Chapter-6|Electromagnetic Induction (EMI)| NCERT 12th Physics:
Lenz’s Law:
According to Lenz’s law, the polarity of induced EMF is such that, it tends to produce a current which opposes the change is magnetic flux that produced it.
Or
The direction of induced EMF or current is always such that, it opposes the cause of it’s generation or the cause producing it.
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As the magnet is moved toward the loop, a particular amount of current is induced in the loop. The current produce it’s own magnetic field.
When the north pole of the magnet moves towards a stationary loop, an induced current I flows in anticlockwise sense. The anticlockwise sense corresponds to the generation of north pole which opposes the motion of the approaching north pole of the magnet.
When the north pole of the magnet is moved away from the loop, the current I flows in the clockwise sense, which corresponds to the generation of south pole. The induced south pole opposes the motion of the receding north pole.
The direction of current induced in all above cases remain same.
Lenz’s Law and Conservation of Energy:
Lenz’s law is in accordance with the law of conservation of energy.
When north pole of the magnet is moved towards the coil, the front face of the coil acquires north polarity. Thus work has to be done against the force of repulsion in bringing the magnet closer to the coil.
When north pole of the magnet is moved away, south pole developed on the front face of the coil. Therefore work has to be done against the force of attraction in taking the magnet away from the coil.
This mechanical work in moving the magnet with respect to the coil changes into electrical energy producing induced current. Hence energy transformation takes place. When we do not move the magnet, work done is zero. Therefore induced current is also zero. Hence, Lenz’s law obeys the law of conservation of energy.