Physics - Lenz's law of Electromagnetic Induction

LENZ LAW OF ELECTROMGNETIC INDUCTION

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It was discovered by a Russian scientist H.F.Lenz in 1835. He discovered a simple law giving the direction of the induced current produced in the circuit.
The law states that the induced current produced in the circuit always flows in such a direction that opposes the change or cause that produces it.
If the coil has N number of turns φ is the magnetic flux linked with each turn of the coil then the total magnetic flux linked with the coil at any time is Nφ.
e = -d(Nφ)/dt
e = -Ndφ/dt
e = N(φ2-φ1)/t

LENZ’S LAW IS A CONSEQUENCE OF CONSERVATION OF ENERGY.

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Experiment to prove Lenz’s law

Requirements

 1. Copper coils are wound on a cylindrical cardboard.
 2. The two ends of the coil connected to the sensitive galvanometer.
 3. The magnet.

OBSERVATION

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 1. The magnet is moved towards the coil.
 2. Upper face of the coil acquires north polarity.
 3. So work has to be done to move the magnet further against the force of repulsion.
 4. When we withdraw the magnet away from the coil it acquires south polarity.
 5. Now the work is done against force of attraction.
 6. When the magnet is moved, the number of magnetic lines of force linking the coil changes, which causes induced current to flow through the coil.
 7. The direction of the induced current according to Lenz’s law is always opposite to the motion of magnet.
 8. The work done in moving the magnet is converted into electrical energy.
 9. This energy is dissipated as heat energy in the coil.
 10. If on the contrary, the direction of the current were to help the motion of the magnet, it would start moving faster increasing the change of magnetic flux linking the coil.
 11. This results in increase of induced current.
 12. Hence kinetic and electrical energy would be produced without any external work.
 13. But it is impossible.
 14. Therefore induced current always flows in such a direction to oppose that cause.
 15. Thus it proved Lenz’s law is the consequence of conservation of energy.
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