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Physics > Electromagnetic Induction > Faraday’s and Lenz’s Law
Electromagnetic Induction

Faraday’s and Lenz’s Law

You must have always come across the security checks at airports or at railway stations. You must have also used tape recorders to record your voice and to listen to the music. All these things work on the principle of Faraday’s law. Now, what is Faraday’s law? We will study this law in detail.

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Faraday’s Experiment

To understand the Faraday’s law, let us first carry out an experiment in which we have a coil attached to a galvanometer and a bar magnet.

Now, this coil does not have a source of current, that means there is no battery attached and no current circulates inside the coil.  When the bar magnet is moved towards the coil, the galvanometer starts showing deflection. That means there is a current induced in the circuit. Was there any battery? NO!

But just because the bar magnet was in motion, emf has been induced in the coil. This is an electromagnetic induction. Now let the magnet move towards the direction of the coil with velocity ”v”. What is observed is that, till the bar magnet was in motion, only at that time the galvanometer shows deflection.

The moment ”v” becomes 0 again, the galvanometer shows ”0” deflection.  So if v = 0, emf = 0. Here we have observed that greater the velocity, greater is the induced emf. Also when the direction of ”v” is changed, the galvanometer shows deflection in the opposite direction, that is the current moves in opposite direction.

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The bar magnet is associated with the magnetic flux and the emf which gets induced inside the coil, it’s because of the magnetic flux. From this above experiment, we get two laws:

Faraday’s Laws

Faraday’s First Law

Whenever there is a change in the magnetic flux associated with a coil and emf is induced in the coil.

E \( \propto \) dΦ

Because of this magnetic flux, current is flowing through the circuit and if the current flows through the circuit there is some emf which is getting induced in the circuit.

Faraday’s  Second Law

The magnitude of the induced emf in a circuit is equal to the time rate of change of magnetic flux through the circuit.

|E| \( \propto \frac{dΦ}{(dt)} \)

|E| = \( \frac{dΦ}{(dt)} \)

  • dΦ is the change in magnetic flux
  • dt is the change in time
  • the proportional constant = 1

Rate of change of flux= \( \frac{dΦ}{(dt)} \)

According to the Faraday’s law, there would be an induced emf. So, E = \( \frac{dΦ}{(dt)} \)

Lenz’s Law

Faraday’s  law does not give an explanation to the direction of the current. However, the Lenz law specifies the direction of the current induced inside the coil. Let us understand the Lenz law.

Lenz law of electromagnetic induction states that, when an emf induces according to Faraday’s law, the polarity (direction) of that induced emf is such that it opposes the cause of its production. According to Lenz’s law,

E = – \( \frac{dΦ}{(dt)} \)

The negative sign shows that the direction of the induced emf and the direction of change in magnetic fields have opposite signs. Suppose we have a coil and a bar magnet.

The moment we take the bar magnet towards the coil, emf is induced in the coil that is the galvanometer shows deflection. The direction of the induced current will be such that it opposes the motion of the magnet towards the coil.

Questions For You

Q1. Lenz’s law of electromagnetic induction corresponds to the

  1. law of conservation of charge
  2. The law of conservation of energy
  3.  Law of conservation of momentum
  4. The law of conservation of angular momentum

Answer: B. Lenz’s law of electromagnetic induction compounds to the law of conservation of energy.

Q2. Two identical coaxial coils and carrying an equal amount of current in the same direction are brought nearer. The current in

  1. P increases while in Q decreases
  2. Q increases while in P decreases
  3. Both P and Q increases
  4. Both P and Q decreases

Answer: D. Two identical coaxial coils P and Q carrying an equal amount of current in the same direction are brought nearer. The current in both P and Q decreases as per Lenz’s law.

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