Lenz's Law

What is Lenz’s Law?

Lenz’s law states that

The induced electromotive force with different polarities induces a current whose magnetic field opposes the change in magnetic flux through the loop in order to ensure that the original flux is maintained through the loop when current flows in it.

Named after Emil Lenz, Lenz’s law depends on the principle of conservation of energy and Newton’s third law. It is the most convenient method to determine the direction of the induced current. It states that the direction of an induced current is always such as to oppose the change in the circuit or the magnetic field that produces it.

Lenz’s Law Formula

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

\(\begin{array}{l}Emf=-N\left ( \frac{\Delta \phi }{\Delta t} \right )\end{array} \)

Where,

Emf is the induced voltage (also known as electromotive force).

N is the number of loops.

\(\begin{array}{l}\Delta \phi \,\,Change \,\,in\,\, magnetic\,\, flux. \end{array} \)

\(\begin{array}{l}\Delta t\,\, Change\,\, in\,\, time.\end{array} \)

Following is the table with links to other Physics-related laws:

Clausius Statement	Kelvin-Planck Statement
Law of Conservation of Mass	Fourier’s Law

Lenz’s Law Applications

Lenz’s law applications are plenty. Some of them are listed below-

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

Lenz’s Law Experiment

To find the direction of the induced electromotive force and current we look to Lenz’s law. Lenz proved some experiments in accordance with his theory.

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.

To produce the current, force is exerted by the magnet in the loop. To oppose the change, the current on the magnet must exert a force on the magnet.

Frequently Asked Questions – FAQs

Q1

How is Lenz’s law conservation of energy?

Lenz’s law is based on the law of conservation of energy. From the definition of Lenz’s law, we know that the induced current is always opposed by the cause that produces it. Therefore, there is extra work done against the opposing force. The work done against the opposing force results in the change in the magnetic flux and hence the current is induced. The extra work done is known as electrical energy, and hence the law of conservation of energy is satisfied.

Q2

What is the difference between Lenz’s law and Faraday’s law?

Lenz’s law is about the conservation of energy applied to the electromagnetic induction, whereas Faraday’s law is about the electromagnetic force produced.

Q3

What is the prime importance of Lenz’s law?

Lenz’s law is used to determine the direction of the induced current.

Q4

What does the negative sign indicate in Lenz’s law?

The negative sign in Lenz’s law indicates that the induced emf in the coil is in the opposite direction of the magnetic flux, which is linked with the coil.

Q5

Where is Lenz’s law used?

Lenz’s law is used to explain how electromagnetic circuits obey the conservation of energy and Newton’s third law.

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