Magnetic Fields - Review
Magnetic fields are produced by
•
magnetised magnetic materials such as iron
•
electric currents
I
Magnetic Fields - Review
Magnetic Fields - Review
A magnetic material contains magnetised regions called domains
• if the magnetic domains are randomly oriented, the material is not
magnetised
• if the magnetic domains become aligned, for example due to an
external magnetic field, the material becomes magnetised
Magnetic Fields - Review
The magnitude of the magnetic field produced by an electric
current depends on
• the magnitude of the current - the greater the current, the
stronger the magnetic field
• the distance from the conductor - the greater the distance
from the wire, the weaker the magnetic field
• the shape into which the conductor is formed - e.g. a coil
Magnetic Fields - Review
Magnetic fields are represented by magnetic lines of force
•
direction of the field - indicated by an arrow pointing in the
direction in which the north pole of a magnet points in the
field
•
magnitude of the field indicated by the spacing
of the field lines - closer
spacing represents a
stronger field
Magnetic Fields - Review
Magnetic fields are represented by
• arrows - closer spacing  stronger B field
X X X X
• crosses – representing a field into the page
• dots – representing a field out the page
X X X X
X X X X
Magnetic Fields - Review
Bar magnet
Straight conductor
Solenoid
Magnetic Fields - Review
Remember the directions using
the right hand grip rule - but
NEVER quote this rule in the
exam - it is just a memory aid!
Straight conductor
Magnetic Fields - Review
Remember the directions using
the right hand grip rule - but
NEVER quote this rule in the
exam - it is just a memory aid!
Animations - see
RHRule.avi
MagneticFieldWire.avi
Magnetic Fields - Review
Magnetic Fields - Review
Magnetic Fields - Review
Magnetic Fields - Review
The magnetic field “close to” the end of a bar magnet is uniform
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Magnetic Fields - Review
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The magnetic field inside, and “close
to” the ends of a solenoid is uniform
Background - the cathode ray tube
A cathode ray tube (CRT) is a
highly evacuate glass tube
containing a source of electrons
(cathode), and in which there is a
strong electric field created by a
high voltage between the cathode
and a positive electrode (anode)
at the opposite end of the tube.
Electrons travelling in straight
lines through the vacuum, are
accelerated from the cathode to
the anode by an electric field, E
e
E
Background - the cathode ray tube
A beam of electrons is called a cathode ray. Cathode rays are not visible,
since electrons neither reflect or emit light under these conditions.
A demonstration cathode ray tube contains a sloping phosphorescent
screen, which emits light when electrons strike it, making the path of the
electrons visible.
Background - the cathode ray tube
Cathode ray tubes (CRTs) are a key component in devices including
Television receivers
Computer monitors
Cathode ray oscilloscopes (CROs)
Medical monitors and other scientific equipment based on the CRO
Moving charges experience a force
Electric charges moving in a magnetic field experience a force
except when they move parallel to the magnetic field
The force is a maximum when the
charge moves perpendicular to
the magnetic field - in this case
the magnetic field is into the page
The force is perpendicular to both
the velocity direction and the
magnetic field direction
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+ X X
B X vX X
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F
Moving charges experience a force
The magnitude of the force depends on
•
the magnitude of the charge
•
the velocity of the charge
•
the magnitude of the field
•
the angle between the direction of the field and velocity
Moving charges experience a force
Electric charges moving in a magnetic field experience a force
except when they move parallel to the magnetic field
This effect is called the motor effect
It is the principle underlying the operation of
•
cathode ray tubes (used in TVs and computers)
•
electric motors and generators
•
loudspeakers
First-hand investigation of the motor effect
The motor effect can be demonstrated
by placing a magnet near a cathode ray
so that the field of the magnet is
perpendicular to the velocity of the
cathode rays
X X X
X X X
The observed result, which
demonstrates the motor effect is the
deflection of the cathode ray, in a
direction perpendicular to the
magnetic field and to the direction of
the cathode ray velocity
X X X
First-hand investigation of the motor effect
The motor effect can be
demonstrated by placing a magnet
near a current carrying wire so that
the magnetic field is perpendicular
to the direction of the current flow in
the wire
The observed result, which
demonstrates the motor effect is the
deflection of the wire, in a direction
perpendicular to the magnetic field
and to the direction of the current
X X X
X X X
X X X
e
deflection
Moving charges experience a force
The force on a charge moving in a magnetic field is at right
angles to
• the velocity of the particle
• the magnetic field
A constant magnitude force,
which is always perpendicular
to the velocity of a particle,
results in the particle travelling
in a circular path
F = qvB
This equation is not in the syllabus
Motion of charges in the Van Allen belts
Charged cosmic rays encountering the magnetic field of the
Earth experience a magnetic force, trapping them in regions
called the Van Allen radiation belts
Van Allen radiation belts
Motion of charges in the Van Allen belts
The spiralling paths of the charged cosmic rays is a result of the
particles having components of their motion parallel to the
Earth’s magnetic field, which is unaffected by the field, and
perpendicular to the field, which causes the particles to travel in
circular paths. The combined effect is a spiralling path.
Motion of charges in the Van Allen belts
High energy charged particles interact with the Earth’s
atmosphere at high latitudes (polar regions) to produce auroras.
Moving charges experience a force
A force is produced on a current carrying conductor in a
magnetic field, except when the conductor is parallel to the field.
The magnitude of the force depends on and is directly
proportional to
•
•
•
•
q
the magnitude of the current, I
the magnitude of the magnetic field, B
F = BIl
the length of the conductor, l in the field
the sine of the angle between the field and the conductor
B
If the wire is at an angle to the
field, the force is reduced by
I a factor of sin(q)
F = BIl sin(q)
Moving charges experience a force
Calculate the maximum force on a conductor of length 5 cm in a
magnetic field with an intensity of 2 x 10–4 T when the current in
the wire is 200 milliamperes.
The maximum force is exerted when the conductor is
perpendicular to the field, and is give by the expression
F = BIl
Write the equation first!
F = 2 x 10–4 x 200 x 10–3 x 5 x 10–2
F = 2 x 10–6 newtons
The force is perpendicular to the current and the field directions
Moving charges experience a force
What is the direction of the force on the wire?
How could the force be doubled without altering the length of the wire?
Note first
The force is perpendicular to the current and the field directions
Therefore it must be either into or out of the page
Use whatever aid to memory you have decided to use…
The force is into the page
Since F = BIl, doubling either the current or the magnetic field strength
would double the force on the wire.
Force between current carrying conductors
A force is produced between two
parallel current carrying conductors
I
X XX . . .
. .
B X XX
in
X XX . .
force
I
B
. out
.
force
The force is a force of repulsion
when the currents are in the
opposite directions
F
II
k 1 2
l
d
k = 2 x 10–7 NA–2
Force between current carrying conductors
A force is produced between two
parallel current carrying conductors
force
X XX . .
. .
B X XX
in
X XX . .
I
.
force
B
. out
.
I
The force is a force of attraction when
the currents are in the same direction
F
I1 I2
k
l
d
k = 2 x 10–7 NA–2
Force between current carrying conductors
A force is produced between two parallel current carrying conductors
The magnitude of the force between the conductors is
• proportional to the magnitude of the currents in each wire
• inversely proportional to the distance between the wires
• dependent on the magnetic properties of the medium
between the wires
F
I1 I2
k
l
d
The medium between the wires
determines the value of the constant
k = 2 x 10–7 NA–2 in air or a vacuum
Force between current carrying conductors
A force is produced between two parallel
current carrying conductors
F
II
k 1 2
l
d
• The force is a force of repulsion
when the currents are in the
opposite directions (a)
• The force is a force of attraction when the currents are in the
same direction (b)
Moving charges experience a force
Calculate the force between two straight conductors separated
by a distance of 1.5 cm with a common length of 35 cm between
them when the current in one wire is 200 milliamperes and the
current in the other is in the opposite direction, with a magnitude
of 5000 microamperes.
F
II
k 1 2
l
d
Write the equation first! Then substitute values…
F
2x10 –7 x200x10 –3 x5000x10 –6

0.35
1.5x10–2
F = 4.7 x 10–9 newtons
The force is perpendicular to the current and the field directions
Moving charges experience a force
Calculate the force between the side of a square coil consisting
of 20 turns carrying a current of 2 A and a straight conductor
sharing a common length of 25 cm and carrying a current of 3 A
if the distance between them is 2 cm.
