The Induced Current Always Opposes the Change in Flux
A changing magnetic flux induces an emf, and Lenz's law fixes the direction: the induced current opposes whatever change in flux causes it. That one statement decides the sign of the force on a moving magnet and the direction of the current in a coil.
Electricity questions depend on identifying charge, a conducting path and the condition that controls current or magnetic effect.
Faraday and Lenz
The induced current produces a magnetic field that opposes the change of flux: if the flux through the coil is increasing, the induced field points opposite to the original field, and if it is decreasing, the induced field points along it.
A magnet falling through a ring
As a bar magnet falls through a horizontal metal ring, the induced current creates a force that opposes the motion, both when the magnet approaches and when it leaves. Far from the ring the acceleration is g, but near the ring it is less than g, and because the induced force depends on the position and speed of the magnet the acceleration is not uniform. The kinetic energy lost appears as heat in the ring.
Direction of the induced current
Treat the face of the coil nearest the magnet as a pole. When a pole approaches, the coil face nearest it becomes the same pole, so it repels the magnet; when the pole recedes, the nearest face becomes the opposite pole and attracts it back. Viewed from a pole face, a current that looks anticlockwise makes that face a north pole and a clockwise current makes it a south pole. Use this to check each coil in a diagram against the direction in which the magnet is moving.
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- ✓e = -dΦ/dt; the induced current opposes the change in flux
- ✓Falling magnet near a ring: acceleration less than g and not uniform
- ✓Approaching pole: nearest coil face becomes the same pole; receding pole: the opposite pole
- ✓Anticlockwise current seen from a face means a north pole on that face
Do not say the induced current opposes the magnet itself. It opposes the change in flux, so it repels an approaching magnet but attracts a receding one.
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