What is Lenz Law?

Lenz law is the basic principle that is used to determine the direction of an induced voltage or current.  Lenz law based on the principle of conservation of energy, the Lenz law simply states as below.

Definition of Lenz Law:

Lenz law simply states that the direction of the induced current must be such that its own magnetic field will oppose the action that produced the induced current.


In figure (a), for example, the induced current has the direction that produces a north pole at the left to oppose the motion by repulsion of the north pole being moved in.  This is why it takes some work to push the permanent magnet into the coil.  The work expended in moving the permanent magnet is the source of energy for the current induced in the coil.

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(a) Induced Current Produced by magnetic flux cutting across turns of wire in a coil.

Using the Lenz Law, we can start with the fact that the left end of the coil in figure (a) must be a north pole to oppose the motion.  Then the direction of the induced current is determined by the left hand rule for electrons flow.  If the fingers coil around the direction of electron flow shown, under and over the winding, the thumb will point to the left for the North Pole.

For the opposite case, suppose that the north pole of the permanent magnet in figure (a) is moved away from the coil.  Then the induced pole at the left end of the coil must be a south pole, by Lenz law.  The induced South Pole will attract the North Pole to oppose the motion of the magnet being moved away.  For a south pole at the left end of the coil, then, the electron flow will be reversed from the direction shown in figure.  We could actually generate an alternating current in the coil by moving the magnet periodically in and out.

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