F
II
k 1 2
l
d
Write the equation first! Then substitute values…
F
2x10 –7 x2x3x20

0.25
2x10 –2
F = 3 x 10–4 newtons
The force is perpendicular to the current and the field directions
Moving charges experience a force
Q1. What is the force between two parallel conductors carrying currents in
opposite directions one centimetre apart if the current in one is 10
amperes, in the other is 5 amperes and the common length is 2 m?
(Ans. 0.002 N, repulsion)
Q2. If the distance between the wires was increased to 2 cm, what would
be the new force between the wires? (use the fact that force and
separation are inversely proportional)
F
I1 I2
k
l
d
Write the equation first! Then substitute values…
F 2x10 –7 x10x5

2
1x10 –2
F = 0.002 newtons
The force is one of repulsion
Torque
syllabus
A torque is a force which acts to produce a rotational effect or a moment.
The magnitude of a torque, 
depends on the
• magnitude of the force, F
• distance of the force from the point of rotation, d
  Fd
The motor effect - force on a current-carrying wire
syllabus
Consider a wire carrying a current across a magnetic field, B as shown
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The force on the moving charges in the wire is into the page [don’t say “down”]
This produces a resulting force on the wire that is also into the page
A wire carrying a current in a magnetic field experiences a force due to the
movement of charges in the wire.
The net result is called the motor effect
Torque on a current-carrying loop
syllabus
A rectangular loop of wire carrying a current can be placed in a magnetic
field so that the force on opposite sides of the loop results in a turning force
about an axis between the two sides.
The force on side WZ is into the page
[never say “down” - it is ambiguous]
The force on side XY is out of the page
Q
P
[never say “up” - it is ambiguous]
The net result is a pair of moments
creating a torque which, given a
suitable mechanical arrangement,
may result in the loop’s rotation
about the axis PQ
  Fd
Torque on a current-carrying loop
syllabus
  Fd
The force is measured in newtons
The distance is measured in metres
The torque is therefore in …
newton metres (N.m)
P
Q
Features of a DC electric motor
A coil on which a torque is produced by the interaction of a current and a magnetic field can
be arranged, with other components, to produce an electric motor
A current-carrying coil
A magnetic field
A commutator
A brush
A DC current source
The current in the loop produces a torque on the loop, causing it to rotate
A DC electric motor converts electrical energy to mechanical energy
The importance of the commutator
The coil in position (a) experiences a maximum torque.
The torque causes rotation,
clockwise viewed from above
At the position (b), no torque
is produced
Inertia carries the coil past the position shown in (b) and the commutator
reverses the current direction so that the torque direction remains the same
and the coil continues to rotate.
syllabus
The importance of the commutator
Figure (a) viewed from above
.
x
B
The torque produced causes
the coil to rotate clockwise
.
B
x
In position (b) there is no torque
because there is no current
x
B
.
Inertia causes the coil to rotate past
the position (b) and the commutator
reverses the current in the coil
Principle of a DC electric motor
syllabus
The commutator in an electric motor is the moving component of a motor,
which provides an electrical contact between the external circuit supplying
the energy to the motor and the rotating armature of the motor. Contact is
achieved through brushes made of carbon, which make contact with the
commutator via a smooth contoured surface matching the brushes to the
commutator.
Principle of a DC electric motor
Commutator
This photograph shows the commutator of a motor made using many coils
The use of many coils, each
in a different plane, results in
a more uniform torque being
produced.
The invention of the
commutator was important for
the development of electric
motors, since it is the device
that allows the current in the
coil to be reversed every
180° so that the torque is
always in the same direction.
Principle of a DC electric motor
A DC electric motor converts electrical energy to mechanical energy
Principle of a DC electric motor
• A DC motor’s operation is based on the principle that a current
carrying conductor placed in, and at right angles to, a
magnetic field tends to move in a direction perpendicular to
the magnetic lines of force
• A rectangular coil of wire placed in a magnetic field such that
two sides of the coil always carry a current perpendicular to
the field will experience a torque due to the forces produced
on the sides - the torque causes the coil to rotate
• A DC motor is similar in construction to a DC generator
• A DC motor may be made to act as a DC generator by
mechanically turning the coil in the field (a DC generator is a
DC motor operating “in reverse” - the energy transformation is
reversed)
Principle of a DC electric motor
DC Motor
syllabus
Components of a DC electric motor
DC Motor
Making a DC electric motor
Practical Exercise - Constructing a DC Motor
Motor effect and loudspeakers
A loudspeaker converts electrical energy to mechanical energy.
Alternating current in the coil produces a force that moves the
speaker cone correspondingly. It uses the motor principle.
Research
Motor effect and loudspeakers
Because of the inertia of the speaker
cone, speakers have to be designed
differently to reproduce different
frequency sounds.
The more rapidly the speaker cone
must vibrate, the lower the mass must
be so that the force produced by the
motor effect on the coil can change the
motion of the cone very rapidly.
Research
The centre-reading galvanometer
A centre-reading galvanometer is a very sensitive DC ammeter.
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I
The zero marker on the scale is
in the centre, and current
readings can be either positive
or negative.
Such meters are built using the
convention that if the meter
produces a positive reading
(pointer deflects to the right),
then the current is flowing into
the positive terminal of the
meter.
Motor effect and galvanometers
spiral spring
A galvanometer operates on the same principle as an electric motor
A current in the coil in a magnetic field produces a torque on the coil, which rotates
Equilibrium is achieved by having a spiral spring producing an opposing torque
Research
Motor effect and galvanometers
spiral spring
The torque on the current carrying coil is proportional to the current in the coil
Research
Motor effect and galvanometers
The torque on the current
carrying coil is proportional to
the current in the coil
The torque turns the coil to
which the pointer is attached
Equilibrium is reached, and the
measurement can be
recorded, when the opposing
torque of the spiral spring
equals that of the
electromagnetically produced
torque on the coil
Research
Faraday’s discovery of the generation of electric current
Background
In 1820, Oersted discovered that an
electric current in a wire produced a
force on a compass needle placed
near the wire… thus establishing a
connection between electricity and
magnetism.
Michael Faraday discovered that a
force was exerted on a current
flowing in a conductor in a
magnetic field [1821].
This is the principle behind the
operation of all electric motors.
Faraday’s discovery of the generation of electric current
The credit for generating electric
current on a practical scale goes
to the famous English scientist,
Michael Faraday.
Faraday was greatly interested
in the invention of the
electromagnet, but his brilliant
mind took earlier experiments
still further.
If electricity could produce
magnetism, why couldn't
magnetism produce electricity?
Michael Faraday
Faraday’s discovery of the generation of electric current
While carrying out investigations on how a current in one coil
could produce a current in another coil, Faraday discovered that a
permanent magnet moved in and out of a coil produced an
electric current in the coil.
The current flows in one
direction as the magnet is
pushed into the coil, and
in the opposite direction
when the magnet is pulled
out of the coil.
syllabus
Magnetic field strength and flux density
Seeing the pattern made visible by sprinkling iron filings around
the magnet...
Faraday developed the
model that magnets
were surrounded by
lines of force.
Cutting the lines of force
with a coil produced a
current.
Faraday deduced that the current produced by a magnet inserted
into a coil was greater if there was a greater number of lines of
force were cut by the coil.
Magnetic field strength and flux density
The total number of lines of force is proportional to a quantity
called the magnetic flux.
The amount of flux passing
through the square between
the magnetic poles depends
on the
• area of the square
• the angle it makes to the field
• the strength of the field
Magnetic field strength and flux density
The amount of flux, or number of lines of force per square metre
is called the flux density.
Flux density is another term
for magnetic field strength
syllabus
Magnetic flux, flux density and area
The flux density of this
magnetic field…
exceeds the flux density of…
this magnetic field
The number of flux lines
through the square is less in
the bottom diagram
syllabus
Generation of potential difference by changing flux
Faraday established that the magnitude of the current produced
was dependent upon the number of lines of force cut by the
conductor in unit time.
