Download Motors and Generators by Ian Wilkinson

HSC Physics Module 9.3 Summary
1. Motors use the effect of forces on current-carrying conductors in magnetic
fields
Identify that the motor effect is due to the force acting on a current-carrying
conductor in a magnetic field
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Recall that a current-carrying conductor produces a magnetic field around the conductor
A current-carrying conductor experiences a force in an external magnetic field due to the
interaction of its own magnetic field and the external magnetic field
This force is called the motor effect, as it is the force used in the operation of motors
Discuss the effect on the magnitude of force on a current-carrying conductor of
variations in:
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the strength of the magnetic field in which it is located
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the magnitude of the current in the conductor
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the length of the conductor in the external magnetic field
•
the angle between the direction of the external magnetic field and the
direction of the length of the conductor
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The magnitude of the force is given by the following equation:
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From this equation, we can see that the magnetic force on a conductor depends on a
number of factors:
o The strength of the external magnetic field. The force is directly proportional to the
magnetic field strength, B, measured in Teslas (T)
o The magnitude of the current in the conductor. The force is directly proportional to
the current, I, measured in amperes (A)
o The length of the conductor in the external magnetic field. The force is directly
proportional to the length, l, measured in metres (m)
o The angle between the direction of the external magnetic field and the direction of
the length of the conductor. As it is only the component of the conductor
perpendicular to the external magnetic field that experiences a force, the force is
proportional to the sine of the angle, hence the term sinΘ
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To work out the direction of the force, there are two possible methods: the right-hand palm
rule, or the general right-hand rule
o For the right-hand palm rule, point your thumb in the direction of the component of
conventional current perpendicular to the external magnetic field, and stretch your
fingers in the direction of the magnetic field. Your palm represents the direction of
the force.
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For the general right-hand rule, point your fingers in the direction of the component
of conventional current perpendicular to the field, and curl your fingers in the
direction of the magnetic field. Your thumb will point in the direction of the force
Solve problems and analyse information about the force on current-carrying
conductors in magnetic fields using:
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See above for a detailed explanation of the force on a current-carrying conductor in a
magnetic field
Remember to always include directions and units in questions requiring calculations
Remember to use the correct angle between the conductor and the field
Remember to use the correct directions for current and the magnetic field
o Current flows from positive to negative (i.e. the opposite direction to the movement
of electrons
o A magnetic field points from north to south (i.e. the direction a small north pole
would move)
Describe qualitatively and quantitatively the force between long parallel currentcarrying conductors:
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If two parallel current-carrying conductors are separated by a finite distance, a force will
exist between the two conductors due to the interaction of their magnetic fields
o The direction of the magnetic fields of each conductor can be determined by using
the right-hand grip rule => point your thumb in the direction of conventional
current, and curl your fingers into a fist. The direction of your fingers indicates the
direction of the magnetic field
The diagram below shows two parallel conductors, both with a current in the same direction
Consider the effect of wire 1 on wire 2. The magnetic field due to wire 1 would be into the
page at wire 2, thus by using the right-hand palm rule, wire 2 experiences a force towards
wire 1. The attractive force also occurs for wire 1.
Conversely, as the diagram below shows, if the currents are in opposite directions, wire 2
would experience a force away from wire 1, and wire 1 would experience a force away from
wire 2, as given by the right-hand palm rule
In summary:
o If the currents are in the same direction, the force is attractive
o If the currents are in opposite directions, the force is repulsive
The magnitude of the force is given by the following equation:
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Where
o F = Force between the two conductors [N]
o l = Common length of the parallel conductors [m]
o I1 = Current through wire 1 [A]
o I2 = Current through wire 2 [A]
o d = distance of separation between conductors [m]
o k = constant = 2x10-7NA-2
Therefore the force between two parallel current-carrying conductors is directly
proportional to the magnitudes of the currents in each wire and the common length of the
wires, and inversely proportional to the distance of separation, with the constant of
proportionality k.
