Lenz’s Law


The law of Lenz , also called Lenz-Faraday law, is used in the field of electromagnetism and to determine the direction of the induced current. It can be stated as follows: a change in the state of an electromagnetic system causes a phenomenon whose effects tend to oppose this change. Heinrich Friedrich Lenz, a physicist of German origin, proposed in 1834 that the induced voltages would oppose the sense of the variation generated by the magnetic flux that produces them, in order to create a law by which the conservation of the electric energy.


What is Lenz’s law?

Lenz’s law is associated with the electromagnetic area , since its approach is based on the relationship of changes or tensions produced by variations in the property of magnetic flux from the fact that these variations occur in a direction opposite to the flux that produces them. . In honor of its creator, Lenz’s law bears his surname. Physicist Heinrich Lenz formulated it at the beginning of the 19th century with the aim of conserving energy.

  • What is Lenz’s law?
  • Who proposed Lenz’s law
  • History
  • Formula
  • Lenz’s law applications
  • Example

What is Lenz’s law?

Lenz’s law has the purpose of predicting the direction of the electromotive force generated in an electrical circuit . This takes place from the fact that the electromotive force (or the direction of the current), generated is such that its magnetic consequences oppose or repel the variation of the magnetic field flux where it is produced.


This magnetic flux through a loop grows and the electrical current found manages to generate a negative flux magnetic field, which decreases successively as the original slack increases.

Who proposed Lenz’s law

Lenz’s Law is named after its creator, Heinrich Friedrich Lenz , a German-born physicist with studies in the areas of chemistry and physics . This scientist focuses on studying climatic conditions at first, until 1831 when he decides to focus on the study of electromagnetism, making the most significant contribution throughout his career: «The sense of currents or induced electromotive force is such that it is always opposed to the cause that produces it, that is, to the variation of the flow » .

This is how the theory presented by Faraday completes, in one of the most significant associations to the advancement of physics.


Heinrich Friedrich Lenz devoted himself to the field of physics and chemistry during his career, reaching the position of teacher and later rector at the University and Academy of Sciences of the city of Saint Petersburg. In its beginnings, it studied the Peltier effect , the conductivity of metals and the variation of electrical resistance with temperature. Lenz studies electrical conductivity and then makes the discovery of what is currently the Joule effect , with which he manages to study the independence of electrical variations , in an exchange of experiences and conclusions that allow him to state the well-known law by Lenz in 1834. In this way, Lenz contributes to an important advance in the area of ​​electromagnetism, allowing to determine the direction and direction produced by the variation of energy flows.


Lenz’s law is expressed mathematically as follows:

Lenz's law formula

  • E represents the emf (electromotive force).
  • N represents the number of turns.
  • ∆ϕ represents is in webers and represents magnetic induction flux .
  • ∆t represents the time expressed in seconds.

It should be noted that the sign – is to indicate that the polarity of the voltage is such that it opposes the cause that produced it.

Lenz’s law applications

The alternators are devices that generate electric currents that alternate, hence its name. They consist of two essential elements: the rotor, which causes the assembly to rotate, and the stator, which surrounds the previous one and is in charge of rotating around its axis. These alternators generate an alternating current in order to be used for motors, generating in turn, mechanical energy starting from electrical energy; with only the connection of the brushes of an alternator with another generator. This induces a reorientation and an indefinite rotation of the turn of the second alternator as long as a current is supplied.


One of the examples of Lenz’s law found in everyday life is the approach of the south pole of a magnet to a loop , which causes the induced electromotive force to be negative and opposes the cause that produces it; the current circulates through it in such a way that the loop behaves like a south pole in front of the magnet, which it must try to repel.

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