Investigate how the current can be changed.
What factors does the number of lines of flux
cut by the coil depend upon?
first
Generation of electric current using a coil and magnet
Three factors determine the number of lines of flux cut by the
coil in a given time, and hence the magnitude of the current
produced when the magnet is moved
• the number of turns on the coil
• the strength of the magnet
• the speed at which the magnet is moved
Increasing any of these variables increases the current produced.
The generated potential difference is proportional
to the rate of change of flux through a circuit.
first
Generation of potential difference
The flux through a circuit can be changed in many ways including..
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B
conducing
loop
v
moving a conducting ring or loop such that the flux through the
loop varies, depending on the position of the loop.
At which point/s in the movement of the coil from
left to right does the flux change?
first
Generation of potential difference
There is no flux change as the coil
moves through positions W, Y or Z
The flux only changes as the loop passes through
these positions. As the loop moves through these
positions, a potential difference is generated in the loop
first
Generation of potential difference
The flux through a circuit can be changed by…
A potential difference is
generated across the
ends of the coil through
which the flux changes
moving a magnet in and out of
a solenoid
changing the current in one
coil,the magnetic field of
which passes through another
coil [see transformers]
Flux changes
in this coil
ac
Generation of potential difference
The flux through a circuit can be
changed by rotating a conducting
loop in a magnetic field
loop
coil
south
north
As the angle the loop
makes to the magnetic
field, changes, the flux
changes accordingly.
loop
Generation of potential difference
A conducting rod sliding along a conducting loop generates a
potential difference in the circuit - the loop plus the conducting rod.
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B
While the rod is
moving, the flux
enclosed by the
current loop changes,
inducing a potential
difference causing a
current to flow
clockwise around the
loop
Review - the field produced by a solenoid
The magnetic field produced by a current in a solenoid (coil) is
similar in shape to the magnetic field surrounding a bar magnet
S
N
A current flowing in a completed
circuit through the coil in the
direction indicated results in the
right hand end of the coil being a
north pole.
Use the right hand grip rule as a
memory aid … but never quote
this rule in the exam… it is not a
physical law!
Lenz’s Law
When a flux change induces a current in a conductor, the
induced current produces a magnetic field that opposes the
change in flux that caused the current
v S
The direction of the current in the
solenoid can be deduced from the
magnetic polarity and the direction
of the windings of the solenoid.
If the magnet is moved in
the direction indicated, v…
then by Lenz’s law, the left
hand end of the solenoid
must become a south pole,
opposing the motion of the
magnet - the movement of
the south end towards the
solenoid.
syllabus
Lenz’s Law
When a flux change induces a current in a conductor, the
induced current produces a magnetic field that opposes the
change in flux that caused the current
Reversing the direction of
movement of the magnet
reverses the direction of the
induced current in the coil.
Note that the polarity of the
solenoid opposes the change
that is inducing the current not simply the polarity of the
moving magnet.
Lenz’s Law
What is the direction of the current induced in the loop by
the moving magnet?
N
S
The current induced must
produce a magnetic field that
opposes the movement of the
bar magnet.
The current produces a north pole to the left of the coil and a
south pole to the right.
The current must flow anticlockwise, viewed from the side of the
approaching magnet, to produce a field, which opposes the
change of flux produced by the movement of the magnet.
Lenz’s Law
As the magnet approaches the ring the
magnetic flux increases
Bind
Changing flux
 induces current
 magnetic field
B OPPOSES flux change that created it
The system behaves such that it tries to keep the flux constant
induced B field to the left offsets increased B due to magnet
Lenz’s Law
Consider a metal conducting loop in a magnetic field
A conducting rod is then slid along the metal conducting loop
Lenz’s Law
As the conducting rod moves to the right…
The flux increases through the area enclosed by the circuit
Lenz’s Law
The increasing flux induces a current
The current must produce a field that opposes the changing flux
Lenz’s Law
A current must flow anticlockwise around the loop to produce a
field out of the page, opposing the increasing flux
I
I
Lenz’s Law
A current flowing towards the top of the page in the conducing rod
results in a force on the rod towards the left, opposing its motion
Force
I
Lenz’s Law
While the rod is moving to the right, the flux enclosed by the current loop
decreases. The induced current in the loop must create a current, the field
produced by which opposes the change in the flux. Hence it is into the page.
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B
The induced current in
the loop must be
clockwise to create a
magnetic field into the
page. This is consistent
with Lenz’s law, since
the flux of the field B
through the current loop
is decreasing, the field
created by the induced
current must counteract
this decrease.
Generation of electric current using a coil and magnet
Using a coil, magnet and a centre-reading galvanometer (a very
sensitive ammeter), an electric current can be generated.
When the magnet is pushed into
the coil, a current flows in one
direction - indicated by the
deflection of the meter pointer in
one direction.
When the magnet is moved in
the opposite direction in the
coil, a current flows in the
opposite direction - indicated by
the deflection of the meter
pointer in the opposite direction.
first
Generation of electric current using a coil and magnet
Quiz
Predict the behaviour of the meter if the magnet is passed
through the coil in the direction indicated. The polarity of the
meter connections to the coil is shown.
+
A
–
Generation of electric current using a coil and magnet
Answer
When the magnet begins to move into the coil, the left hand end
of the coil becomes a north pole.
The current must flow away from
the positive meter terminal for this
to happen. The needle therefore
deflects to the left (negative).
As the magnet passes through the
centre of the coil the current
reverses direction.
As it leaves…
+
A
–
Lenz’s Law and the conservation of energy
When a flux change induces a current in a conductor, the induced current produces
a magnetic field that opposes the change in flux that caused the current
v N
Imagine that the motion of the
magnet as shown resulted in the
end of the solenoid becoming a
north pole.
This would produce a force on the magnet, causing it to accelerate towards the
solenoid.
This would increase the rate of flux change through the solenoid, producing a
larger current and solenoid field, causing the magnet to accelerate more.
This feedback cycle would result in an increase in the kinetic energy of the
magnet and the current in the solenoid that required no further input of energy.
This violates the law of conservation of energy. It is therefore impossible. Lenz’s
law is a special example of the law of energy conservation.
Lenz’s Law and back emf
Review
Outline the principle of operation of an electric generator.
• when a flux change is produced through an electric circuit or
conductor, an emf is produced, resulting in an electric current
if there is a complete circuit
• this is the principle on
which an electric
generator operates
• When the plane of the
coil is perpendicular to
the field, the flux is a
maximum
• As the coil rotates and the flux changes, an emf is induced
Review question
Explain the principle of the electric motor
Identify the energy transformations occurring in an electric motor
Current flowing perpendicular to a magnetic field in a coil of wire
produces a force on two sides of the coil which results in a
torque on the coil. The torque produces rotation of the armature
about the axle on which the coil is situated.
The purpose of an electric motor is to convert electrical energy
to mechanical energy. Motors are less than 100% efficient and
some of the electrical energy is converted to heat energy.
Lenz’s law and back emf
An emf is produced in any coil of wire rotated
in a magnetic field because of the change of
flux taking place through the coil.
back emf
Hence an emf is produced as the coil of an
electric motor as the motor rotates. The faster
the rotation, the greater the emf.
Question Do you think the emf produced as the rotor coil spins in an
electric motor opposes or aids the rotation of the motor?
Explain your answer.
The induced emf in an electric motor opposes the rotation of the
motor. If it aided the rotation, it would cause the motor to spin
ever faster with no further input of energy - a violation of the law
of conservation of energy.
Lenz’s law and back emf
DC motor
As an electric motor turns, an emf
opposing the applied voltage is produced
in the rotor coil of the motor.
This induced emf opposing the rotation of
the motor is called back emf.
As the speed of a motor increases, the back emf increases until
the effects opposing the rotation of the motor - the load, friction
and back emf - result in an equilibrium being reached causing the
motor to spin at a constant speed.
Motors sometimes have resistor built in, which limits the current
that can flow in the rotor coil until the motor is spinning fast
enough for the back emf to prevent the motor from burning out
due to excessive current in the rotor coil.