Solve problems using:
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Remember the above formula gives the force per unit length between two parallel currentcarrying conductors, so you must multiply by length to get the total force =>see above for
detailed qualitative and quantitative analysis
Remember to always include directions and units
Remember to use the correct direction for current
Define torque as the turning moment of a force using
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Torque is the turning moment of a force, which can be considered the rotational effect of a
force
It is easier to rotate an object if the force is applied at a greater distance from the pivot axis,
and the force is directed perpendicularly to the line joining the pivot axis to the point of
force application
o For example, it’s easier to loosen a nut with a spanner than by directly applying a
force, as the spanner allows the force to be applied at further distance away from
the pivot
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Thus torque is quantified by the following relationship:
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Torque is calculated using the component of force perpendicular to the rotating arm, so the
following relationship should also be considered:
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Remember to always state the direction of torque and include units
NOTE: Torque is measured in Newton metres [Nm] =>this is NOT equivalent to Joules, as
calculations, as torque is a vector whilst energy is a scalar quantity
Describe the forces experienced by a current-carrying loop in a magnetic field and
describe the net result of the forces
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Consider a current-carrying rectangular loop of wire in a magnetic field, shown below
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The sides of the loop perpendicular to the field will experience a force due to the magnetic
field equal to F=ilB, whilst the sides of the loop parallel to the field will initially experience no
force
As the loop is fixed to a pivot, the force will cause a torque to be applied to the loop, thus it
will begin to rotate in the directions given above
The forces on the loop remains constant throughout rotation as the conductors are always
perpendicular to the field, but the torque does not, as illustrated in the diagrams below
At position 1, the coil is perpendicular to the magnetic field, side WX experiences an
upwards force and side YZ experiences a downwards force, as given by the right-hand palm
rule. The force causes a maximum torque to be applied to the loop, so the loop begins to
rotate.
At position 2, the direction of force remains upwards whilst the plane of the coil has
changed, so the torque on the coil has decreased as the component of force perpendicular
to the rotating arm decreased for both sides WX and YZ
At position 3, the coil reaches is perpendicular to the magnetic field, so the component of
force perpendicular to the rotating arm is zero, so no torque is applied to the coil, even
though sides WX and YZ still experience a force. The coil continues to rotate however, under
its own momentum.
At position 4, the direction of the current has changed due to the split-ring commutator (see
the DC motor section below), thus the forces acting on each side have reversed, and the coil
continues to rotate in the same direction. If the current had not changed direction however,
the torque would be applied in the opposite direction to rotation, so the coil’s rotation
would slow, and eventually reverse direction. The coil would then continue to oscillate
At position 5, the coil has completed half a rotation, and the pattern above will repeat
If torque is plotted against time, the graph produced would be the absolute value of a cosine
curve due to the commutator
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NOTE: Whilst sides XY and ZW do not initially experience a force as they are parallel to the
field, as the coils rotates their position changes, so they do experience a force. But as the
force is not in the direction of rotation, there is no effect on the overall rotation of the
moment.
NOTE: If the coil had originally was initially perpendicular to the field, it would not rotate as
no torque would be applied to the coil
Solve problems and analyse information about simple motors using:
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Consider the coil shown below
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Recall the equation for the force on a current-carrying conductor:
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Motors can have many wires wound around at once, so the force is given by:
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As the conductor is always perpendicular to the field, sinΘ=1
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The magnitude of the torque on each side of the coil is given by
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As there are two sides experiencing a force, the torque is given by
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Since lxw=A, the area of the coil, the total torque acting on the coil can be expressed as
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Where:
o τ = Torque on the coil [Nm]
o n = Number of coils
o B = Magnetic field strength [T]
o I = Current through coil [A]
o A = Area of coil [m2]
o Θ = Angle between the plane of the coil and the magnetic field
From the above relationship, it can be seen that there are several ways to increase the
rotation speed of a motor by increasing the net torque
o Increasing the force on each side of the coil. This can be done by…
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Increasing the current in the coil (or increasing the voltage of the external
circuit)
 Producing a stronger external magnetic field
 Using a soft iron core as part of the armature, in the centre of the loop
o Increasing the area of the coil, either by increasing width, length, or both
o Using more than one coil mounted on the armature
o Increasing the number of coils wound onto the armature (with extra split-ring
commutators as necessary) to maintain maximum torque
o Having stators with curved magnetic poles so torque is at a maximum for the longest
possible duration, as well as for smoother operation
NOTE: When asked to analyse motors, make sure to mention F=BIlsinΘ and then discuss
torque => do not base a discussion around the formula above
Describe the main features of a DC electric motor and the role of each feature
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An electric motor is a device that transforms electrical potential energy into rotational
kinetic energy by passing a current through a coil in a magnetic field.
