What is Lenz's Law?

Lenz’s law named after Emil Lenz who formulated it during 1834. It depends on the principle of conservation of energy and Newton’s third law and 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 Law Definition

Lenz’s law states that 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 original flux is maintained through the loop when current flows in it.

Lenz Law Formula

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

\(Emf=-N\left ( \frac{\Delta \phi }{\Delta t} \right )\)

Where,

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

N is the number of loops.

\(\Delta \phi \) Change in magnetic flux.

\(\Delta t\) Change in time.

Following is the table with links of other Physics related laws:

Lenz Law Applications

Lenz 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 Law Experiment

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

Lenz’s Law

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 flows through the coil increases, the magnetic flux will increase. The direction of the flow of induced current would be such that it opposes when the magnetic flux increases.

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 the 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 goes on decreasing that 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 current force is exerted by the magnet in the loop. To oppose the change a force must be exerted by the current on the magnet.

Practice Questions on Lenz’s Law

Q1: State Lenz’s law
Ans: Lenz’s law states that 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 original flux is maintained through the loop when current flows in it.

Q2: Write the Lenz law formula and explain the terms.
Ans: Lenz law formula is given by- \(Emf=-N\left ( \frac{\Delta \phi }{\Delta t} \right )\) Where,

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

N is the number of loops.

\(\Delta \phi \) Change in magnetic flux.

\(\Delta t\) Change in time.

Q3: What is the prime importance of Lenz law?
Ans: Lenz law is used to determine the direction of the induced current.

Q4: What are the applications of Lenz law?
Ans: The applications of Lenz law are

  • Eddy current balances
  • Metal detectors
  • Eddy current dynamometers
  • Braking systems on train

Q5: Where is Lenz law used?
Ans:Lenz’s law is used to explain how electromagnetic circuits obey the conservation of energy and Newton’s third law

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Practise This Question

A conducting ring is placed around the core of an electromagnet as shown in fig. When key K is pressed, the ring