## Lenz Law and Conservation of Energy – Class 12 | Chapter – 6 | Physics Short Notes Series PDF for NEET & JEE

Lenz Law and Conservation of Energy: Lenz’s law is a fundamental principle in electromagnetism that describes the direction of the induced current in a conductor when a magnetic field changes. Lenz’s law is closely related to the law of conservation of energy, which states that energy cannot be created or destroyed, only transformed from one form to another.

## Lenz Law and Conservation of Energy

According to Lenz’s law, the direction of the induced current in a conductor is such that it opposes the change in the magnetic field that produced it. This means that if a magnetic field is increasing, the induced current will produce a magnetic field that opposes the increase. Similarly, if a magnetic field is decreasing, the induced current will produce a magnetic field that opposes the decrease.

The effect of Lenz’s law is to conserve energy. When a magnetic field changes, energy is required to induce an opposing current in the conductor. This energy is provided by the work done on the system that is producing the magnetic field. The work done to produce the changing magnetic field is equal to the energy stored in the field, and the work done by the induced current is equal to the energy dissipated from the field.

By opposing the change in the magnetic field, the induced current reduces the rate at which energy is transformed from the magnetic field to the conductor, thereby conserving energy in the system. This principle is important in many applications of electromagnetic theory, including electric motors, generators, and transformers.

## Applications of Lenz Law

Lenz law has a number of important applications in various fields, including:

• Electromagnetic braking: Lenz’s law is used in electromagnetic braking systems, such as those used in trains and roller coasters. The braking system uses the principle of electromagnetic induction to create a magnetic field that opposes the motion of the conductor, resulting in a braking force.
• Eddy current testing: Lenz’s law is used in non-destructive testing techniques, such as eddy current testing. In this technique, a changing magnetic field is applied to a conductive material, and the resulting eddy currents create an opposing magnetic field that can be measured to detect flaws or defects in the material.
• Transformers: Lenz’s law is a fundamental principle in the operation of transformers. When an AC current flows through the primary coil of a transformer, it creates a changing magnetic field, which induces a voltage in the secondary coil. Lenz’s law ensures that the induced voltage in the secondary coil is in the opposite direction to the original voltage in the primary coil.
• Induction heating: Lenz’s law is used in induction heating, where a changing magnetic field induces eddy currents in a conductive material, resulting in heating due to the resistance of the material to the flow of current.
• Electric generators: Lenz’s law is fundamental to the operation of electric generators, where a changing magnetic field is used to induce an electromotive force (EMF) in a conductor, resulting in the generation of electricity. Lenz’s law ensures that the direction of the induced EMF is such that it opposes the change in magnetic field, thereby conserving energy.

## Formula of Lenz Law

Lenz Law is reflected in the formula of Faraday’s law. Here the negative sign is contributed by Lenz’s law. The expression is –

Emf = -N (Δø / Δt)

Where,

• Emf is the induced voltage (also known as electromotive force).
• N is the number of loops.
• Δø is change in the magnetic flux
• Δt is change in time.

## Lenz Law Experiment

### First Experiment

In the first experiment, he concluded that when the current in the coil flows in the circuit, the magnetic field lines are produced. As the current flow through the coil increases, the magnetic flux will increase. The direction of the flow of induced current would be such that it opposes the increase in magnetic flux. ### Second Experiment

In the second experiment, he concluded that when the current-carrying coil is wound on an iron rod with its left end behaving as N-pole and is moved towards coil S, an induced current will be produced.

### Third Experiment

In the third experiment, he concluded that when the coil is pulled towards the magnetic flux, the coil linked with it decreases, which means that the area of the coil inside the magnetic field decreases. According to Lenz’s law, the motion of the coil is opposed when the induced current is applied in the same direction.

## How is Lenz law conservation of energy?

Lenz law is related to the conservation of energy because it is a consequence of the law of conservation of energy. Lenz law states that the direction of an induced electromotive force (EMF) in a conductor is such that it opposes the change in magnetic flux that produced it. This means that when a magnetic field changes in strength or orientation, it creates an EMF in a nearby conductor, which in turn generates a current that creates a magnetic field that opposes the original change in magnetic field.

The opposition of the induced EMF to the change in magnetic flux results in a dissipation of energy in the form of heat. This dissipation of energy is consistent with the law of conservation of energy, which states that energy cannot be created or destroyed, but can only be converted from one form to another. In this case, the energy of the changing magnetic field is converted into thermal energy, which is dissipated in the conductor. Therefore, Lenz’s law is a manifestation of the conservation of energy principle.

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