend me by bothering the man well if you watch my previous video you will see how much I actually agree with dr. Levine I would never try to discredit his lectures for no reason he has around 300 lectures in his channel and I suggest everyone watches them they are great in the process of this back and forth a lot of people left comments with majority of them being pretty respectful to both sides but there are always these oddballs who leave bad comments I'm sure dr. Lewin received his fair share of unnecessary bad comments from my viewers as I did from his fans here is a few of them written for me obviously the balding skin with a single eyebrow does not understand potential theory in nonk it feels electro boom shut up on the topic didn't you take an applied electromagnetics class in your degree as another EE I'm embarrassed for you and when I left a comment under dr. Levine's video somebody said don't worry dude pray to Allah he will solve your problem go to a mosque nearby he knows kvl better than this walter lewin moreover both Khrushchev and Faraday are in heaven so just request Allah to ask them for a solution and my hero our young Kasim said what the hell is wrong with you boy I chuckled both of us chose believe on the internet and should deal with the consequences personally I don't care about bad comments but I do apologize to dr. Lwin for receiving a similar wave that wasn't my intention it's not a political debate and we are not looking for votes it's about science and facts and in science we don't need people who blindly follow somebody we need people who put their preference aside and look at hard facts and judge accordingly in his third video although he still didn't agree with me which is totally fine dr. Lewin did a good job of calming both sides down allowing for a more civilized discussion and I appreciate it that's the end of the drama but don't leave now hold my hand let's go through this journey together it's a long and tough journey but coming out of it we will learn much more let's do it so as dr. Lewin suggested I contacted his ex colleague professor John Winston Belcher a current MIT professor a great man with his own series of scientific awards and achievements he spent so many hours with me on this and made this beautiful MIT quality report which I share with you in the description I'll get to that a bit later now let's go all the way back if you have watched my electro boom 101 I showed that voltage is basically defined as the amount of potential energy available to move unit charge or V is equal to the energy acting on charge divided by charge and of course work or energy is equal to the force applied to the charge times the distance that the force travels with the charge now what happens if the charge moves in a closed loop in the presence of this force it is like gravity it doesn't matter if you move sideways because the gravity always points down the mass will lose its potential energy as it gets closer to the earth and will gain the same amount back when you return to the same point so the sum of all energies stored or produced by gravity is zero in a closed loop basically you take out only what you put in that's called the law of conservation of energy for the same reason the sum of all energies acting on the charge in a closed loop must be zero we can divide these by Q to get voltage and we arrive at the original definition of Kirchhoff's voltage law some people call him Kirk off I call him Kirchhoff's it is based on the law of conservation of energy and says at any point in time the sum of all voltages in a closed loop must be zero let's create an electric force you know if you have a tiny charge you in the presence of a stationary big charge Q there will be an electric force between them repelling or attracting depending on the polarity let's assume they attract for the sake of simplicity I'll avoid vector and curl math let's keep it simple so it's easy to understand according to Coulomb's law this force is equal to some constant times the tiny Q times the big Q divided by a square of their distance the tiny Q starts moving due to this force let's assume charge Q moves by a very small distance of Delta L where we can assume the force almost remains the same because like a short force is a function of distance so the work done with that force in that tiny distance or Delta W is equal to the force in that spot times the distance for extremely small distances we can write this as DW equal to the F times DL and if the charge moves a distance of L from point A to B to get the total energy of work we integrate the energy which is integral of F DL from point A to B so the voltage between points a and B would be that energy divided by Q or integral of F TL divided by q q is not a function of L so we can take it inside an electric force divided by charge is defined as electric field which is equal to a constant times Q divided by square of distance and from these we conclude that voltage from point A to B is equal to the integral of e DL from point A to B and like I said before if we move the charge in a closed loop the of all energies and so all the voltages in the loop will be zero so in an electric field only the sum of all voltages is the closed loop integral of e DL equal to zero later on Michael Faraday revolutionized our understanding of magnetic fields and with the help of James Maxwell they came up with the Faraday's law and Maxwell's equations I of course fully comply to those laws and have nothing against them according to Faraday's law if we have an open surface surrounded by a closed loop the closed loop integral of e DL which is a voltage generated by electric fields around a loop also known as electro-motive force is equal to the rate of change of magnetic flux Phi through this surface in time and this magnetic flux is all the B magnetic fields passing through the surface enclosed by the loop and is equal to the magnetic field times the surface area