Lenz's law
/ LENZ /
When induction makes a current appear, which way does it flow? Lenz's law gives the answer in one line: the induced current always flows so as to OPPOSE the change that created it. Nature pushes back, rather like a crowd resisting being shoved. It tells you the direction of every induced current.
Precisely, the induced current flows in whatever direction makes its own magnetic field fight the change in flux. If the flux through the loop is increasing, the induced current sets up a field that opposes the increase; if the flux is decreasing, the current tries to prop it up. This opposition is exactly what the minus sign in Faraday's law, EMF = - dPhi/dt, is saying. Underneath, Lenz's law is simply energy conservation: if the induced current helped the change along, you would get energy for free.
It explains why a magnet dropped down a copper pipe falls in gentle slow motion (induced eddy currents oppose it), how magnetic braking works, and why a generator takes effort to turn. Be careful of a common misconception: the induced current opposes the CHANGE in flux, not the field itself — if the flux is falling, the induced current actually tries to keep the field going.
Push the north pole of a magnet toward a coil and the near face of the coil becomes a north pole to repel it, so you must push against a force. Pull the magnet away and that face turns south to attract it — again opposing your motion.
The coil always fights the magnet's motion, whichever way you move it.
Common misconception: that the induced current opposes the field. It opposes the CHANGE in flux — if the flux is falling, the induced current actually acts to maintain the field, not to cancel it.