Back emf - its relation to the supply voltage
The back emf produced in an electric
motor opposes the applied supply
voltage used to operate the motor
As the speed of the motor increases,
the back emf also increases, opposing
the effect of the applied emf and hence
limiting the speed of the motor.
back emf
–
+
The motor reaches an equilibrium at its operating speed, with the
torque generated by the motor being in equilibrium due to the
combined effects of the torque produced by the current in the
rotor coil, the load the motor is driving, the frictional effects within
the motor and the effect of the back emf
syllabus
Back emf - its relation to the supply voltage
Demonstration
Connect a small DC electric motor to
a battery with an ammeter in series
When the motor is first turned on,
there is a surge of current.
The current quickly stabilises at a
lower value.
When the motor is held so that it cannot turn, the current
increases dramatically - 5 to 10 times greater.
Explanation: When the motor is prevented from turning, there is
no back emf and so the effective potential is greater than when
the motor is running, so the current is greater.
syllabus
Lenz’s law and eddy currents
Eddy currents are produced in any conductor through which there is a changing flux
Definition
Eddy currents are circulating currents, or current loops, within a
conducting material, produced as a result of potential differences
induced by a changing magnetic flux through the conductor.
Lenz’s Law Applies
A flux change through a conductor induces a potential difference
The induced potential difference causes a current to flow
The current produces a magnetic field
This magnetic field opposes the original change in flux
syllabus
Induction cook tops
Induction cook tops induce an electric current in a special metal
cooking vessel using the principle of Faraday’s law of induction.
The induced currents are referred to as “eddy currents”
The current in the cooking vessel directly heats the cooking
vessel, because of its low resistance.
There is no direct electrical
contact between the cooking
vessel and the cook top.
current  heat
A changing magnetic field,
produced by alternating current in
induced voltage
coils below the cooking surface
Changing B field
induces voltages in the cooking
AC current in coils
vessel causing eddy currents
Reference: http://stuweb.ee.mtu.edu/~mtromble/induction/cooktops.html
Induction cook tops
Components of a CookTek Induction-Efficient Pan*
(1)
(2,3,4)
(5)
(6)
(7)
18/10 stainless steel is non-reactive to food and easy to clean.
1145 aluminium with a layer of 3004 aluminium between for even heat
distribution.
18/10 stainless steel for superior bond.
Magnetic stainless steel for efficient induction.
18/10 stainless steel resists pitting and rusting.
A changing magnetic field
produced by coils in the cook
top induce eddy currents in
the metal cooking vessel
* from CookTek Online
Reference: http://stuweb.ee.mtu.edu/~mtromble/induction/cooktops.html
Induction cook tops
Summary of operation
4. Currents produces heat
3. Induces voltage in metal vessel
2. Produces changing B field
1. AC current in coils
Reference: http://stuweb.ee.mtu.edu/~mtromble/induction/cooktops.html
syllabus
Eddy currents and switching devices
A ground fault circuit breaker uses a sensing coil to detect
magnetic flux changes produced by eddy currents in an iron
ring to switch off the current if there is an earth related fault.
A sudden change in the
current to the earth
(ground) connection
induces changing eddy
currents in the iron ring.
A current is induced in
the sensing coil.
This current activates
the circuit breaker,
isolating the device from
the electricity supply.
xx
syllabus
Eddy currents and magnetic braking
Induced eddy currents produce magnetic fields opposing the
change in flux that causes the potential difference.
These opposing magnetic effects can be used to produce a
braking effect on the object, the movement of which resulted in
the changing flux that induced the eddy currents.
Applications include
• slowing of maglev trains
• slowing the “space probe 7” ride at Australia’s Wonderland
syllabus
Investigating eddy currents and magnetic braking
1. Using a neodymium magnet, hold the magnet just clear (1 mm)
of a long non-magnetic metal surface (such as aluminium) and
quickly move the magnet quickly across the surface.
Describe your observations
Explanation: The relative movement between the magnet and the
metal conductor causes a change in flux through the conductor.
Thus a current is induced in the conductor, which produces a
magnetic field that opposes the changing flux produced by the
movement - hence opposing the movement. The induced current,
and hence the magnetic braking stops when the movement stops.
Eddy currents
When a metal disk moves in
a magnetic field the induced
emf results in currents known
as “eddy currents”
Eddy currents
Eddy currents result in heating of the
metal plate
Eddy currents can be reduced by
cutting slots in the metal plate.
The slots act like open switches and
prevent the flow of eddy currents and
hence the loss of heat energy.
This principle is important in reducing
eddy current heat losses in motors
and transformers.
Investigating eddy currents and magnetic braking
1. Drop a neodymium magnet through a copper pipe
with a diameter just a little larger than the magnet.
Describe your observations
Explanation: As the neodymium magnet falls
through the copper tube (a non-magnetic material),
the changing flux induces eddy currents in the
copper.
The eddy currents produce magnetic fields that
oppose the cause of the flux change - the falling
magnet, thus opposing the movement and causing
the magnet to fall slowly through the tube.
I
Investigating eddy currents and magnetic braking
The south end of the falling magnet is pointing
upward and north end is facing downwards.
To oppose this motion (the cause of the flux change
through the copper tube), by producing an upward
force, the induced eddy currents must produce a
north pole above the eddy current as shown.
S
I
The eddy currents produce magnetic fields that
oppose the movement, causing the magnet to fall
slowly through the tube.
Eddy currents above the falling magnet will flow in
a direction that results in the south pole being
attracted upwards - as per Lenz’s law.
syllabus
Eddy currents and electricity meters
Eddy currents are induced in a metal disc in the electricity meter.
These cause the disc to rotate.
syllabus
Identifying the main generator components
In addition to the
identified components
slip rings
of the generator, there
must be some method
of mechanically turning
the coil in the magnetic
field.
brushes
This is may be done
using a turbine driven
by water or steam, or a
belt, as in the case of
the alternator in a car.
syllabus
magnets
coil
magnetic
field
Generation of alternating current
Three methods can be used to generate AC in the lab
• Move a magnet in and out of a solenoid in a circuit
• Move a long wire back and forth across the Earth’s field lines
• Rotate a coil, connected to a pair of slip rings, in a B field
Which method would be most appropriate in a
commercial power station?
Three things are essential to generate a potential difference
• A conductor
• A magnet
• Relative motion
first
Generation of alternating current
When the magnet is pushed in and
out of the coil, an alternating current
is induced in the completed circuit
through the microammeter.
Note!
• The conductor - the coil connected to a completed circuit
• The magnet
• Relative motion - the magnet or coil must be moved
first
The Earth’s Magnetic Field
The Earth’s field is similar in form to that of a bar magnet.
The angle the Earth’s field
makes to the the surface
of the Earth depends on
latitude.
Near the poles the field is
almost vertical, while at
the equator, the field is
parallel to the ground.
first
The Earth’s Magnetic Field at Different Locations
Around Sydney, the angle of the field is about 30° to the
ground, pointing in a northerly direction.
At the South Pole
Sydney
At the Equator
A conductor perpendicular to the field, moved back and
forth perpendicular to the field will have an alternating
current produced in it if it is connected to a circuit.
first
Rotation of a coil in a magnetic field
syllabus
first
Comparison of generator and motor structure
Generators and motors have the same key components. A
motor can be run as a generator, converting mechanical to
electrical energy - although not very efficiently.
Comparison of generator and motor structure
Generator structure
Motor structure
Generators and motors have the same key
Key components
Key components
components. A motor can be run as a generator,
Magnets
Magnets
converting mechanical to electrical energy
Coil
Coil
Commutator or slip rings
Commutator or slip rings
Armature, stator and rotor
Armature , stator and rotor
Comparison of generator and motor function
syllabus
Demonstrations
• A loudspeaker (in principle the same
as an AC motor) can be operated as a
microphone (in principle the same as
an AC generator)
• Connect the inputs of two moving coil galvanometers to each other
and pick up one of the meters and gently rock it so that the needle
moves back and forth.
The needle on the other meter moves
correspondingly, because the meter being
rocked is acting as an AC generator, and the
other meter is acting as a motor. (moving coil
meters and motors work on the same
principle - called the motor principle)
Describing the operation of an AC generator
An AC generator consists of one or more coils in a magnetic
field. When the coil is turned, usually by a turbine driven by
water or steam, the change in magnetic flux through the coil
induces a potential difference across the ends of the coil.
