Lenz’s Law of Electromagnetic Induction, Formula, Experiment, Application

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.

History

Lenz’s law states that the direction of the current 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,

Lenz’s Law is named after the German scientist H. F. E. Lenz in 1834. Lenz’s law obeys Newton’s third law of motion (i.e to every action there is always an equal and opposite reaction) and the conservation of energy (i.e energy may neither be created nor destroyed and therefore the sum of all the energies in the system is a constant).

Lenz’s Law Formula

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

Emf=−N(ΔϕΔt)

Where:

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

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

Clausius StatementKelvin-Planck Statement
Law of Conservation of MassFourier’s Law

Conservation of Energy

To obey the conservation of energy, the direction of the current induced via Lenz’s law must create a magnetic field that opposes the magnetic field that created it. In fact, Lenz’s law is a consequence of the law of conservation of energy.

Why’s that you ask? Well, let’s pretend that wasn’t the case and see what happens.

If the magnetic field created by the induced current is the same direction as the field that produced it, then these two magnetic fields would combine and create a larger magnetic field. This combined larger magnetic field would, in turn, induce another current within the conductor twice the magnitude of the original induced current.

And this would, in turn, create another magnetic field that would induce yet another current. And so on. So we can see that if Lenz’s law did not dictate that the induced current must create a magnetic field that opposes the field that created it – then we would end up with an endless positive feedback loop, breaking the conservation of energy (since we are effectively creating an endless energy source).

Lenz’s law also obeys Newton’s third law of motion (i.e to every action there is always an equal and opposite reaction). If the induced current creates a magnetic field that is equal and opposite to the direction of the magnetic field that creates it, then only it can resist the change in the magnetic field in the area. This is in accordance with Newton’s third law of motion.

Lenz’s 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.

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 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 a force must be exerted by the current on the magnet.

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

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