[Physics] Why does AC induction motor rotate the same way

electricityelectromagnetic-inductionelectromagnetisminduction

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.

Best Answer

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.