Lecture 9: Electromagnetic Induction Ch' 22

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Lecture 9 Electromagnetic Induction (Ch. 22)

• 04-13-07

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Induced EMF and Induced Current
Relative motion between the magnet and the coil
is needed to generate a current it does not
matter which one moves.
The current in the coil is called an induced
current because it is brought about (or
induced) by a changing magnetic field. Since a
source of emf (electromotive force) is always
needed to produce a current, the coil itself
behaves as if it were a source of emf. This emf
is known as an induced emf. Thus, a changing
magnetic field induces an emf in the coil, and
the emf leads to an induced current.
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Induced EMF and Induced Current
Cruise Control System
An emf can be induced by changing the area of a
coil in a constant magnetic field. Here the shape
of the coil is being distorted so as to reduce
the area. As long as the area is changing, an
induced emf and current exist they vanish when
the area is no longer changing. If the distorted
coil is returned to its original shape, thereby
increasing the area, an oppositely directed
current is generated while the area is changing.
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Motional EMF EMF induced in a moving conductor
RHR 1
When a conducting rod moves at right angles to a
constant magnetic field, the magnetic force
causes opposite charges to appear at the ends of
the rod, giving rise to an induced emf. The
induced emf causes an induced current I to appear
in the circuit.
The separated charges on the ends of the moving
conductor give rise to an induced emf, called a
motional emf because it originates from the
motion of charges through a magnetic field. The
emf exists as long as the rod moves. If the rod
is brought to a halt, the magnetic force
vanishes, with the result that the attractive
electric force reunites the positive and negative
charges and the emf disappears.
Electric Force
Magnetic Force
Example 1
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Motional EMF and Electrical Energy
Motional emf arises because a magnetic force acts
on the charges in a conductor that is moving
through a magnetic field. Whenever this emf
causes a current, a second magnetic force enters
the picture See RHR-1
Violates Conservation of energy
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Magnetic Flux Motional EMF and Magnetic Flux
Motional emf, as well as any other type of
induced emf, can be described in terms of a
concept called magnetic flux. Magnetic flux is
analogous to electric flux, which deals with the
electric field and the surface through which it
passes.
The quantity BA is given the name magnetic flux
and is represented by the symbol   (Greek
capital letter phi) thus        . The
magnitude of the induced emf is the change in
flux              divided by the time interval
           during which the change occurs
General Expression for Magnetic Flux
UNIT Weber 1 Wb  1 T  m2
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Graphical interpretation of magnetic flux
Example 4
The magnitude of the magnetic field in (a) is
three times as great as that in (b), because the
number of magnetic field lines crossing the
surfaces is in the ratio of 31.