remember I showed in the presence of electric fields only the closed loop integral of e DL is zero what as soon as we have magnetic fields through the loop this won't be zero anymore and we have to follow Faraday's law now watching dr. Levine's first response video I noticed that he believes this to be Chris offs voltage law if that was in fact the definition of KVL I would hundred percent agree with dr. Lewin that it is a special case of Faraday's law where D Phi over DT is equal to zero which means there are no magnetic fields but that's not the definition of KVL KVL says that the sum of all voltages in a loop must be zero but then you might say that the voltage is integral of e DL but that's not true voltage is any energy per unit charge not just energy from electric sources is that it first dr. Lewin believed that voltage is only defined by electric forces because looking at his second response and other videos it seems he doesn't think of dy over DT as a different entity you see he's showing F minus ma equals zero is the same as F equals MA what he's trying to explain is that in this equation force is equal to mass times acceleration ma is just the value of F and is not a separate entity so writing it as f minus ma equals to zero is pretty pointless which is true but similarly he thinks that minus D Phi over DT is only the value of this integral and it's not a separate entity what that's not true these two are two different entities that exist in a circuit with their values equal in a closed loop like when you balance your weight with your friend on a seesaw the value of your torques is equal but you are two different people but it returns to the definition of voltage what if voltage is defined as the effect of electric fields only now I know that dr. on the wing defines it like that and with that definition all his videos make perfect sense but who's right in his second video he refers to a paper written by Robert Romer published in American Journal of physics back in 1982 I read the paper and have a copy of it in my description if you're interested it's a great paper focused on what the meters would measure based both on math and actual experiments and they both match and also they match dr. Levine's experiments and mine what Romer does is that like dr. Levine and I he has a loop of wire with two different resistors one at each side and a changing magnetic field passes through the middle and also he has two identical meteors connected across the same point in the circuit shown as these resistive components he imposes some specific conditions on his setup and shows that meters read different values each meter reads the voltage across the resistor on their side I did the same experiment in my previous video I showed that it was true and provided some explanation in my setup I had to impose my own conditions because unlike Romer my coil was pretty small so the fields could close before passing the main loop or could create uneven fields so in my setup the condition was that the probe wires had to run very closely to the loop wires to make sure they are exposed to the same magnetic fields as the main loop which is the same idea as in Romer setup now I can easily explain it mathematically this is also explained beautifully in professor Belcher's and he created he provides such great detail that you can see how the charges move in the loop with your own eyes in my setup I twist the wires at this point all the way to the scope to make sure that the scope and this extension of wire are not affected by the field but for calculations we can model the scope as this tiny resistive component across these points the scope resistor can be one mega and our loop resistors are much smaller in kilo ohm range and the connecting wires have insignificant resistance we can assume zero if the field is going through the page that way the induced current is this way according to Faraday's law the closed loop integral of e DL over the main loop is equal to minus D Phi over DT the change of magnetic flux through the loop we can split this integral into two pieces one from A to B and one from B to a we know that the integral of e DL over ideal inductors and wires is zero because they have no resistance and only work with magnetic fields so the integral of e DL from point A to B is only the voltage across resistor one and from B to a is only voltage across resistor two so we get V r1 is equal to minus D Phi over DT minus V r2 now if you go through a different loop through the probe wires and the scope on this side and main loop on the other side we get a similar thing that the scope voltage on this side is equal to minus D Phi over DT minus v r2 because the same flux passes through our loops so basically the voltage of scope 1 is equal to V r1 and based on similar calculations on the other side voltage of scope 2 is equal to minus v r2 math matches our readings I never disagreed with dr. Lu in on this and confirmed it in my previous video in his paper Roman never disputes KVL but in a section titled voltage across an inductor he expresses his confusion and discomfort around what happens to the fields in an inductor then he mentions two books that help him find peace one of those books is the fine man lectures on physics by Fineman Layton and sands if you don't know dr. Fineman you should really check his Wikipedia page he was a fine man pun intended a huge scientist and an all-around great with many contributions to science both professor Belcher and I read Roma's paper and chapter 22 of fine mans book on AC circuits and we also found peace let me explain the book explains that for every component like an inductor or a capacitor it is best to use a lumped model meaning that all the fields are contained within the component and don't leak outside doing this makes it easy to calculate the voltage across the component and the voltage across the inductor is minus L di over DT despite the fact that the integral of e DL through