In a coal fired power
station, the generators are
turned turbines operated
by steam produced from a
boiler.
Describing the operation of an AC generator
In a simple AC generator consisting of a single coil having many
turns, the output of the coil is connected by brushes to an
external circuit by a pair of slip rings.
slip rings
The AC potential difference produced has a frequency equal to the
frequency of rotation of the coil.
Describing the operation of an AC generator
In the AC generator below, in which positition/s of the armature is
there a potential difference between the slip rings? [ B and D ]
Identify the positive terminal.
+
+
Describing the operation of an AC generator
Two Types of AC Generators
• Revolving armature
– rotor is an armature which is rotating inside a stationary
electromagnetic field
– seldom used since output power must be transmitted through sliprings and brushes
• Revolving field
– dc current is supplied to the rotor which makes a rotating
electromagnetic field inside the stator
– more practical since the current required to supply a field is much
smaller than the output current of the armature
Describing the operation of an AC generator
Revolving Armature
Describing the operation of an AC generator
Revolving Field
Describing the operation of an AC generator
syllabus
• The AC generator converts mechanical energy to electrical
energy in the form of an alternating current
• Mechanical energy is used to turn a coil in a magnetic field
• The magnetic flux change through the coil induces a potential
difference across the ends of the coil
• The circuit with the rotating coil is completed by slip rings that
connect the rotating coil to the stationary external circuit
• Alternating current flows with a frequency equal to the
frequency of rotation of the generator
Describing the operation of an AC generator
Generation of AC electricity for commercial distribution
See excellent website - http://www.tampaelectric.com/eduction/TEEDElecGen.html
syllabus
Transmission line energy losses
Energy losses occur from transmission lines because they have a
small resistance and they carry a current. E = P x t = I2R x t
The energy loss is proportional to the square of the current, and so
high voltages are used on the transmission line to minimise the
current, requiring the use of step-up and step-down transformers.
Describing the operation of a DC generator
A DC generator converts mechanical to electrical energy.
Mechanical energy is used to rotate the armature / coil assembly
of a DC generator e.g. using a steam or water turbine.
The rotation of the coil in a magnetic field produces a flux change
through the coil, which induces a potential difference across the
coil.
The ends of the coil connect to
the external circuit via a split
ring commutator.
The DC generator produces a
varying DC voltage, as shown
in the graph.
Describing the operation of a DC generator
To convert the output of a DC generator to a constant voltage, a
smoothing circuit must be used.
This typically contains a capacitor placed across the varying DC
output.
This capacitor stores energy while the voltage is high.
When the voltage drops, the
stored energy in the capacitor is
released, maintaining the
voltage at a constant level.
capacitor
Describing the operation of a DC generator
syllabus
Identify the generator in the diagram below.
What component makes this identification possible?
This is a DC generator since the coil is connected to the external
circuit using a split ring commutator.
Describing the operation of an AC generator
A model generator - this one has a split ring commutator
What kind of generator is this? This is a DC generator
In what way would an AC
generator differ from this
generator?
An AC generator has slip
rings instead of a split ring
commutator.
syllabus
Producing an electric current with a voltaic cell
syllabus
A voltaic cell produces an electrical potential by exploiting the
different attraction which atoms have for their electrons.
Chemists call this the reduction potential of the element.
A simple voltaic cell uses
two different metal
electrodes in a conducting
solution called an
electrolyte. Two electrolyte
solutions are use in the cell
shown here. (zinc and
copper sulfate)
Note
first
Producing an electric current with a voltaic cell
Carry out an investigation to measure the potential difference
produced by two different pairs of metals used in a simple
electrolytic cell as shown below. Record which metal electrode
was the positive electrode in each case.
Tabulate your results, and
those of other groups in your
class.
Cu(+)/Zn __ volts
Fe(_)/Zn
__ volts
Cu(_)/Fe
__ volts
Ni(_)/Zn
__ volts
first
Producing an electric current with a voltaic cell
syllabus
Carry out an investigation to measure the potential difference
produced by two different pairs of metals used in a simple
electrolytic cell as shown below. Record which metal electrode
was the positive electrode in each case.
Typical results
Cu(+)/Zn 0.9 volts
Fe(+)/Zn
0.5 volts
Cu(+)/Fe 0.4 volts
Ni(+)/Zn
0.8 volts
first
Quiz
Three different metal electrodes, A, B and C were
used in pairs to make three electrochemical cells.
Two of the results obtained are as follows.
A and B
A and C
0.5 volts
0.2 volts
A
B
A
C
B
C
A positive
C positive
Predict the voltage that was produced using
electrodes B and C.
C is positive relative to A
A is positive relative to B
Therefore C must be positive relative to B
The voltage produced will be 0.7 volts
Quiz
Predict, at the instant shown in the diagram, the
potential difference across the ends of the loop.
Justify your prediction.
Hint: Look carefully at the direction of motion
of the conductor in the magnetic field.
The potential difference is zero.
Since the conductor at this instant is moving
parallel to the magnetic field lines, there is no force
produced on the charges in it, so there is no PD.
Comparison of benefits of AC and DC generators
syllabus
• AC generators produce a voltage that can be readily transformed
• AC generators are simpler and more reliable (they have fewer parts)
• AC can be used for operating motors suitable for a range of applications
Reference: http://www.phys.unsw.edu.au/~jw/HSCmotors.html#links
Comparison of benefits of AC and DC generators
syllabus
Thomas Edison pioneered DC and George Westinghouse and
Nikola Tesla championed AC.
AC is easily transformed permitting transmission over long
distances with less energy loss, but AC is more deadly than DC.
Reference:
“The war of the currents, or let’s Westinghouse him” by Ira Flatow
Edison’s electric chair, using AC, was
used in the 1892 execution of Charles
MacElvaine.
It was part of Edison’s public relations
campaign to portray AC power as a
menace to public safety.
More
The effects of generators on the environment
• Generators need an energy input to operate, usually either
• Water turbine
• Steam turbine
The effects of generators on the environment
Water turbines
• Usually require the building of large dams, which may
arguably have a very significant effect on the environment
• e.g. Tasmania, Snowy Mountains, China, Brazil
Large areas covered by water when dams are built inevitably
involve the destruction of habitat and species.
Rotting vegetable matter in dams contributes further to
carbon dioxide and methane production - both greenhouse
gases.
Dams may interfere with the movement of animals, affecting
breeding and food supplies.
The effects of generators on the environment
The Three Gorges Dam in China…
When completed in 2009, this will be the largest dam in the world.
The effects of generators on the environment
The Three Gorges Dam in China…
Environmental issues
• Inundation
• Flood concerns
• Increased earthquakes
• Water pollution
• Sedimentation
• Species affected
• Human resettlement
The effects of generators on the environment
Brazil … Tucurui Dam
Hydroelectric generators produce over 90% of Brazil’s electrical energy
hydroelectric energy production
Generators at a large Canadian
hydroelectric power station
Hydroelectric power station
The effects of generators on the environment
Steam turbines
• Steam turbines can use existing bodies of water, without the
need for dams, however the water needs to be heated
requiring either…
• The burning of fossil fuels, coal (in NSW), gas or oil,
resulting in the production of large quantities of carbon
dioxide, as well as other pollutants. The carbon dioxide
may be linked to global warming - it is a greenhouse gas.
• The operation of nuclear power plants (which are used to
heat water to steam to operate steam turbines) resulting
in radioactive waste products, which must be safely
stored for long periods.
The effects of generators on the environment
Cooling towers at a nuclear power plant
Thermal pollution and gaseous emissions
The effects of generators on the environment
Location of nuclear power stations
The effects of generators on the environment
syllabus
Steam turbines
Steam turbines produce thermal pollution.
Not all the heat produced can be used efficiently.
Hot waste water must be cooled before it can be recycled into
the environment. Despite cooling, water enters Lake Macquarie
in NSW from adjacent power stations at a higher temperature
than the lake itself.
This promotes growth of algae, especially when it is in
combination with nitrogen and phosphorous based chemicals
entering the system from homes and farms, acting as fertiliser
for the algae.
