ELECTROMAGNETIC INDUCTION- LAWS OF ELECTROMAGNETIC INDUCTION

DEFINITION
In 1830 Joseph Henry and Faraday observed that an E.M.F. is setup in a coil placed in a magnetic     field whenever the flux through the coil changes. This effect is called Electromagnetic Induction.     If the coil forms a part of a close circuit, the E.M.F. causes a current to flow in the circuit.    E.M.F. setup in the coil is called "induced E.M.F" and the current thus produced is termed as     "Induced Current".    Experiments show that the magnitude of E.M.F. depends on the rate at which the flux through     the coil changes. It also depends on the number of turns on the coil.    There are various ways to change magnetic flux of a coil such as;    (1) By changing the relative position of the coil with respect to a magnet.    (2) By changing current in the coil itself.    (3) By changing current in the neighbouring coil.    (4) By changing area of a coil placed in the magnetic field etc.
FARADAY’S LAW OF ELECTROMAGNETIC INDUCTION
Faraday was the first scientist who performed a number of experiments to discover the facts and    figures of electromagnetic induction, he formulated the following law:    When magnetic flux changes through a circuit, an emf is induced in it which lasts only as long as the    change in the magnetic flux through the circuit continues.    Quantitatively, induced emf is directly proportional to the rate of change of magnetic flux through     the coil. i.e.
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Average emf = -NDf/Dt                   Where N = number of turns in the coil.    The negative sign indicates that the induced current is such that the magnetic field due to it opposes    the magnetic flux producing it.
LENZ’S LAW
Lenz’s law infact describes that in order to produce an induced emf or induced current some external    source of energy must be supplied otherwise no current will induce.    Lenz’s law states that"    "The direction of induced current is always such as to oppose the cause which produces it".    That is why a –ve sign is used in Faraday’s law.
EXPLANATION
Consider a bar magnet and a coil of wire.
When the N-pole of magnet is approaching the face of the coil, it becomes a north face by the induction of current in anticlockwise direction to oppose forward motion of the magnet. b. When the N-pole of the magnet is receding the face of the coil becomes a south pole due to a    clockwise induced current to oppose the backward motion.