To understand the generated EMF, we have a rule called Lenz
rule. This is a fundamental rule and it works basing on law of conservation of
energy.
According to the Lenz law, a changing magnetic field produces a induced
EMF and induced current in a closed coil. This induced EMF opposes the change
in the magnetic flux that creates it.
If we are moving a magnet towards a closed coil, it generates
a change in the magnetic field. If you are moving a north pole towards the
coil, the current is generated in the closed coil such that it generates
another north pole on the face of magnet coming towards it with North Pole so
that it opposes the change.
If we are getting south pole of a magnet towards a closed coil,
the induced current in that closed coil intern generate another magnetic field
with south pole so that it opposes the change in the magnetic flux.
Thus it totally tells us that the induced emf and induced
current opposes the change in the magnetic flux that is causing it and that can
be shown below in the diagram.
If a bar magnet is coming towards a current carrying coil in
the perpendicular plane, an induced EMF is generated. The generated potential
difference always oppose the change in the magnetic flux. Thus when north pole
of the bar magnet is approaching it, north pole is formed over the surface and
anti clock wise current is generated. Even when the north pole is going away
from the coil, to oppose that change a south pole is formed on the face of the
coil and hence a clock wise induced current is generated there. In that sense,
the magnet always experience a acceleration less than the acceleration due to
gravity.
If the coil is not a closed coil, there is no induced EMF and
hence, the magnet falls through it with an acceleration equal to acceleration due to gravity.
The magnitude of induced EMF varies even with the variation
of the temperature of the coil. If the temperature increases, the resistance of
the coil decreases and hence induced emf increases.
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