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Date Submitted: 03/29/2016 01:43 AM
7. Faraday’s Law
Electromagnetism
1
7. FARADAY’S LAW
7.1 Faraday’s Law of Induction
7.2 Motional emf
7.3 Lenz’s Law
7.4 Induced emf and Electric Fields
7.6 Generators and motors
7.5 Eddy Currents
Electromagnetism
2
7.1 Faraday’s Law of Induction
• Consider
a
bar
magnet places near a
wire loop connected
to a voltmeter. The
voltmeter deflects
when the magnet is
moved towards or
away from the coil.
However, when the
magnet is stationary,
there
is
no
deflection.
Electromagnetism
3
7.1 Faraday’s Law of Induction
• There
is
an
induced
electromotive force (emf)
when there is a relative
motion between the magnet
and the coil.
• The relative motion gives rise
to a changing magnetic field
in the loop, which leads to the
induced emf. This is known
as electromagnetic induction.
• When
the
magnet
is
stationary, there is no change
in the magnetic field through
the loop with time, hence
there is no induced emf.
Electromagnetism
4
7.1 Faraday’s Law of Induction
• An emf is induced in a circuit
when the magnetic field
through the circuit changes
with time.
• The effect is described by the
Faraday’s law of induction:
the emf induced in a circuit is
directly proportional to the
time rate of change of the
magnetic flux through the
circuit.
d B dA
dB
dt
dt
Electromagnetism
5
7.1 Faraday’s Law of Induction
• Induced emf , ε, is in the unit of volt (V).
• An emf can be induced in the circuit in several ways:
a) Change |B | with time.
b) Change the area A enclosed by the loop with time.
c) Change the angle θ between the vectors B and A with time.
d) Any combination of the above.
• No emf is produced when dΦB/dt = 0.
• If the circuit consists of N identical loops, the total induced emf
is given by
d B
N
dt
Electromagnetism
6
7.1 Faraday’s Law of Induction
• Example: A coil consists of 200 turns of wire. Each turn is a
square of side 18 cm, and a uniform magnetic field directed
perpendicular to the plane of the coil is turned on. If the field...