The DC electric motor can be simplified into several key components:
o A source of emf and an external circuit with resistors if necessary
o The stator, which is all the stationary components of the motor, namely the magnets
o The rotor, which consists of a coil wounded on an armature that is free to rotate on
an axle
o A split-ring commutators and brushes, which change the direction of the current as
necessary
Part
Source of emf
Magnets (stator)
Description
Consists of a source of emf,
such as a battery or a power
supply. The supplied current
flows in one direction (DC),
and is often connected to
external variable resistors to
reduce the chances of excess
current flowing, which would
burn out the motor.
Can either be permanent
Role
Provides a source current to
flow in the external circuit,
which allows for a force to be
produced inside the coils,
hence a torque. The electric
potential energy supplied by
the emf is converted to kinetic
energy in the motor
Provide the magnetic field
Armature and axle
(rotor)
Coils (rotor)
Split-ring
commutators and
brushes
magnets or electromagnets
(see below for greater detail).
The poles of the magnet are
curved to fit around the
armature, and are fixed to the
body of the motor.
The armature consists of a
cylinder of laminated iron
mounted on an axle that is
free to rotate.
Consist of many loops of
conductive wire wound onto
the armature. The ends of the
wires are connected to the
bars of the armature.
required for a force to be
applied to the coils, thus
causing the motor effect.
Curved poles allows for
maximum torque on the
rotating coil.
The armature strengthens the
magnetic field, and provides a
base for the wire coils. The
axle allows the rotor to rotate.
Create rotational kinetic
energy from the torque
supplied by the motor effect.
Having many loops of wire
increases the force on each
side of the coils, thus
increasing torque.
The commutator consists of a Causes the current to reverse
split metal ring that is in
direction every half-rotation,
contact with graphite brushes. which allows the rotor to
Each part of the split metal
rotate in one direction only. If
ring is connected to either end current flowed in one
of the coil, so the current
direction only, the rotor would
through the rotor reverses
oscillate rather than rotate
direction every time torque
falls to zero. Graphite is used
in the brushes as it conducts
electricity and acts as a
lubricant. See the diagram of
the commutator below.
Identify that the required magnetic fields in DC motors can be produced either by
current-carrying coils or permanent magnets
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The required magnetic fields in DC motors can be produced by either permanent magnets
or current-carrying coils
Permanent magnets are used in smaller motors. The poles of the magnet curved to
maximise torque, and are fixed to the body of the motor
Electromagnets, which are used in large DC motors, can be created using a soft iron shape
that has coils of wire around it. The current that flows through the armature coil can be used
in the electromagnet coils. The iron core strengthens the magnetic field.
Below is one possible arrangement for an electromagnet in a DC motor.
Identify data sources, gather and process information to qualitatively describe the
application of the motor effect in:
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the galvanometer
•
the loudspeaker
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GALVANOMETER
A galvanometer is a device used to measure the magnitude and direction of small DC
currents passing through a particular point.
A schematic diagram of a galvanometer is shown below
The coil consists of many loops of wire wrapped around an iron core, and is connected in
series with the rest of the circuit so that the current in the circuit flows through the wire
When the current flows, the coil experiences a force due to the presence of the external
magnetic field (the motor effect). This force exerts a torque on the coil, causing it to rotate
around the pivot.
