Why does AC induction motor rotate the same way?

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Let's start from the ground up. The experiment below illustrates the basic principle of an AC induction motor and I assume is what you are referring to (if not, it still works).

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When the magnetic rotates, there will be a induced current in the aluminium disc. By Lenz's Law, this current will act in such a way as to oppose the change in relative motion that caused it. The result of that the aluminium disc will appear to chase the spinning magnet.

The part you get stuck at is why the disc is following the magnet? Thinking in terms of relativity, by following the magnet you are decreasing the relative motion between the disc and the magnet. Thus by decreasing the relative motion you are essentially minimising this change (Lenz law) to stay stationary.

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John Smith
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Updated on October 26, 2020

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  • John Smith
    John Smith about 3 years

    Suppose an aluminium disc is suspended where it can freely rotate.

    A magnet is placed above (not touching) the aluminium disc and made to spun.

    This obvious causes a changing magnetic field.

    By Faraday's Law, this will induce current in the disc below that oppose the motion of the magnet by Lenz's Law.

    However, why does the disc then still spin in the same direction as the movement of the magnet?

    I understand that the disc will induce a secondary magnetic field that slows the rotation of the magnet, but how come the disc still follows the magnet?

    Please help! All is appreciated.

  • John Smith
    John Smith about 8 years
    But doesn't Lenz's Law always say that it should oppose the motion which induced the current? Shouldn't it spin the other way instead? Sorry for my ignorance.
  • Daniel Griscom
    Daniel Griscom about 8 years
    The currents resist the motion of the magnetic field through the disc, distorting the magnetic field which transmits a force to the magnet, resisting the magnet's motion. However, in the process an equal and opposite force is applied to the disc, pushing it in the same direction as the motion of the magnet.
  • John Smith
    John Smith about 8 years
    I think I get it now! Is it because if the disc moved the other way, then the relative motion is instead increased? So to decrease the RELATIVE motion they must chase each other! Right?
  • silenceislife
    silenceislife about 8 years
    @JohnSmith Yes. Decreasing relative motion = analogous to becoming stationary i.e. minimising change so they can become still again by chasing each other and trying to 'catch up' per se.
  • silenceislife
    silenceislife about 8 years
    No problem, don't forget to mark it as answered