The insulation of high voltage transmission lines
Glass or ceramic insulators are used on both high voltage and
lower voltage transmission lines. These can be seen on most
electricity poles. The higher the voltage, the more insulation is
required.
detail
High voltage transmission lines
High voltage transmission lines are the backbone of the
national electricity grid in Australia.
Voltages are stepped up at the power station (from
23 kV typically produced by the generators to 330 kV or
500 kV) for transmission across the electricity grid.
The electricity grid is a network of interconnected
transmission lines and power stations.
Finally, the voltage is stepped down progressively at
points in the grid, ultimately to 240 V for domestic use.
High voltage transmission lines
The insulation of high voltage transmission lines
The higher the voltage, the more insulation is required
ceramic insulators
ceramic
insulators
conducting
bypass
power
lines
Detail of the ceramic
insulators on this high
voltage transmission line
tower are shown on the left
The insulation of high voltage transmission lines
Reference: ElectricitySaskpower.pdf
The insulation of high voltage transmission lines
Ceramic insulators on transmission lines are very varied in their design
Identify the supporting structures, insulators and conducting wires in these
photographs
Generally the greater the voltage, the greater the separation must be the
separation between the current carrying wires and the supporting structures.
Additional reference: ElectricitySaskpower.pdf
Lightning protection of high voltage transmission lines
syllabus
High voltage transmission lines are
excellent lightning targets because
• they are metal conductors
earthed
conductors
• they are usually the tallest object in
the vicinity
Transmission lines must be protected
from lightning strikes because
• strikes produce voltage surges that
damage both the supply system and
connected appliances
• they are expensive to repair
• damage interrupts energy supply
high voltage
transmission lines
High voltage transmission lines are protected by earthed conductors
connecting the highest points of the supporting towers to each other
Human health and high voltage power lines
• Some studies appear to show a weak association between exposure to
power-frequency magnetic fields and the incidence of cancer however…
• Epidemiological studies done in recent years show little evidence that
power lines are associated with an increase in cancer
• A connection between power line fields and cancer remains biophysically
implausible
• "The scientific evidence suggesting that [power-frequency
electromagnetic field] exposures pose any health risk is weak.”
(A 1999 review by the U.S. National Institutes of Health )
• "Laboratory experiments have provided no good evidence that extremely
low frequency electromagnetic fields are capable of producing cancer,
nor do human epidemiological studies suggest that they cause cancer in
general.”
(A 2001 review by the U.K. National Radiation Protection Board (NRPB))
Reference: Power Lines and Cancer.pdf taken from http://www.mcw.edu/gcrc/cop/powerlines-cancer-FAQ/toc.html
Human health and high voltage power lines
syllabus
Do power lines produce electromagnetic radiation?
To be an effective radiation source an antenna must have a length comparable to its
wavelength. Power-frequency sources are clearly too short compared to their wavelength
(5,000 km) to be effective radiation sources. Calculations show that the typical maximum
power radiated by a power line would be less than 0.0001 microwatts/cm^2, compared to
the 0.2 microwatts/cm^2 that a full moon delivers to the Earth's surface on a clear night.
How do radiofrequency radiation and microwaves cause biological effects?
A principal mechanism by which radiofrequency radiation and microwaves cause biological
effects is by heating (thermal effects). This heating can kill cells. If enough cells are killed,
burns and other forms of long-term, and possibly permanent tissue damage can occur.
Cells which are not killed by heating gradually return to normal after the heating ceases;
permanent non-lethal cellular damage is not known to occur. At the whole-animal level,
tissue injury and other thermally-induced effects can be expected when the amount of
power absorbed by the animal is similar to or exceeds the amount of heat generated by
normal body processes. Some of these thermal effects are very subtle, and do not
represent biological hazards.
Reference: Power Lines and Cancer.pdf taken from http://www.mcw.edu/gcrc/cop/powerlines-cancer-FAQ/toc.html
The purpose and principle of a transformer
A transformer is an electromagnetic
device consisting of two conducting
coils, isolated electrically from each
other, but linked magnetically.
• The primary purpose of a transformer is to change a voltage
from one value to another, using magnetic induction
• A secondary purpose of a transformer is to isolate one part of
a circuit from another physically and electrically, while allowing
the transfer of energy from one part to the other
The purpose and principle of a transformer
• A transformer is a device that
transfers energy by
electromagnetic induction
• Primary and secondary windings
(insulated from each other
electrically) are wound onto a
ferromagnetic core
• Used to raise voltage (“step-up transformer”) or lower voltage
(“step-down transformer”)
• Voltage is raised when the primary winding has fewer turns
than the secondary winding, and voltage is lowered when the
primary winding has more turns than the secondary winding
The purpose and principle of a transformer
1. A changing current in the
primary coil, produces a
changing magnetic flux
Transformers operate
on the principle of
Faraday’s law
2. A The changing flux from
the primary coil, induces a
potential difference across
the secondary coil
The purpose and principle of a transformer
1. The changing current in the
primary coil, is usually
achieved by applying an
alternating voltage, resulting
in an alternating current (AC)
syllabus
3. The field from the primary coil is
intensified and concentrated
(also referred to as increasing
the flux linkage) through the
secondary coil by an iron core
AC
output
AC input
2. As the alternating current changes
magnitude and direction, a magnetic
field is produced, which changes in a
corresponding manner
4.The changing flux through the
secondary coil, induces a
potential difference across
the secondary coil
Investigation - examining a transformer
Transformers frequently have more than two coil windings.
first
Quiz
Examine a transformer and record
the input and output information
using a diagram.
What type of primary input is
required to produce a secondary
output on a transformer?
Ouch!!
Predict
Would connecting a battery to the
primary coil of a transformer
produce an output voltage?
Investigation - modelling a transformer
Construct a simple transformer by winding a primary coil of 20
turns and a secondary coil of 100 turns onto a soft iron bar
Apply a low voltage (2-4 volts) AC to the 20 turn coil
Measure and record the input and output voltages
Apply a low voltage (2-4 volts) AC to the 100 turn coil
Measure and record the input and output voltages
Investigation - modelling a transformer
Typical results
20 turn coil input voltage (primary)
__ volts
100 turn coil output voltage (secondary) __ volts
100 turn coil input voltage (primary)
__ volts
20 turn coil output voltage (secondary)
__ volts
Faraday’s original transformer (left)
Faraday’s Observations of Induction
Quiz
Predict what will happen when the switch is closed.
Explain the processes underlying your prediction.
Model of Faraday’s experiment demonstrating induction
Faraday’s Observations of Induction
Three levels of answers:
Elementary: The compass needle will be
deflected when the switch is closed.
Better: The compass needle will deflect
momentarily, tending to align parallel to
the axis of the coil in which it is located. It
will then return to the position shown in
the diagram.
Best: The compass needle will deflect momentarily, with the north end,
tending to align parallel to the axis of the coil in which it is located, and
pointing to the right. It will then return to the position shown in the diagram.
Faraday’s Observations of Induction
Explanation
Closing the switch causes a current to flow in
the coil connected to the battery.
As the current changes from zero to a steady
value, the magnetic field intensity increases.
The iron torus links the changing flux from the primary coil connected to the
battery to the secondary coil.
The flux change through the secondary coil induces a current, because the
circuit is closed, producing a magnetic field parallel to the axis of the
secondary coil, causing the compass to deflect in that direction.
Faraday’s Observations of Induction
Detailed Explanation
The current flows clockwise around the
N N
primary circuit when the battery is connected.
This produces the magnetic polarity shown,
I
I
with the magnetic field intensity increasing.
The current induced in the secondary, must
flow in the direction shown, producing a What happens when the switch
magnetic field with a flux opposing the is opened after being closed?
increasing flux of the primary.
The magnetic polarity and the current of the secondary is therefore as shown.
The magnetic field inside the secondary coil increases, and its direction is to the
right, deflecting the north end of the compass in that direction.
When the current in the primary reaches a constant value, the flux change is
zero, so no current is induced in the secondary coil. The compass returns to its
original position.
Comparing step-up and step-down transformers
A step-down transformer has less
turns on the secondary coil than
on the primary coil
By comparison, a step-up
transformer has more turns on
the secondary coil than on the
primary coil
Comparing step-up and step-down transformers
Step-down transformers are found in all electronic
devices that can be run from the domestic 240 V
AC domestic supply, since all electronic devices
require low voltages to operate the semiconductor
components.
