Define and explain Lenz’s Law

Lenz’s law and direction of induced EMF

The direction of the induced current is always such to oppose the change which causes the current.


Consider a coil in which current is induced by the movement of a bar magnet towards in coil. The magnetic field produces in the coil. one face of the coil acts as the north pole and the other as the South pole as shown in the figure.

According to the right-hand rule, from the side of the bar magnet anti-clockwise induced current flows through the coil. The left face of the coil becomes the north pole and the right face becomes the south pole. The north pole of the coil repels the north pole of the bar magnet and this is why it opposes the motion of the bar magnet.

According to Lenz’s law, the push of the magnet is the change that produces the induced current, and the current acts to oppose the push. On the other hand, if we pull the magnet away from the coil the induced current will oppose the ” pull” because the face of the coil towards the bar magnet becomes the south pole which attracts the bar magnet.

How is Lenz’s law related to the law of conservation energy

Consider a rod of length L that moves towards the right magnetic field B and induced current flows through the loop in the anti-clockwise direction. the magnitude of the magnetic force on it is given by,

F = IL B Sin 90°

According to the right-hand rule, the direction of Fm is opposite to that of V and it tends to stop the rod, as shown in the figure.

An external dragging force equal to Fm in magnitude opposite in direction must be applied to keep the rod moving with constant velocity. This dragging force provides energy for the induced current to flow. This energy is the source of induced current. Thus electromagnetic induction is exactly according to the law of conservation of energy. In the figure, Lenz’s law forbids the induced current directed cock wise.


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