A spring provides resistance to the torque, and the needle comes to rest when the torque on
the coil equals the torque from the spring
The iron core of the coil increases the magnitude of the force, which allows for more precise
readings
The magnets are curved around the coil and iron core. This produces a radial magnetic field,
which ensures that the field is always perpendicular to the flow of current along the sides of
the coil. Thus the coils experience a force given by F=nBIl, and a torque of τ=nBIA. Applied
torque is then in proportion to current, which allows the scale to be linear.
LOUDSPEAKER
Loudspeakers are devices that transform electrical energy into sound energy.
A loudspeaker consists of a circular magnet that has one pole on the outside and the other
on the inside, as show below.
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An amplifier connected to the coil produces an alternating current that represents the sound
signal to be produced
When the current passes through the coil, the coil experiences a force due to the magnetic
field (the motor effect), which causes it to vibrate back and forth depending on the direction
of the current
The vibrating coil causes the cardboard cone to vibrate, which produces the sound waves we
hear
The frequency of the input signal from the amplifier affects the pitch of the sound produced,
and the amplitude of the signal affects the volume of sound
Perform a first-hand investigation to demonstrate the motor effect
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METHOD
Two strong bar magnets were clamped horizontally about 1cm apart with opposite poles
facing each other. A wire was suspended between the two magnets so that the lower end
was free to move. The wire was connected to a DC voltage supply and a light. The current
was switched on briefly, and any movement was noted. The supplied voltage was
incrementally increased, and any further movements were noted.
SAFETY:
Connecting a wire to the DC supply can cause it to burn out, which can damage equipment
=> ensure that there is significant resistance in the circuit by connecting a light bulb in series
to the circuit
RESULTS
The wire moved perpendicularly to the magnetic field when the current was turned on
When the supplied voltage was increased, the wire moved further
ACCURACY/RELIABILITY/VALIDITY
Accuracy is largely irrelevant, as no quantitative measurements were taken
The experiment was repeated several times for each supplied voltage, and similar
observations were noted, thus it was reliable
A zero reading was taken, thus the movement can be attributed to the current in the wire
All other possible variables were controlled (e.g. strength of magnets, wire used), so the
experiment was largely valid
2. The relative motion between a conductor and magnetic field is used to
generate an electrical voltage
Outline Michael Faraday’s discovery of the generation of an electric current by a
moving magnet
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One of Michael Faraday’s numerous discoveries was electromagnetic induction, which is the
generation of an electric current by a changing magnetic field
Faraday showed that when he moved a magnet near a wire coil, a current temporarily
flowed within the coil, which was measured by deflections on a galvanometer
Faraday noticed that this current was only induced whilst the magnet was moving, but no
current was induced when the magnet was stationary
He also demonstrated that moving a north pole out of a coil produced a current in the
opposite direction to moving a north pole into a coil at the same speed. In addition, moving
a south pole into a coil produced a current in the same direction as moving a north pole out
of a coil at the same speed.
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Faraday also observed that the magnitude of the induced current depended on the speed at
which the magnet was moving towards or away from the coil. The faster the magnet was
moving, the greater the induced current
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Faraday’s discovery of electromagnetic induction was significant as it led to a new
understanding in electromagnetism, and led to the development of generators and
transformers, which are essential in modern electrical distribution systems
Define magnetic field strength B as magnetic flux density
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Below is a magnetic field diagram for a bar magnet
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The field lines on the diagram represent magnetic flux, which is a measure of the number of
magnetic field passing through a given area => the more lines there are in a given area, the
greater the magnetic flux passing through that area
Recall that the magnetic field strength is indicated by the spacing of the flux lines => the
greater the density of flux lines (i.e. the closer the spacing of flux lines), the stronger the
magnetic field
We can thus consider a magnetic field strength (B) in terms of magnetic flux density, which is
the number of flux lines passing through a unit area
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Describe the concept of magnetic flux in terms of magnetic flux density and surface
area
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As stated above, magnetic flux, ΦB, is a measure of the number of magnetic field lines
passing through a given surface area, A, and the field strength, B, is a measure of the density
of magnetic flux
If the particular area is perpendicular to the field lines, then
Where:
o ΦB = Magnetic flux [Weber, Wb]
o B = Magnetic flux density (or magnetic field strength) [Tesla, T]
o A = Area [m2]
The above relationship shows that magnetic flux is the number of field lines passing through
a perpendicular surface area for a given magnetic flux density
If the surface area and magnetic field are not perpendicular, then the magnetic flux equals
the component of the magnetic flux density that is perpendicular to the area A. Thus a
maximum flux through an area occurs when flux and area are perpendicular
Describe generated potential difference as the rate of change of magnetic flux
through a circuit
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Faraday noted in his experiments on electromagnetic induction that a magnet had to be
moving to induce a current in a coil. Further experiments, such as winding two coils onto an
iron ring and inducing current in the second circuit, showed that the magnetic field through
a coil has to be changing in order for a current to be induced.