Step-down transformers are used in the electricity
supply grid to reduce the high voltages (up to 330
kV) used when transmitting energy to lower
voltages (240 V) for domestic use
Comparing step-up and step-down transformers
Step-up transformers are used at power stations to
convert the lower voltage generator output (~600 V)
to higher voltages (up to 330 kV) for transmission
across the grid.
Step-up transformers are needed to convert 240 V
to several thousand volts needed to operate
fluorescent lights.
TVs and other devices containing a cathode ray
tube (CRT) require high voltages (up to 50 kV) for
their operation.
X-ray machines require high voltages, requiring the
use of step-up transformers for their operation.
Comparing step-up and step-down transformers
Compare the number of turns
on the secondary coil to the
number of turns on the
primary coil of this transformer
This transformer has less
turns on the secondary coil
than the primary coil since it is
a step-down transformer
The ratio of the number of
turns on the primary to the
secondary coil is 240:6.3
Comparing step-up and step-down transformers
syllabus
The induction coil
A low-voltage, pulsed DC (6 V) applied to the primary coil (typically
having less than a hundred turns), produces a high voltage
(~30 kV) across the secondary coil (having thousands of turns)
Induction coil
Pulsed DC is used because the rate of change of flux is much
greater than that produced by a 6 V alternating current.
+
–
The induction coil voltage supply*
• the applied DC voltage causes a current to flow
• the current produces a magnetic field
• the field attracts the magnetic reed switch
• the circuit is broken, switching off the current
• the reed switch springs back, completing the circuit
reed switch
+
iron cored coil
+
DC supply
–
I
–
I
* not explicit in HSC syllabus
Induction coil
This induction coil is producing a
voltage of about 20 kilovolts
spark
A device similar to
this is used to
produce the spark
to ignite the petrol
in a car engine.
Transformer primary-secondary voltage relationship
The voltage input and output of a
transformer are related to the number of
turns on the primary and secondary coils by
np = number of turns on the primary coil
ns = number of turns on the secondary coil
Vp = primary voltage (input)
Vs = secondary voltage (output)
syllabus
Vp n p

Vs ns
Solving primary-secondary voltage problems
A transformer was found to be transform 220 V AC to 5.5 V AC.
If the number of turns on the secondary coil was 20, how many
turns must there have been on the primary coil?
Vp n p

Vs ns
220 n p

5.5 20
np = 800 turns
220 V
5.5 V
Comparing step-up and step-down transformers
There are 126 of turns on the
secondary coil of this
transformer.
How many turns are there on
the primary coil?
Vp n p

Vs ns
240 np

6.3 126
There are 4800 turns on the primary coil.
Transformers
Identify the component labelled A
and outline its function.
Describe one structural feature of
this component and explain the
reason for this feature.
A
Component A is a soft iron core. Its two main functions are
(1) to increase the flux through the primary coil and
(2) to provide a more effective flux linkage between the primary
and the secondary coils.
The iron core is usually laminated to reduce eddy currents in the
core caused by the flux changes through it, and thus to minimise
heat losses and to increase the efficiency of the transformer.
Conservation of energy and voltage transformations
The law of conservation of energy
Energy cannot be created or destroyed, but
only transformed from one form to another.
Step-up transformers increase the voltage applied to the primary
coil, resulting in a greater voltage across the secondary, however
the energy available at the output of the secondary coil is always
less than the energy applied to the primary coil.
Analysing the energy relationships in a transformer…
Energy is the product of power and time.
Power is the product of voltage and current.
P  VI
Conservation of energy and voltage transformations
Energy cannot be created or destroyed, but
only transformed from one form to another
transformer
energy input
Input
240 V
Output
12 V
energy output
energy losses
• The energy output of a transformer is always less than the input
• Energy losses occur because eddy currents induced in the
transformer core by the alternating current, result in resistive
heat losses (the transformer core heats up)
• The ratio of the energy output to the energy input, expressed as
a percentage is called the efficiency of the transformer.
Conservation of energy and voltage transformations
Energy cannot be created or destroyed, but
only transformed from one form to another.
transformer
energy input
Input
240 V
Output
12 V
energy output
energy losses
Consider a simplified case in which…
• The transformer output remains at 12 V, regardless of the load
• The energy losses (mainly heat) are constant, say 30%
i.e. the efficiency of the transformer is constant at 70%
Conservation of energy and voltage transformations
Energy cannot be created or destroyed, but
only transformed from one form to another.
transformer
energy input
Input
240 V
Output
12 V
energy output
energy losses
If the lamp is a 36 watt lamp (12 V) then the output current is 3 A
What is the input current?
efficiency 
output
input
0.7 
36W
input
The input power is 51.4 watts
P  VI
The input current is therefore 0.21 A [ I = P / V ]
Conservation of energy and voltage transformations
Energy cannot be created or destroyed, but
only transformed from one form to another.
transformer
energy input
Input
240 V
Output
12 V
energy output
energy losses
If a 48 W lamp (12 V) is connected instead of the 36 W lamp, then
the output current is 4 A. What is the input current?
efficiency 
output
input
0.7 
48W
input
P  VI
The input power is 68.6 watts
The input current is therefore 0.29 A [P=VI]
Conservation of energy and voltage transformations
Energy cannot be created or destroyed, but
only transformed from one form to another.
transformer
energy input
Input
240 V
Output
12 V
energy output
energy losses
• The output current of a step-down transformer is more than the
input current
• The output power of a step-down transformer is less than the
input power
• If the load (resistance) on the output is decreased, the output
current increases accordingly and the input current increases
Conservation of energy and voltage transformations
Energy cannot be created or destroyed, but
only transformed from one form to another.
transformer
energy input
Input
240 V
Output
12 V
energy output
energy losses
What would be the effect of increasing the number of turns on
the secondary coil, with the same light globe attached?
Vp n p

Vs ns
syllabus
Conservation of energy and voltage transformations
Energy cannot be created or destroyed, but
only transformed from one form to another.
transformer
energy input
Input
240 V
Output
12 V
energy output
energy losses
• Increasing the number of turns on the secondary coil would increase the
transformer output voltage
• With the same lamp, the current would be more and hence more power
would be produced and the lamp would be brighter
• A greater output current would result in a greater input (primary) current
• Energy would still be lost in the core as heat [How much more?]
Electricity substations and transformers
Electricity generated at a power
station is usually produced at a
voltage ranging from a few
hundred volts to 10s of kV (Eraring
power station at Lake Macquarie
has four 660 MW generators with
an output of 23 kV.
It is transformed to 330 kV or 500
kV for transmission over the grid.
High transmission voltages are used to
minimise heat losses in the transmission lines.
Electricity substations and transformers
The voltage must be transformed to lower voltages, usually a few
thousand for local distribution, and then to 240 volts for domestic use.
Electricity substations are used to transform voltages in the grid
Electricity substations and transformers
The distribution of electrical energy involves substations responsible
for stepping up voltage for transmission and stepping it down for use
Electricity substations are used to transform voltages in the grid
Domestic appliances and transformers
Many domestic appliances today have semiconductor (electronic)
components requiring low voltage DC for their operation.
Electronically operated domestic appliances require both a
• transformer to change 240 volts to about 5-20 volts
• rectifier to change the low voltage AC to DC
Appliances with no transformer
Appliances with a transformer
kettle, hot water heater, toaster,
older room heaters, hair dryers,
incandescent lights, old model
refrigerators, some clothes
dryers
TV, stereo, computer, CD
player, clock radio, fluorescent
lights, home security systems,
microwave oven, answering
machines, air conditioner, fax
machines, washing machines,
microwave oven
Domestic appliances and transformers
A 2002 transformer development…
The need for transformers in the electricity grid
Transformers are required at the
power station to step up the relatively
low voltage from the generators (100
V) to high voltages (330 kV) for
distribution over the grid.
