in the link,starts from minute 3.30 : https://www.youtube.com/watch?v=1xFRtdN5IJA&list=PLyQSN7X0ro2314mKyUiOILaOC2hk6Pc3j&index=22

we have a magnet, a dc source, and a circular winding that has little length but a lot of turns. Now, if we were to hold the winding so that It can not turn due to lorentz force and so there will be no induced EMF, In that case Lewin says there would be huge amount of current drawn from the DC source. But if we free the winding so that winding would turn freely, current drawn from the source will be 40 times smaller than the first case.

My question is, when I tried to write down the equation I couldn't come up with the same result. Here is my calculations:

B:magnetic field of the magnet on the winding(assumed to be constant)

A: area of circle

N: number of turns

Q: angle between the surface normal of the winding and magnetic field

Rs: resistance of the source

Vs: voltage of the source

note: winding has also resistance but it is neglected

  1. -(magnetic flux change)= EMF= B.A.N.sinQ
  2. closed line integral of Electric field of the circuit = B.A.N.sinQ= I.Rs-Vs

according to "2)" current drawn from the source must oscillate within the range that also includes the case where the winding is not allowed to turn. If to simplify, for the first case , if the current is 30 ampere for the second case, it must oscillate between 20 ampere and 40 ampere. Because magnetic field is hugely dependent on the magnet unlike if there were no magnet.


1 Answer 1


If the coil does not move, the DC current is determined by the resistance in the circuit. If the coil is spinning, the “back emf” produced by the changing flux limits the current. Normally, the commutator supplies current to the part of the coil which is near a position where it supplies a maximum torque (always in the same direction).

  • $\begingroup$ but, you did not say anything about my equations. assuming they are correct, induced emf limits the current in which, the current measuered for non-moving winding is included. So that kind of limiitation is like min 20 ampere and max 40 ampere and so it does includes 30 ampere. $\endgroup$
    – ozgun can
    Aug 19, 2021 at 20:56
  • $\begingroup$ OK, I think I get your point. If all the coils in the winding have area vectors in the same direction, and the B field is uniform, then the back emf will be sinusoidal and when it goes through zero, the current will be large. (This is not a problem in a normal DC motor which has coils in two or more directions.) The motor in the video is poorly described, and the numbers you quote do not relate to that motor. $\endgroup$
    – R.W. Bird
    Aug 20, 2021 at 17:42
  • $\begingroup$ one more question related with the direction of the tork of the motor. If I am seeing that right, direction of the current is constant. What that will cause is that tork will reverse half of the time. So in all, motor shouldn't have revolved but it does, how so? $\endgroup$
    – ozgun can
    Aug 21, 2021 at 8:01
  • $\begingroup$ As I mentioned, in a normal DC motor, the split commutator switches the current to a coil which is in position to maintain the torque in the same direction. I can't tell from the video how their little motor works. $\endgroup$
    – R.W. Bird
    Aug 21, 2021 at 14:04

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