We can consider the changing magnetic field as the rate of change of magnetic flux through
the circuit
Faraday also noted that the faster the magnetic flux was changing, the greater the induced
current
This led to the development of Faraday’s Law of Induction, which states that the induced
emf in a circuit is equal in magnitude to the rate of change in magnetic flux. That is,
The negative sign indicates the direction of induced emf by Lenz’s Law, which is explained
below. The n represents the number of turns of wire in the coil.
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Since ΦB=B⊥A, a change in flux can be caused by a change in the magnetic field strength, B,
or in the area of the coil that is perpendicular to the magnetic field, or both
The generated potential difference can be increased by:
o Decreasing the distance between the conductor and the magnetic field, as the flux
lines are closer together nearer the magnet
o Increasing the strength of the magnet, as there are more flux lines in the same space
in a stronger magnetic field than in a weaker field
o Increasing the speed of the relative motion between the conductor and the
magnetic field, as the conductor cuts more flux lines per unit time
o Increasing the angle between the direction of motion of the conductor and the
direction of the magnetic field from near zero towards 90 degrees, as the conductor
cuts the maximum number of flux lines per second when its motion is perpendicular
to the field
o Increasing the number of turns in the coil, as the number of coils experiencing a
changing flux increases
Account for Lenz’s Law in terms of conservation of energy and relate it to the
production of back emf in motors
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Lenz’s law states that an induced emf in a closed conducting loop will appear in such a
direction that it opposes the change that it produced
This is a consequence of the Principle of the Conservation of Energy, which states that
energy cannot be created nor destroyed, only transformed.
Lenz’s law appears in Faraday’s Law as the negative sign
Consider the diagram below
By Lenz’s Law, the current induced in the coil produces a north pole towards the north pole
of the magnet, thus opposing the change in flux, and repelling the magnet away from the
coil
If Lenz’s Law were not true, and a south pole was induced instead, the magnet would be
attracted towards the coil. This would lead to an even greater change in flux, so the induced
current would continue to increase, and eventually create infinite ‘free energy’ without
input work. This violates the Law of Conservation of Energy, and thus would not occur.
Another method to validate Lenz’s Law is to consider the energy before and after an emf is
induced in the coil. The magnet is moving towards the coil with an initial velocity v, and its
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overall energy is equal to its kinetic energy. As it nears the coil, an emf is induced in the coil,
which is a form of electromagnetic energy, so the total energy of the system equals the EM
energy and the magnet’s kinetic energy. By the conservation of energy, energy before has to
equal energy after, thus the magnet’s kinetic energy has decreased, so the induced current
must have opposed its motion.
To work out the direction of induced currents, simply create a magnetic field such that it
‘repels’ the change in the magnetic field, and the direction of the current necessary to
produce this field is the induced current.
For example, consider the coil below that is moving out of a magnetic field.
The induced current will oppose the decrease of flux within the coil, so the induced current
will produce a magnetic field that is into page within the coil. Thus a clockwise current is
induced.
More examples of applying Lenz’s Law are shown below
BACK EMF IN ELECTRIC MOTORS
Electric motors use an input voltage to produce a current in a coil to make the coil rotate in
an external magnetic field. As the coil is rotating in a magnetic field, the flux through the coil
is changing over time, thus an emf is induced => this emf is called back emf.
