Electrical Engineering ⇒ Topic : Lenzs Law

Lenz's Law

The emf induced by variation of flux or magnetic field is termed as statically induced emf. A statically induced emf may be (i) mutually induced, or (ii) self induced.

The direction of statically induced current or voltage may be found by Lenz's law formulated by the Russian physicist Heinrich Lenz. According to Lenz's law, the induced current always develops a flux that opposes the change responsible for inducing this current, or the counter emf or back emf, always has a polarity which opposes the force that created it.

The induced emf is given a minus sign in order to take into consideration the fact that the counter emf opposes the change in flux thus

Lenz's Law

It can be stated as:

The induced current will flow in such a direction that it will oppose the cause that produces it.

The explanation of the above law is discussed hereinafter:

The current flows in coil 1 is varied by the rheostat as shown in Figure (a). The e.m.f. is induced in coil 2 due to variation of flux in coil 1. The induced e.m.f. between P and Q will be such that the current will flow from P to Q via the resistance R so that it will oppose the flux that is linked with it. In this case, P will be of +ve polarity and Q will be of -ve polarity. The induced e.m.f. is given by

                                                                                                       figure (a)

(1) Lenz's law:-Emil Lenz, a German scientist, gave the following simple rule (known as Lenz's law) to find the direction of the induced current :

The induced current will flow in such a direction so as to oppose the cause that produces it i.e.the induced current will set up magnetic flux to oppose the change in flux.

Note that Lenz's law is reflected mathematically in the minus sign on the R.H.S. of Faraday's second law viz. e = - N dΦldt.

The negative sign simply reminds us that the induced current opposes the changing magnetic field that caused the induced current. The negative sign has no other meaning.

Let us apply Lenz's law to Figure (a) Here the N-pole of the magnet is approaching a coil of several turns. As the N-pole of the magnet moves towards the coil, the magnetic flux linking the coil increases. Therefore an e.m.f. and hence current is induced in the coil according to Faraday's laws of electromagnetic induction. According to Lenz's law, the direction of the induced current will be such so as to oppose the cause that produces it. In the present case, the cause of the induced current is the increasing magnetic flux linking the coil. Therefore, the induced current will set up magnetic flux that opposes the increase in flux through the coil. This is possible only if the left hand face of the coil becomes N-pole. Once we know the magnetic polarity of the coil face, the direction of the induced current can be easily determined by applying right-hand rule for the coil. If the magnet is moved away from the coil, then by Lenz's law, the left hand face of the coil will become S-pole.Therefore, by right-hand rule for the coil, the direction of induced current in the coil will be opposite to that in the first case.

                                                      FIGURE (A)

It may be noted here that Lenz's law directly follows from the law of conservation of energy i. e.in order to set up induced current, some energy must be expended. In the above case, for example,when the N-pole of the magnet is approaching the coil, the induced current will flow in the coil in such a direction that the left-hand face of the coil becomes N-pole. The result is that the motion of the magnet is opposed. The mechanical energy spent in overcoming this opposition is converted into electrical energy which appears in the coil. Thus Lenz's law is consistent with the law of conservation of energy.