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I've been really stumped on this particular concept. In Case A, when a bar magnet is brought towards a copper coil around a soft iron core, in accordance with Faraday's Law of Electromagnetic Induction, the the pole facing the magnet acquires a North Polarity while the opposing pole acquires a South Polarity. Now by Lenz's Law, the direction of the induced EMF must oppose the cause that produces it, but the current is in the same direction as the moving magnet. The galvanometer deflects in the same direction too. Why doesn't this obey Lenz's Law?

And additionally, in Case B, by merely reversing the winding of the copper coil, the current flows in accordance with Lenz's Law and the Principle of Conservation of Energy.

My question is, why does changing the winding of a coil defy Lenz's Law in Case A and then follow Lenz's law in Case B? Something such as the Principle of Conservation of Energy and Lenz's Law should apply in any case.

The method of winding shouldn't matter that much right?

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  • $\begingroup$ If N approaches, The iron ges an S in direction of N, so your drawing is wrong $\endgroup$
    – trula
    Commented Sep 16, 2023 at 16:58

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Both diagrams are correct and are consistent with Faraday and Lenz.

The magnet moving towards the coil produces a change in the magnetic flux linked with the coil and so an emf is induced in the coil (Faraday).
There is a complete electrical circuit so the induced emf produces an induced current.
That induced current produces a magnetic field (induced N pole at right end of coil) which opposes the motion of the incoming N-pole of the magnet (Lenz).

I am afraid that I do not understand the statement, . . . . . but the current is in the same direction as the moving magnet . . . . ., that you have made.

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  • $\begingroup$ Oh alright thankyou I thought it was the galvanometer deflection which must always oppose the moving magnet So its not the galvanometer, but the polarities on either side which oppose the motion of the magnet Is that correct? $\endgroup$
    – Schrödinger's Cat
    Commented Sep 16, 2023 at 17:23
  • $\begingroup$ In both cases the induced current produces a magnetic field which points towards the right. $\endgroup$
    – Farcher
    Commented Sep 16, 2023 at 18:33
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Your diagrams are correct for the case of the magnet moving towards the iron-cored solenoid. The induced polarities of the core and the sense of the induced current are correct.

Where your confusion seems to lie is in your part-sentence, "...but the current is in the same direction as the moving magnet." The current goes round and round the core, in the same sense (clockwise, viewed from the left-hand end of the core) in both case A and case B. The end of the core, left or right, at which the current enters the coil is irrelevant. In both cases A and B, the sense of the current's rotation around the core gives (using the right hand rule) a field direction in the core such that its North Pole is at its right hand end, opposing the advance of the magnet, as Lenz's law predicts.

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