Back emf opposes the supply emf, otherwise the current would continue increasing, and the
motor would go faster forever. The net voltage across the coil equals supply emf minus the
back emf
The magnitude of back emf is proportional to the rotation speed of the rotor => the faster
the rotor is spinning, the greater the change in flux, thus the greater the back emf.
When a motor is turned on, the rotation speed of the armature increases, thus the back emf
increases. If there is no load on the motor, the back emf will increase until most of the
supply emf has been cancelled, so only a small current continues to flow. This residual
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current is required to overcome any internal friction and resistive heating, thus the motor
operates reaches a constant speed
If a load is attached to the motor, the coil rotates at a slower speed due to the additional
resistive torque, thus back emf is reduced. This means that the applied potential difference
is greater, and a larger current flows through the coils.
If a motor is overloaded, the armature will rotate too slowly and will not produce sufficient
back emf. This would cause the current to be too high, and the motor would burn out due to
resistive heating
Motors are usually protected from initially high currents produced when they are switched
on by variable series resistors, which switch out of the circuit at higher rotation speeds
because the back emf results in a lower current in the coil.
Below is a graph of current against time for a DC motor, with a load applied at point C
Explain that, in electric motors, back emf opposes the supply emf
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As explained above, a back emf is induced in a motor due to the changing magnetic flux
through the motor’s coil as the coil rotates
If back emf was in the same direction as supply emf, the current would continue increasing
and the motor would rotate even faster. Thus the motor would create infinite energy, which
would violate the Law of Conservation of Energy.
Therefore, by Lenz’s Law, the back emf opposes the supply emf, as it opposes the change in
flux through the circuit
Explain the production of eddy currents in terms of Lenz’s Law
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Recall that the electrons in a conductor are mostly free to move
When a conductor experiences a changing magnetic flux, small loops of current called eddy
currents are induced within the conductor, which produce their own magnetic fields
The direction of eddy currents can be determined by Lenz’s law, i.e. the currents flow in a
direction that produces a magnetic field to oppose the change in magnetic flux
Eddy currents can be induced from the following changes in flux:
o A magnetic field acts on part of a metal object, and there is relative movement
between the magnetic field and the object
o A conductor is moving in an external magnetic field
o A metal object is subjected to a changing magnetic field
Consider the sheet of metal moving out of a magnetic field, as shown below:
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On the left edge of the magnetic field, charged particles in the metal experience a force
because they are moving relative to the magnetic field (remember to consider the forces
acting on positive charges, no electrons)
To the right of the edge of the magnetic field, charged particles experience no force because
there is no relative motion to a magnetic field
The accumulation of positive charges at the top of the sheet causes the charges to flow
downwards in the metal that is outside the field, which causes an eddy current to be
induced
By using the right-hand grip rule, we can see that the eddy current produces a magnetic field
into the page to oppose the decreasing magnetic flux. The induced magnetic field is
attracted to the external magnetic field, thus the sheet of metal experiences a force that
opposes its motion
Consider the north pole of a magnet moving over and close to the face of an aluminium
plate
By Lenz’s Law, the circulation of an eddy current ahead of the moving magnet will produce a
north pole that will repel the moving magnet. Similarly, an induced eddy current behind the
moving magnet will produce a south pole that will attract the moving magnet.
Together, these two poles oppose the motion of the magnet over the aluminium plate
Gather, analyse and present information to explain how induction is used in
cooktops in electric ranges
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An induction cooktop consists of a coil below a ceramic surface, as shown below
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The coil is connected to a high frequency AC supply, so the coil produces a rapidly changing
magnetic field.
The rapidly changing magnetic flux through the ceramic surface induces eddy currents in
metallic pans, which heats the pan due to resistive heating
The heating is maximised if the pan is made of ferromagnetic metal of high resistance, as
this increases the power loss due to resistive heating. Induction cooktops will not work with
Pyrex glass or ceramic cookers, as these materials are insulators, so no eddy currents will be
induced in these cookers.
