Eletromagnetic Induction
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INTRODUCTION
In year 1820 Orested discovered the magnetic effect of current. Faraday gives the thought that reverse of
this phenomenon is also possible i.e. current is also produced by magnetic field. Faraday threw a magnet
in a coil which is connected by a sensitive galvanometer when the magnet passes through the coil the galvanometer
gives instantaneous deflection
Magnetic Flux
The magnetic flux (f) linked with a surface held in a magnetic field (B) is
defined as the number of magnetic field lines crossing that area (A).
If q is the angle between the direction of the field and normal to the
area, (area vector) then f = B.A
r r
= BA cosq
Flux Linkage
If a coil has more than one turn, then the flux through the whole coil is the sum of the fluxes through the
individual turn.
If the magnetic field is uniform, the flux through one turn is f = BA cosq
If the coil has N turns, the total flux linkage f = NBA cosq
Magnetic field lines are imaginary, magnetic flux is a real scalar physical quantity with dimensions
ELECTROMAGNETIC INDUCTION
Michael Faraday demonstrated the reverse effect of Oersted experiment. He explained the possibility of producing
emf across the ends of a conductor when the magnetic flux linked with the conductor changes. This was
termed as electromagnetic induction. The discovery of this phenomenon brought about a revolution in the field
of electric power generation.
FARADAY'S EXPERIMENT
Faraday performed various experiments to discover and understand the phenomenon of electromagnetic
induction. Some of them are :
When the magnet is held stationary anywhere near or inside the coil,the galvanometer does not show any deflection
When the N-pole of a strong bar magnet is moved towards the coilthe galvanometer shows a deflection right to the zero mark.
When the N-pole of a strong bar magnet is moved away from the coilthe galvanometer shows a deflection left to the zero mark
If the above experiments are repeated by bringing the S-pole of the, magnet towards or away from the coil, the direction of current in the coil is opposite to that obtained in the case of N-pole
The deflection in galvanometer is more when the magnet movesfaster and less when the magnet moves slower.
FARADAY'S LAWS OF ELECTROMAGNETIC INDUCTION
Based on his experimental studies on the phenomenon of electromagnetic induction, Faraday proposed the
following two laws.
*First law
Whenever magnetic flux linked with a closed circuit changes, an emf is induced in the circuit. The induced emf
lasts so long as the change in magnetic flux continues.
* Second law
The magnitude of emf induced in a closed circuit is directly proportional to rate of change of magnetic flux
linked with the circuit. If the change in magnetic flux in a time dt is df
LENZ'S LAW
The Russian scientist H.F. Lenz in 1835 discovered a simple law giving the direction of the induced current
produced in a circuit. Lenz's law states that the induced current produced in a circuit always flow in such a
direction that it opposes the change or cause that produced it. If the coil has N number of turns and f is the
magnetic flux linked with each turn of the coil then, the total magnetic flux linked with the coil at any time = Nf
LENZ'S LAW - A CONSEQUENCE OF CONSERVATION OF ENERGY
Copper coils are wound on a cylindrical cardboard and the two ends of the coil are
connected to a sensitive galvanometer. When a bar magnet is moved towards the
coil (fig.). The upper face of the coil near the magnet acquired north polarity.
Consequently work has to be done to move the magnet further against the force of
repulsion. When we withdraw the magnet away from the coil, its nearer face acquires
south polarity. Now the work done is against the force of attraction.When the
magnet is moved, the number of magnetic lines of force linking the coil changes,
which causes an induced current of flow through the coil. The direction of the induced
current, according to Lenz's law is always to oppose the motion of the magent.
The work done in moving the magnet is converted into electrical energy. This energy
is dissipated as heat energy in the coil. Therefore, the induced current always flows
in such a direction to oppose the cause. Thus it is proved that Lenz's law is the
consequence of conservation of energy.
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