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Electromagnetic Induction
Faraday
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Discovered basic principle of
electromagnetic induction
Whenever the magnetic field around a
conductor is moving or changing
magnitude, a current is induced in the
conductor
Torus Ring
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When switch is turned on, a magnetic field is
created in coil A and the entire iron ring
becomes magnetized
Sudden increase in magnetic field causes a
current to momentarily be induced in coil B
Once the field becomes steady in the ring,
induced current no longer exits
When switch is turned off, the sudden
demagnetization causes current to be again
momentarily induced but in opposite direction
Factors Affecting Current
Induced
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Number of loops
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Rate of motion of magnetic field
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More loops, greater current
Faster motion, greater current
Strength of magnetic field
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Stronger field, greater current
Faraday’s Law
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Amount of emf induced is
proportional to:

Rate of change in magnetic field (called
flux)
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Flux is directly proportional to B and A
Unit of flux is the Weber (Wb)
 = BA cos
Number of loops in the wire
Rate of change
 = -N (/t)
Example

A conductive wire consisting of 3 loops
and enclosing an area of .020 m2 is
perpendicular to a uniform magnetic
field of .030T. If the field goes to zero in
.0045sec, what is the magnitude of the
induced emf?
Example

The magnetic flux through a 60 turn
coil of wire is reduced from 35Wb to
5.0Wb in .10sec. The average induced
current is .0036 A, what is the wire’s
resistance?
Direction
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emf acts in direction opposite to the
flux
Induced emf gives rise to current whose
magnetic field opposes original field
Lenz’s Law
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Current flows in a direction such that
the induced field they create opposes
the action of the inducing field
Work done moving a magnetic field
against its opposing force is
transformed into electric energy