Transformers are required at local
substations to step down the very
high voltages from transmission lines
to lower voltages (11 kV) for
suburban distribution. Finally, local
transformers step the voltages down
further for domestic use (240 V)
syllabus
High voltage transmission
Suburban step down transformer
The need for transformers in the electricity grid
Voltage produced by the power
station generators ~ 23 kV
The need for transformers in the electricity grid
MANY transmission lines to
the step-up transformers
The need for transformers in the electricity grid
Step-up transformers at
the power station ~ 330 kV
The need for transformers in the electricity grid
Step-down transformers
for industry ~ 415 volts
The need for transformers in the electricity grid
Local distribution
~ 110 kV - 33 kV
The need for transformers in the electricity grid
The need for transformers in the electricity grid
Domestic transformer
~ 220 - 240 volts
The need for transformers in the electricity grid
All energy comes in via
the home meter box
The need for transformers in the electricity grid
Below-ground distribution is
becoming popular
Above-ground is still common
The need for transformers in the electricity grid
Step-up transformer at a power station.
A suburban step-down transformer
syllabus
syllabus
Eddy currents and energy losses in transformers
To increase the magnetic flux produced
by the primary coil of a transformer, a soft
iron core is used
The changing flux induces eddy currents
in the iron core, which results in resistive
heat losses, and therefore inefficiency of
the transformer.
To reduce the eddy currents, the core of
a transformer is usually laminated, that is,
made up of many layers of soft iron,
electrically insulated from each other
Transformers and their impact on society
Analyse the impact on society of the development of transformers
• The first practical transformer, using AC, was developed in 1883
• Prior to this, direct current was seen as being the logical way to distribute
energy using electricity
• AC triumphed, and by the early 1900s, its future impact on society was
inevitable
• Transformers permitted the long-distance transfer of electrical energy with
low resistive energy losses
• Without the high voltages possible through the use of transformers, the
electrical wires required to transmit large amounts of electrical energy
would have to have been too large to be practical
Transformers and their impact on society
Animation! electricToaster.avi
syllabus
Transformers and their impact on society
syllabus
Transformers were a key to establishing electrical energy as the driving
force behind technological and industrial development in the 20th century.
• Electrical energy rapidly became the means of lighting homes and cities,
with its distribution facilitated by the use of transformers
• Electrically operated machines thus replaced less efficient machines,
resulting in the rapid growth of industry and commerce
• Communication networks grew rapidly as a result of electrical energy and
its intimate association with radio, then television and ultimately the
computer revolution of the late 20th century
• Every home has dozens of appliances that make use of transformers,
permitting a host of electronic devices to be operated from the mains
Describing the main features of the AC electric motor
The main features of AC motors
• current direction is reversed by the
alternating voltage used, rather than by a
split ring commutator as in a DC motor
• the motor speed is determined by the AC
frequency, rather than by the magnitude of
the applied voltage, as with a simple DC
motor
There are two common types of AC motor
• synchronous motors
• induction motors
Describing the main features of the AC electric motor
Synchronous AC motors
An alternating voltage is applied to the rotor
coils via a pair of slip rings.
The stator field may be produced by either
permanent magnets, or it may be produced
by a DC electromagnet.
This type of motor is called “synchronous”
because the speed is synchronised to the
frequency of the applied voltage.
The frequencies may be different, but they bear a simple relationship to
each other.
Heavy loads cause synchronous motors to slow down too much and the
frequencies will no longer be synchronised - hence the motor does not work
efficiently. It may fail altogether.
Describing the main features of the AC electric motor
AC induction motor
• This is simplest and most rugged type of electric motor
• Induction motors have current carrying coils wound on the stator, and a
rotor assembly which has no electrical connections to the power supply.
• The AC induction motor is
named because the electric
current flowing in the rotor is
induced by the alternating
current flowing in the stator.
• The power supply is connected
only to the stator. The combined
electromagnetic effects of the
applied alternating current and
the induced rotor current
produce the torque
Describing the main features of the AC electric motor
Main features of the AC electric induction motor
Describing the main features of the AC electric motor
Stator: The fixed part of an AC motor, consisting of copper windings within
steel laminations.
Rotor: The rotor is the rotating component of an induction AC motor
Describing the main features of the AC electric motor
Stator: The fixed part of an AC motor, consisting of copper windings within
steel laminations.
Rotor
The rotor is the rotating component
of an induction AC motor.
The rotor consists of conducting
cast-aluminium rods.
These rods are short-circuited by end
plates completing the so called
“squirrel cage”, which rotates when
the moving magnetic field induces
current in the conductors.
Describing the main features of the AC electric motor
Rotor
The squirrel cage may have a
laminated, cylindrical soft iron core,
electrically insulated from the squirrel
cage.
The iron core intensifies the
magnetic field of the stator, inducing
a larger current in the rotor, resulting
in a larger torque.
The purpose of the laminations is to
reduce heat losses due to eddy
currents induced in the core, thus
making the motor more efficient
Describing the main features of the AC electric motor
A squirrel cage rotor
Describing the main features of the AC electric motor
These two photographs show two
views of the rotor from an AC
electric motor.
Shaft
squirrel cage rotor
These conductors run parallel
to the shaft, the full length of
the squirrel cage
syllabus
Investigating the principle of the AC induction motor
The neodymium magnets
rotating above the metal
plate produce a changing
flux through the plate.
The flux change
induces eddy currents
in the plate.
The eddy currents produce
a magnetic field which
results in an interaction with
that of the rotating magnets,
producing a torque on the
metal plate, which rotates.
Principle of the induction motor
Discuss why most motors are AC induction motors
AC induction motors have a simple design, requiring no
brushes, commutator or slip rings for their operation, since
there is no electrical contact between the rotor and the power
supply.
AC induction motors, because of their simplicity are cheaper to
manufacture as well as being very reliable.
They are well suited to applications requiring a constant torque
and rotational speed - common criteria in applications such as
fans, fridges, washing machines, clothes dryers, air conditioners.
syllabus
AC electric motors and power tools
Despite what the syllabus states…
AC motors are used for high power applications.
Three phase AC induction motors are widely used for high power
applications, including heavy industry.
However, such motors are unsuitable if multiphase is unavailable, or
difficult to deliver, as in the case of electric trains.
Many electric train systems run on DC, because it is easier to build power
supply lines requiring just one active conductor for DC.
AC electric motors and power tools
Power tools and some appliances use
synchronous AC motors with brushes.
Brushes introduce energy losses (plus arcing and
ozone production).
Power tools using AC induction motors produce
low torque at low speeds - this can be a problem.
Eddy currents induced in the rotor core result in
energy losses and the possibility of overheating.
These motors are sometimes called ‘universal
motors’ because they can operate on DC as well
as AC.
DC operated
electric drill
syllabus
AC electric motors and power tools
Power tools can operate on DC as well as AC
however the simplest type use a DC motor.
syllabus
AC induction motors and their advantages
Simple design
Low cost
Reliable operation
syllabus
Conversion of electrical energy
Gather, process and analyse information to identify
some of the energy transfers and transformations
involving the conversion of electrical energy into other
forms in the home and in industry
Discussion in class!
Coils from an AC fan motor (right)
syllabus
syllabus
AC vs DC motors*
AC most common type for
power applications,
simple, cheap, constant
speed operation
DC easily controlled, variable
speed operation, with or
without brushes, also
function as generator
commutator
Brushes
Stationary windings
(stator)
Armature (rotating unit)
Internal view of the “Universal” motor
used in the SKIL electric hand drill - the
so-called universal motor can be
operated using either AC or DC voltage
* Not specifically required by syllabus, however AC motor advantages must be understood
The end
Westinghouse and Tesla
George Westinghouse was a famous
American inventor and industrialist
who purchased and developed Nikola
Tesla's patented motor for
generating alternating current. The
work of Westinghouse and Tesla
gradually persuaded Americans that
the future lay with AC rather than
DC (Adoption of AC generation
enabled the transmission of large
blocks of electrical, power using
higher voltages via transformers,
which would have been impossible
otherwise). Today the unit of
measurement for magnetic fields
commemorates Tesla's name.
Return
What chemists do
The electrochemical cell shown here separates the
two electrodes into separate solutions so that the
chemistry of what is being done can be more
easily understood.
It is not necessary to have two different electrolytes
in two containers in order to produce a potential
difference.
Return
A word from the creator
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Greg Pitt of Hurlstone Agricultural High School.
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