# Electromagnetic induction- Lenz's Law

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#1
Hiya,

I've read in the textbook that the magnetic field of the coil opposes the magnetic field of the magnet entering the coil. So if magnet has its north pole going towards the coil the magnetic field in the coil is north and the current goes anticlockwise.

When the magnet is being pulled out the induced current is in the clockwise direction. This corresponds to an induced south pole in the coil which opposes the magnet moving away- this is the bit that I don't understand, why does it oppose the magnet moving away if it's the north end?

Thanks
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#2
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4 years ago
#3
(Original post by VioletPhillippo)
Hiya,

I've read in the textbook that the magnetic field of the coil opposes the magnetic field of the magnet entering the coil. So if magnet has its north pole going towards the coil the magnetic field in the coil is north and the current goes anticlockwise.

When the magnet is being pulled out the induced current is in the clockwise direction. This corresponds to an induced south pole in the coil which opposes the magnet moving away- this is the bit that I don't understand, why does it oppose the magnet moving away if it's the north end?

Thanks
Lenz's law is a consequence of the energy conservation laws.

The magnetic field is a store of energy and anything that tries to change the equilibrium status will be opposed.

We know from Faraday that an e.m.f. will be induced in a conductor (coil) cutting lines of magnetic flux. Moving a permanent magnet towards a conductor will cause the conductor to cut the magnets lines of flux and an e.m.f. will be induced. i.e. The moving magnet has introduced an increasing magnetic field into the domain of the coil.

The induced e.m.f. gives rise to a current in the conductor. Most importantly, that induced current will be accompanied by it's own increasing magnetic field which will always oppose the field which created it in the first instance. i.e. the induced field will try to prevent the incoming field from increasing further.

Pulling the magnet out of the coil induces the opposite effect. The induced e.m.f. reverses such that the induced magnetic field once again tries to oppose the collapsing field as the magnet is pulled out of the coil. i.e. this time, the fields try to reinforce each other to prevent the collapse.

If Lenz's law were not true and the fields repelled each other when pulled apart, the current in the coil would carry on increasing and would therefore violate the energy conservation laws and as Scotty often retorts: 'yeah cannae break the laws of physics captain'.
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#4
(Original post by uberteknik)
Lenz's law is a consequence of the energy conservation laws.

The magnetic field is a store of energy and anything that tries to change the equilibrium status will be opposed.

We know from Faraday that an e.m.f. will be induced in a conductor (coil) cutting lines of magnetic flux. Moving a permanent magnet towards a conductor will cause the conductor to cut the magnets lines of flux and an e.m.f. will be induced. i.e. The moving magnet has introduced an increasing magnetic field into the domain of the coil.

The induced e.m.f. gives rise to a current in the conductor. Most importantly, that induced current will be accompanied by it's own increasing magnetic field which will always oppose the field which created it in the first instance. i.e. the induced field will try to prevent the incoming field from increasing further.

Pulling the magnet out of the coil induces the opposite effect. The induced e.m.f. reverses such that the induced magnetic field once again tries to oppose the collapsing field as the magnet is pulled out of the coil. i.e. this time, the fields try to reinforce each other to prevent the collapse.

If Lenz's law were not true and the fields repelled each other when pulled apart, the current in the coil would carry on increasing and would therefore violate the energy conservation laws and as Scotty often retorts: 'yeah cannae break the laws of physics captain'.
Thanks so much
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