Electrical Law

LENZ'S LAW

STATEMENT

Lenz’s law states that the direction of the current (or emf) induced in a conductor by a changing magnetic field is such that the magnetic field created by the induced current opposes the initial changing magnetic field which produced it. 
The direction of this current flow is given by Fleming’s right hand rule.

Lenz’s law is based on Faraday’s law of induction. 

This change in the magnetic field may be caused by changing the magnetic field strength by moving a magnet towards or away from the coil, or moving the coil into or out of the magnetic field. 

Where:

• ε = Induced emf
• δΦB = change in magnetic flux
• N = No of turns in coil

The negative sign indicates that the induced EMF (ε) and the change in magnetic flux (δΦB) have opposite signs. The formula for Lenz’s law is shown above.

EXPLANATION

To better understand Lenz’s law, let us consider two cases:

Case 1: When a magnet is moving towards the coil.

When the north pole of the magnet is approaching towards the coil, the magnetic flux linking to the coil increases. According to Faraday’s law of electromagnetic induction, when there is a change in flux, an EMF and hence current is induced in the coil and this current will create its own magnetic field.

Now according to Lenz’s law, this magnetic field created will oppose its own or  opposes the increase in flux through the coil and this is possible only if approaching coil side attains north polarity, as we know similar poles repel each other. Once we know the magnetic polarity of the coil side, we can easily determine the direction of the induced current by applying right hand rule.

 In this case, the current flows in the anticlockwise direction.

Case 2: When a magnet is moving away from the coil

When the north pole of the magnet is moving away from the coil, the magnetic flux linking to the coil decreases. According to Faraday’s law of electromagnetic induction, an EMF and hence current is induced in the coil and this current will create its own magnetic field.

Now according to Lenz’s law, this magnetic field created will oppose its own or  opposes the decrease in flux through the coil and this is possible only if approaching coil side attains south polarity, as we know dissimilar poles attract each other. Once we know the magnetic polarity of the coil side, we can easily determine the direction of the induced current by applying right hand rule. 

In this case, the current flows in a clockwise direction.

APPLICATIONS

• Eddy current balances
• Metal detectors
• Eddy current dynamometers
• Braking systems on train
• AC generators
• Microphones

Video Link

https://youtu.be/ddT7Wrzkn0M