# Thread about the MIT prof claiming an incorrect usage of KVL in RL circuits

1. Jul 8, 2006

### leright

There was a thread where the thread starter pointed out a video lecture where a MIT prof claimed that KVL was incorrectly used in the analysis of self inductive circuits in almost all textbooks. I thought it was an interesting read and wanted more opinions on the idea, however I cannot find the thread. Does anyone recall this thread, as I feel it would be a great topic for discussion. I enjoy threads that poke and prod at common scientific innaccuracies in textbooks.

Sorry this is vague, but I would really like to revive the disscussion.

2. Jul 9, 2006

### vanesch

Staff Emeritus

Probably the prof tried to point out that the KVL is the "lumped element" form of rot E = 0, while of course in a self, there is a dB / dt and hence rot E is not 0.
As long as "most of the self behaviour" is inside a lumped, eh, self, you can however close your eyes and think of England, and apply the KVL correctly. However, from the moment that the circuit loop itself is "the self" (such as in high-frequency circuitry) you get into troubles with KVL. Was this the idea ?

Maybe I'm just guessing wrong...

3. Jul 9, 2006

### rbj

the problem is, of course, that the changing magnetic field from the "L" in the RL circuit induces e.m.f. in the loop making the sum of voltages around the loop (at an instant of time) not add to zero. actually, this induced voltage could be lumped into the voltage of the inductor and KVL would be happy. the inductor could also be positioned in such a way that there is no net field in the loop (the axis of the coil is at a right angle to the axis of the loop).

a similar problem can be postulated with an RC circuit with the changing E field inducing a changine B field which induces a net e.m.f. in the loop.

4. Jul 10, 2006

### leright

You are on the right track. The problem he was analyzing was an simple RL series circuit. The self inductance wasn't the circuit loop, but it was the inductor in the loop. The prof had a problem with the use of KVL in this analysis since, as you pointed out, KVL employs the property that E-fields have a curl of zero. But then I think, as rbj pointed out, changing e-fields within a capacitor cause a curl in E also, and voltage sources (such as batteries) produce curls in E. So the inductor is only one case where KVL is improperly used, and if the prof is going to split hairs about such issues then he should point these things out also.

That lecture confused me pretty significantly, and the fact that it's an MIT professor concerns me, since if an MIT prof says it it it holds merit.

Lecture #20 is the lecture in question. http://ocw.mit.edu/OcwWeb/Physics/8-02Electricity-and-MagnetismSpring2002/VideoLectures/index.htm [Broken]

Man, Lewin sure is a great lecturer though.

Last edited by a moderator: May 2, 2017