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Chapter 21
Electromagnetic Induction
and Faraday’s Law
20.10 Applications: Motors
An electric motor
takes advantage of
the torque on a
current loop, to
change electrical
energy to
mechanical energy.
20.10 Applications: Motors
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There are 2 types of motors: DC &
AC
When a current is passed through
a wire loop (armature) in a
magnetic field, there is a magnetic
force on armature, which makes a
180o turn
In order to make 360o rotation,
split-ring commutator (attached to
armature) allows current to change
direction in armature (due to
contact with brushes)
A change in direction of current
means magnetic force in opposite
direction and thus a further 180o
turn
When current supplied to
commutator is AC, commutator
has two separate parts (slip ring
commutator), each connected to
one end of armature
Thus there is a reversal of current
supplied by voltage source (no
need for split-ring commutator)
Use 3rd Right Hand Rule for
determining direction of Fm
DC Motor
Split-ring commutator
20.10 Applications: Motors
AC Motor
Slip-ring commutator
21.1 Induced EMF
Note: EMF or Electromotive Force (Ɛ) is actually a voltage produced by a battery
or magnetic field
Faraday observed that a steady current in X produced no
current in Y. When the current was starting or stopping in X,
current was produced in Y. He concluded only a changing
magnetic field can produce an electric current. This is
called an induced current.
21.2 Faraday’s Law of Induction;
Lenz’s Law
The induced emf in a wire loop is proportional
to the rate of change of magnetic flux (Ф)
through the loop.
21.2 Faraday’s Law of Induction;
Lenz’s Law
The magnetic flux proportional to the total
number of lines passing through the loop.
21.2 Faraday’s Law of Induction;
Lenz’s Law
Faraday’s law of induction:
[N loops]
If the magnetic flux through a coil(s) of wire changes
with time, an emf is induced in the coil(s). The
magnitude of the induced emf equals the time rate of
change of the magnetic flux through the loop times
the number of loops, N, in the coil.
21.2 Faraday’s Law of Induction;
Lenz’s Law
The minus sign gives the direction of the
induced emf:
An induced emf always gives rise to a current
whose magnetic field opposes the original
change in flux (Lenz’s Law).
Lenz’s Law is the ‘electromagnetic’ version of the Law
of Conservation of Energy
In (a) the magnetic field and flux are increasing. The current moves in
the direction to oppose that – to decrease the magnetic field. In (b) the
magnetic field and flux are decreasing. Again, the current moves in the
direction to oppose that. In (c) there is no change in flux, so there is no
induced emf.
Induced EMF
Therefore, a changing magnetic field induces
an emf.
(Faraday’s experiment used a magnetic field
that was changing because the current
producing it was changing; the previous
graphic shows a magnetic field that is
changing because the magnet is moving.)
21.2 Faraday’s Law of Induction;
Lenz’s Law
Magnetic flux will change if the area of the
loop changes:
Current increases in the direction shown (clockwise) to maintain
original flux.
21.2 Faraday’s Law of Induction;
Lenz’s Law
Magnetic flux will change if the angle between
the loop and the field changes:
Flux is decreasing so the current will go in the clockwise direction to
increase flux.
21.5 Electric Generators
A generator is the opposite of a motor – it
transforms mechanical energy into electrical
energy. This is an AC generator:
The axle is rotated by an
external force such as
falling water or steam.
The brushes are in
constant electrical
contact with the slip
rings.
21.5 Electric Generators
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There are 2 types of generators:
AC & DC
A wire loop rotated in MF by
external means (i.e. crank)
As loop rotates, motion of wire
loop changes in the magnetic field
and thus an induced current is
produced
The end of loop connected to slipring commutator
Connections to external circuit
made by stationary brushes in
contact with commutator
When loop is 1/2 way through
rotation in MF, the current flows
one direction
When completing other 1/2
rotation, the current flows in
opposite direction producing
alternating current (AC)
For direct current (DC) generation,
split-ring commutator used to
produce pulsating (or rectified)
direct current
AC Generator
21.5 Electric Generators
DC Generator
21.7 Transformers and Transmission
of Power
A transformer is a
device that changes
the magnitude of an
AC voltage using EM
induction.
This is a step-up
transformer – the emf
in the secondary coil
is larger than the emf
in the primary:
21.7 Transformers and Transmission
of Power
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Transformers consist of 2 coils of wire
wound around a core of soft iron
The coil connected to input AC voltage
source (primary) has N1 turns
The 2nd coil connected to a resistor
(secondary) has N2 turns
If N2 > N1, then V2 > V1 step-up
transformer
If N2 < N1, then V2 < V1  step-down
transformer
21.7 Transformers and Transmission
of Power
Transformers work only if the current is
changing; this is one reason why electricity
is transmitted as ac.