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what is Lenze's law? explain its application.

Lenz's law is a common way of understanding how electromagnetic circuits obey Newton’s third law and the conservation of energy. Lenz's law is named after Heinrich Lenz, and it says:

An induced electromotive force (emf) always gives rise to a current whose magnetic field opposes the original change in magnetic flux. There is an induced current in a closed conducting loop if and only if the magnetic flux through the loop is changing.  The direction of the induced current is such that the induced magnetic field always opposes the change in the flux.

If you wrap your fingers around the coil in the direction of the current, your thumb  points north is called Right Hand Rule.Lenz's law is shown with the minus sign in  Faraday/s law of induction,  which indicates that the induced emf () and the change in magnetic flux () have opposite signs.

Taking a permanent magnet and putting a coil in front of it, with the north pole nearest the coil, as the magnet is brought closer to the coil, this will increase the flux through the coil. Then, by Lenz's law, the current will be in counterclockwise direction from the north end of the magnet when looking into the coil from the south pole of magnet. If the magnet is brought away from the coil, this will decrease the flux through the coil. Therefore, the current should be induced in the clockwise direction from the north end of the magnet. By keeping at rest but increasing the field strength of the magnet, the flux through the coil will be increased: thus the induced current should be in the counterclockwise direction from the north end of the magnet. This case is analogous to the case where we moved the magnet towards the coil. Similarly, if the magnet is kept at rest but the field strength of the magnet decreases, the current will be induced in the clockwise direction from the aforementioned position.

A number of real world applications of Lenz’s law, including electromagnetic braking, induction cooktops and back emf in electric motors.

Electromagnetic Braking

Stationary electromagnets induce eddy currents into the spinning rotor. These eddy currents create magnetic fields which oppose the original change in flux (Lenz’s law). This slows the rotor. More force can be applied by increasing the current in the electromagnets.

Induction cooktops

Induction cooktops are a useful application of eddy currents. They have coils beneath the surface of the cooktop, which produce a changing magnetic field. If a saucepan is sitting on top, small, localised emf’s will be induced, resulting in eddy currents flowing. The saucepan is intentionally made from electrically resistive metal, so the eddy currents experience high

resistance as they flow. The resistance causes the electrical energy to be dissipated as heat, thus warming the saucepan.

Back EMF

We have a conductor experiencing a changing magnetic field (the rotating wire loop). Therefore

there will be an induced EMF (but negligible eddy currents since the wires are thin). However, the wire already experiences an EMF, provided by the battery or power source, which makes the motor spin. This is called supply EMF. The induced EMF must oppose the supply EMF. Let’s imagine that it didn’t. As the motor spins, the induced EMF would add to the supply, making more electricity flow through the motor. This would make it turn faster, and induce more EMF, making more electricity flow, making it turn even faster, etc. Again we would be getting energy from nowhere and violating the conservation of energy. So this induced EMF must oppose the supply EMF. We call the induced EMF back EMF.