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Electromagnetic Induction

We have learnt that an electric current produces magnetic field. The reverse is also possible, i.e., we can use a magnetic field to produce an electric current.
This was discovered independently in the year 1831 by Michael Faraday in England and Joseph Henry in America. Both discovered that electric current can be produced in a wire, which is in the form of a loop or coil, by simply moving a magnet in and out of it. No other voltage source is required. The same effect is observed if a part of a wire loop is moved through the magnetic field of a magnet.
The phenomenon of inducing an emf by changing the magnetic field in a coil is called electromagnetic induction. The current produced is called the induced current. The emf responsible for the induced current is called the induced emf. Electric generators, which are used to produce electricity, work on this principle.
Faraday’s Laws of Electromagnetic Induction
First Law
Whenever there is a change in magnetic flux associated with a closed circuit, an emf is induced, and hence current flows in the circuit.
Second Law
The magnitude of the induced emf is directly proportional to the rate of change of magnetic flux associated with the circuit.
Lenz’s Law
This law gives the direction of the induced current. ‘The induced emf drives an induced current in such a direction as to oppose the change responsible for its production’.

Applications of Electromagnetic Induction

Electric Generator An electric generator works on the principle of electromagnetic induction. A coil of insulated wire is rotated with a uniform speed in the uniform field of a permanent magnet. Since the coil cuts the magnetic flux lines in the field, an emf is induced, and hence current flows in it. The direction of the induced current is determined by Fleming’s right-hand rule. The strength of the induced current depends on
  1. the number of turns in the coil,
  2. the cross-sectional area of the coil and
  3. the speed of rotation of the coil.
In an AC generator, the induced current continuously varies both in direction and magnitude. It alternates its direction of flow twice during every rotation of the coil. The induced current in a DC generator is unidirectional, i.e., it flows in the same direction as long as the coil rotates.
Transformers A transformer is a device that enables us to increase (step up) or decrease (step down) the magnitude of an alternating emf or voltage.
For a transformer, the input and output current and voltages are related as
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where Es is the voltage produced in the secondary coils, Ep is the voltage in the primary coil, Np is the number of turns of primary coil, Ns is the number of turns of secondary coil, Is is the current in the secondary coil and Ip the current in the primary coil.

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