Electromagnetic Induction
Create electric current from changing
magnetic fields
Objectives
Explain how a changing magnetic field
produces an electric current
Define electromotive force and solve
problems involving wires moving in a
magnetic field
Describe how an electric generator works
and how it differs from a motor
Recognize the difference between peak and
effective voltage and current
Objectives
State Lenz’s Law and explain “back-EMF”
and how it affects the operation of motors and
generators
Explain self-inductance and how it affects
circuits
Describe a transformer and solve problems
involving voltage, current, and turns ratios
Electromagnetism
Force on a moving charged particle in a
magnetic field
Force on a current-carrying wire in a
magnetic field
Voltage (and current) induced in a wire
forced to move in a magnetic field
Voltage (and current) induced by a
changing magnetic field
Basis of motors and generators, and
electromagnetic waves
Electromagnetic Induction
Faraday’s Law
Lenz’s Law
The AC Generator
The DC Generator
Back EMF in a Motor
The Transformer
Faraday’s Law
Faraday’s Law:
Moving a conductor through magnetic flux lines causes an emf
to be induced in the conductor.
• Relative motion between a conductor and a
magnetic field induces an emf in the conductor.
• The direction of the induced emf depends on the
direction of motion of the conductor with respect
to the field.
• The magnitude of the emf is directly
proportional to the rate at which magnetic flux
lines are cut by the conductor.
• The magnitude of the emf is directly
proportional to the number of turns of the
Textbook, Pg. 582
conductor crossing the flux lines.


 N
t
Induced Voltage
Voltage induced in a wire moving
perpendicular to a magnetic field
E = induced voltage
(electromotive force)
B = magnetic field
strength
L = length of wire
v = velocity of wire
moving perpendicular to
magnetic field
E  BLv
Induced Voltage
EMF Induced by a Moving Wire
An emf is induced in a wire that is
moving perpendicular to a magnetic
field.
Where:
 = the induced
emf
B = strength of the
magnetic field
l = length of the
wire
v = velocity of
motion
q = angle of the
wire relative to
the flux lines
  Blvsin q
Example
A straight wire 0.20 m long moves at a constant speed of 7.0 m/s
perpendicular to a magnetic field of strength 8.0 x 10-2 T
What EMF is induced in the wire?
The wire is part of a circuit that has a resistance of 0.50 Ω. What is
the current through the wire?
Known:
v = 7.0 m/s
L = 0.20 m
B = 8.0 x 10-2 T
R = 0.50 Ω
Unknown:
EMF = ? (V)
I = ? (A)
EMF  BLv
c
hb ge j
 8.0 x102 T 0.20m 7.0 m s
 011
. V
Textbook, Pg. 584-5
V EMF
Practice Problems 1-4
I 
R
R
011
. V

 0.22 A counterclockwise
0.50
The AC Generator
An electric generator converts mechanical energy into electric
energy.
The three main
components are:
a field magnet
an armature
slip rings with brushes
If the armature is rotating
with a constant angular
velocity in a constant
magnetic field, the
magnitude of the induced
emf and current varies
sinusoidally with respect to
time.
 inst   maxsin 2ft
iinst  imaxsin 2ft
“Effective” Current & Voltage
Power is the product of voltage and current
Both vary with an alternating current
Average power is half the maximum
“Effective” current and “effective” voltage
Equivalent constant voltage and current that would
generate the same average power
Veff2
Pavg  1 2 Pmax  I eff2 R 
R
2
I eff2 R  1 2 I max
R  I eff  1 2 I max
I eff  0.707 I max
Likewise, Veff  0.707Vmax
Textbook, Pg. 588-9
Practice Problems 5-8
The DC Generator
A simple ac generator can be
converted easily to a dc generator
by substituting a split-ring
commutator for the slip rings.
Lenz's Law
Lenz’s Law: An induced current will flow in such a direction
that it will oppose by its magnetic field the motion of the
magnetic field that is producing it.
Fleming’s Rule: If the thumb,
forefinger, and middle finger of the
right hand are held at right angles
to each other, with the thumb
pointing in the direction in which
the wire is moving and the
forefinger pointing in the field
direction (N to S), the middle finger
will point in the direction of induced
conventional current.
Textbook, Pg. 590
Back EMF in a Motor
• In an electric motor, a magnetic torque turns a currentcarrying loop in a constant magnetic field.
• A coil rotating in a magnetic field induces an emf that opposes
the cause that gave rise to it.
• Thus, every motor is also a generator.
According to Lenz's law,
such an induced emf must
oppose the current
delivered to the motor. For
this reason, the emf induced
in a motor is called back
emf or counter emf.
The Transformer
The basic parts of a simple
transformer are:
A primary coil connected
to an ac source.
A secondary coil
connected to a load.
primary voltage
primary turns

A soft iron core.
secondary voltage secondary turns
A transformer that produces a higher secondary
voltage is called a step-up transformer.
 p Np

 s Ns
A transformer that produces a lower secondary
voltage is called a step-down transformer.
Textbook, Pg. 594
New Terms
induced emf
ac generator
dc generator
electromagnetic induction
back emf
step-up transformer
step-down transformer
transformer efficiency
Equations


 N
t
  Blvsin q
 inst   maxsin 2ft
iinst  imaxsin 2ft
primary voltage
primary turns

secondary voltage secondary turns
 p Np

 s Ns
Summary
Michael Faraday discovered that if a
wire moves through a magnetic field, an
electric current can flow
The current produced depends upon the
angle between the velocity of the wire and
the magnetic field
Maximum current occurs when the wire is
moving at right angles to the field
Electromotive force (EMF) is the potential
difference created across the moving wire
EMF is measured in volts
Continued on next slide
Summary
The EMF in a straight length of wire moving
through a uniform magnetic field is the
product of the magnetic field (B), the length of
the wire (L), and the component of the
velocity of the moving wire (v), perpendicular
to the field
A generator and a motor are similar devices
A generator converts mechanical energy to
electrical energy
A motor converts electrical energy to mechanical
energy
Continued on next slide
Summary
Lenz’s Law states that an induced
current is always produced in a
direction such that the magnetic field
resulting from the induced current
opposes the change in the magnetic field
that is causing the induced current
Self-inductance is a property of a wire
carrying a changing current
The faster the current is changing, the
greater the induced EMF that opposes that
change
Continued on next slide
Summary
A transformer has two coils wound
about the same core
An AC current through the primary coil
induces an alternating EMF in the
seondary coil
The voltages in alternating-current
circuits may be increased of decreased by
transformers
Credits
•Physics, Sixth Edition
•Paul E. Tippens
•Physics: Principles and Problems
•Paul W. Zitzewitz