the inductor is zero this is a real voltage across a magnetic component and doing so the book shows that the sum of all voltages in a loop is zero and KVL holds so professor Belcher also concluded that dr. Fineman himself and I have the same definition for voltage and the KVL holds in all cases although maybe I shouldn't assume myself at the same rank but then you might ask if KVL and Faraday's law do the same thing why do I push for KVL it's because KVL is much easier to work with over time and frequency domains see if we have a circuit like this knowing that KVL holds it's very easy to calculate the circuit impedance which is that resistor in series with these two in parallel which is the impedance of the inductor times impedance of the capacitor over this Plus that which is equal to that I can easily calculate my LC resonance frequency by finding where this becomes zero I just replace s with J Omega and calculate my frequency Fineman book explains that by making suitable approximations in a lumped model it is possible to ignore the great complexities of the fields inside the object a separation is made between what happens inside and what happens outside this is important to know because even if the fields are not contained and leaked outside we still have an inductor and defined voltages everywhere and KVL holds but this creates huge complexity in our calculations because now the leaking fields affect all the components and wires in the circuit that we have to take into account which is almost impossible because now there are so many new variables introduced that we might easily overlook our simple loop and probe was the easiest example of uncontained fields the fields closed around the entire circuit and effect everything every piece of wire and even the resistor itself becomes an inductor and the secondary of a transformer one of the examples people brought to disprove my conclusions was that what if the entire loop was made of a uniform resistive element and we take these two points and the induced current I would create two different voltages across the same points based on Ohm's law what you must understand is that a real resistor is only ideal if it's infinitely small as soon as it has any dimensions it at least has an inductance series with it especially something this long remember to make it easy you have to produce a model using lumped elements you pull out the resistance and the secondary inductance and put them in series for each half of the circuit if you assume a uniform field through a loop and D 1 and D 2 are the length of each side this is what you get an unknown voltage across these points and KVL holes there is an interesting experiment in one of Professor Belcher's assignments that kind of looks like this and shows the effect of uncontained fields we have an AC supply in series with a resistor and a loop of wire and on this side we measure some voltage and on the other side we measure zero volts here's what I have the supply connects with this alligator clips to this resistor and the loop and I'm measuring across the loop with these wires when I'm probing on the power supply side I read around 40 millivolts and now if I move the wires to the other side of the loop hmmm I'm reading 20 millivolts instead of zero it's half of the previous voltage I should read zero volts but instead I'm reading half of this voltage because I introduced a new probing problem both my AC supply and probe are coming from the same scope see if you can figure out what's going on on the other hand if I create an inductor with contained fields mic winding around the toroid core all the fields are contained in the core and my readings are not affected by where I probe here I replace my loop with a toroid and a few windings and you see it doesn't make any difference if I probe on this side or the other side okay let me explain why we didn't reach zero on the first test if our AC supply and the probe are isolated of course we read zero volts they're a simple engineer like me would explain it by the fact that every piece of wire is an inductor if we have a voltage V induced on the main loop we would also have half a V induced on equal length of probe wires exposed to the same fields that subtract from V and we read zero volt difference there a more proper scientists like Roma would say that the scope voltage is the integral of e DL over the main loop on the scope side which is zero now in my setup the AC supply and the scope ground are not isolated and shorted through the scope the simple engineer would say or sure you just shorted across that voltage on one of the probe wires so the scope would read half the voltage that's not wrong but it's a bit misleading what if we short across the probe wire with a piece of wire that runs closely to the probe wire it doesn't make any difference and we still read zero there because the fields closed around all of them the more accurate reason is that the ground through the scope creates a large loop if Phi is the magnetic flux through the loop half of it closes on the top side through air and half of it on the bottom through the ground loop so like the case of terrowin where all the fields were contained in the core here the majority of the fields are contained around the probe wire and are too far to loop around the shorting ground wire and affected this is as if the probe wire is an inductor shorted by a piece of wire there is a string current running through it but the voltage across it is 0 this is how we are shorting the induced voltage across the bottom probe wire and read half a voltage at the output instead of zero you can of course write the Faraday's law around the loops and get the same results Thank You professor Belcher for helping me with this

Channel: ElectroBOOM

Views: 1,361,381

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. John Winston Belcher, Feynman

Id: Q9LuVBfwvzA

Length: 19min 54sec (1194 seconds)

Published: Mon Dec 03 2018