The cooktop surface is made of insulating ceramic material, so no eddy currents are induced
in the surface, and the surface remains relatively cool
Induction cooktops are much more efficient than gas stove tops, as the heat is generated in
the pan itself. Induction cooktops also heat faster than gas stove tops, and the heat is
distributed much more evenly
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Gather secondary information to identify how eddy currents have been utilised in
electromagnetic braking
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Consider a rotating metal disk that has part of it influenced by an external magnetic field, as
shown below:
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As the disk is rotating, the flux changes for a point entering and exiting the magnetic field.
This causes eddy currents to be induced in the disk that oppose the change in flux, thus
causing the disk to smoothly slow down.
Electromagnetic braking is also used in many free-fall amusement rides. A copper plate
attached to the ride capsule passes between strong electromagnets near the bottom of the
ride, which causes eddy currents to be induced in the copper plate that produce magnetic
fields to oppose the motion of the ride.
Electromagnetic braking is used in trams and trains. An electromagnetic is switched on so
that it an external magnetic field affects part of a metal wheel or the steel rail below the
vehicle. If applied to the rail, the magnetic field induces eddy currents to produce a like pole
in front of the magnet, and an opposite pole behind the magnet, which oppose the forward
motion of the train.
The advantage of electromagnetic braking is that the strength of induced eddy currents is
proportional to the speed of the vehicle, so the braking force is reduced as the vehicle slows,
resulting in smooth braking
The disadvantage of electromagnetic braking is that the loss in kinetic energy is transformed
to heat energy produced by the eddy currents, which can cause damage to equipment if the
heat is no dissipated properly
Perform and investigation to model the generation of an electric current by moving
a magnet in a coil or a coil near a magnet
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METHOD
A 500 turn solenoid was attached to a microammeter. The north pole of a bar magnet was
rapidly moved in and out of the coil, and any readings on the microammeter were noted.
The south pole of the magnet was then rapidly moved in and out, and any further
observations were recorded.
The magnet was then held stationary, and the coil was moved into the magnet. Any
observations were noted.
No specific safety applies to this experiment apart from basic laboratory safety
RESULTS
When the magnet moved in and out of the coil, a current was registered on the ammeter.
The current was in opposite directions for moving into the coil and moving out of the coil,
and also for moving a north pole into the coil and moving a south pole in.
Moving the coil near the magnet produced similar readings to moving the magnet into the
coil
No current was induced in the coil when the magnet was stationary
ACCURACY/RELIABILITY/VALIDITY
Only observations were recorded, so accuracy is not a consideration
The experiment was repeated several times and produced similar, expected results, thus the
investigation was reliable
A control reading was taken, which was holding the magnet stationary in the coil
All other variables (e.g. number of turns in solenoid, material of wire) were controlled, thus
the experiment was valid.
Plan, choose equipment or resources, and perform a first-hand investigation to
predict and verify the effect on a generated electric current when:
•
the distance between the coil and magnet is varied
•
the strength of the magnet is varied
•
the relative motion between the coil and the magnet is varied
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METHOD
A 500 turn solenoid was attached to a microammeter. A bar magnet was then moved in and
out of the coil, and the reading on the microammeter was recorded.
Using the same magnet, the magnet was moved at varying distances to the coil keeping its
oscillation speeds as similar as possible.
Keeping the same distance and oscillation speed, the strength of the magnet was varied by
using one and two, bar magnets together, ensuring the like poles were together.
Keeping the same distance and using one bar magnet, the relative motion between the coil
and the magnet was varied
The readings on the microammeter were recorded for each tested variable.
No specific safety applies to this experiment apart from basic laboratory safety
RESULTS
Variable
Magnitude
Current induced (μA)
Distance
Close
50 000
Far
10 000
Strength of magnet
One bar magnets
50 000
Two bars magnets
100 000
Speed of magnet
Slow
10 000
Medium
50 000
Fast
100 000
The induced current was stronger when:
o The distance between the coil and the magnet was smaller
o The strength of the magnet was greater
o The speed of the magnet was faster
ACCURACY/RELIABILITY/VALIDITY
The results obtained were estimates from the fluctuating ammeter, and were a guide to
verifying the predicted trends, thus were not accurate or reliable
The accuracy of the readings could have been improved by using a multimeter or
oscilloscope, which could have shown the maximum current for each experiment
The trends corroborated with others in the class and with expected results, thus the trends
obtained were reliable
For each set of readings, all other variables were kept constant (e.g. number of turns in
solenoid, material of solenoid), and a control reading was taken, thus the results were valid
3. Generators are used to provide large scale power production
Describe the main components of a generator
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A generator is a device that transforms mechanical kinetic energy into electrical energy.
