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Lenz's Law: Definition, Formula and Applications

Lenz's law: definition, formula and applications

Lenz's law states that when generating an electromotive force (emf) caused by a change in magnetic flux according to Faraday's law, the induced emf polarity generates a magnetic current that opposes the variation it produces.

This law is based on Faraday's law of induction which states that when a variable magnetic field is connected to a coil, an electromotive force (induced voltage) is induced in it. In other words: the magnitude of the electromotive force induced in the circuit is proportional to the variation of the flow change.

The law is a consequence of the principle of conservation of energy (energy cannot be created or destroyed) and Newton's third law (there is always an equal and opposite reaction to every action).

Lenz's law is named after the work done by Heinrich Lenz in 1834. 

Lenz's Law Formula

The flux generated by a uniform magnetic field when passing through a flat circuit can be calculated with the following formula:

Φ = B · S · cos (α)

where

  • Φ is the magnetic flux expressed in Wb.

  • B is the magnetic induction expressed in T.

  • S is the flat surface of the conductor.

  • α is the angle formed by the direction of the field and the surface of the conductor.

Lenz's Law Applications

  • This law indicates that the change in flow and the induced voltage have opposite signs. In Faraday's law a physical interpretation of the choice of the sign can be made

  • Electric generators and motors: When a current is induced in an electric generator, the direction of this induced current is such that it opposes and causes the generator to rotate. This means that the generator needs more mechanical energy. in the case of engines. In the case of electric motors, the same thing happens but in the opposite direction.

  • In induction plates and electromagnetic brakes.

  • To understand the concept of magnetic energy storage in an inductor: When connecting an emf through a coil, it generates an electric current through it and a rear emf appears that opposes the cause that produces it. The external source has to do work to overcome the elements that oppose the flow variation. This work can be done by the emf that is stored in the inductor.

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Published: December 26, 2021
Last review: December 26, 2021