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If I have a permanent magnet falling through a coil, North pole first, what is the direction of the induced current and how do I figure it out by myself using Lenz's Law and Flemming's hand rules

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2 Answers 2

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Try to make it more obvious which way your coil is winded by 'hiding' the part of the coil which goes behind the wires.

I'll assume that it's wounded clockwise when looking from the top, in which case the induced current will be going from the top of your coil to your ampmeter (right to left on the top wire of your drawing) when the magnet is entering the coil:

current at the beginning

When the magnet is leaving the coil at the bottom, the current will flow in the other direction. And when the magnet is completely inside the coil, there is no current (as the current induced from the N pole crossing the turns gets cancelled by the current induced from the S pole crossing the turns above). See these questions and answers for more details:

Bar magnet dropped through coil

Lenz law - Direction of induced current

You can see the explanation on this nice image from Wikipedia with a magnet and a coil, and the hand rule:

Lenz law demonstration

You can also watch this video which covers the very question you asked: What Happens when a Magnet Falls Through a Coil?

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Step 1: Determine the direction of the force that will oppose the motion of the magnet. In your example this would be an upwards force on the bar magnet since it is moving down. This is where you are applying Lenz’s Law.

Step 2: Determine the polarity of the coil that will produce this opposing force. In your example this would be a north pole at the top end of the coil to repel the incoming magnet, or a north pole at the bottom end of the coil to attract the south pole of the bar magnet once it has passed through the coil.

Step 3: Use one of Fleming’s Hand Rules to determine the direction of current flow to produce the coil polarity in step 2.

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