The diagrams below show a DC generator and an AC generator
FEATURE OF GENERATORS
Component
Description
Stator (magnets)
Consist of two curved
permanent magnets with
opposite polarity, or an
electromagnet (coil wound
around iron core) in largescale generation
Rotor (coils and
armature)
The coil consists of many turns
of wire wound on the
armature. The armature is a
cylinder of laminated iron
mounted on an axle.
Split-ring commutator
and brushes
Used in DC motors. Consists of
semi-circular metal contacts
mounted on the axle. The
graphite brushes provide
electrical contact to the splitring.
Slip ring commutators
and brushes
Used in AC motors. Consist of
two circular metal contacts on
the axle, one to each end of
the circuit connected to the
rotor coils. The graphite
brushes provide electrical
contact.
Can be provided by driving a
turbine with steam
Energy source
Role
Provides the external magnetic
field, so a current is induced due
to the changing flux through the
rotating coil. The radial magnetic
maximises current, as the sides of
the rotor coil are travelling
perpendicular to the magnetic
field for longer
Provides the internal circuit which
allows the production of
electricity. Torque is applied to
the armature to make the rotor
spin. Many turns of wire are
present to maximise the total flux
change as many areas are present
Reverses the current every halfrotation when the change in flux
through the coil is zero, thus
producing current in one direction
only. Graphite is used in the
brushes as graphite conducts
electricity and is a lubricant
Allows and alternating current to
be produced, as the direction of
the current is reversed every halfrotation of the coil. The slip-rings
allow the current to vary as it does
in the coil, without tangling the
wires
Provide the mechanical energy to
rotate the coil
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OPERATION OF GENERATORS
A generator is able to produce electricity by using Faraday’s Law of Induction
As the coil is forced to rotate in the magnetic field, the flux through the coil is changing as
the plane of the coil is changing. Thus the change in magnetic flux induces a current in the
coil.
Below are graphs of the emf and flux through an AC generator and a DC generator
When the plane of the coil is parallel to the magnetic flux lines, it is cutting the maximum
number of flux lines, thus a maximum current is induced
When the plane of the coil is perpendicular to the magnetic flux lines, no flux lines are being
cut, thus there is no current being induced
When the coil passes a minimum change in flux, the plane of the coil is facing the opposite
direction to its original direction, thus the current induced is in an opposite direction. In a DC
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motor, the split-ring commutator changes the direction of the current supplied to the
external circuit at this point, thus current in the external circuit is in one direction only
The direction of the current can be determined by Lenz’s Law. The current induced in the
coil will produce a magnetic field that opposes the change in flux, so the current can be
determined by using the right-hand grip rule. Also, the current induced will oppose the
rotation of the coil, thus the direction can also be determined via the right-hand palm rule.
If given a graph of magnetic flux through a generator coil against time, the direction of the
current can be determine by considering emf as the negative gradient of the flux graph,
according to Faraday’s Law and Lenz’s Law
A DC generator can produce a more steady current by having the multiple coils set in
different directions and connected to multiple commutators. The superposition of the outof-phase currents leads to a more constant current
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The rotation speed of AC and DC generators affects both the frequency of the current
produced, and the maximum emf induced. When the coil is rotated faster, the rate of
change of flux through the coil when it is parallel to the external magnetic field is greater,
thus the induced emf is greater.
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NOTE: When asked to discuss induced currents in generators, ensure to mention emf, as
Faraday’s Law of Induction concerns induced emf, not current
LARGE-SCALE AC GENERATORS
A typical generator in a power station has an output of 22kV, which would require the use of
massive coils. This would place huge forces on bearings if they were required to rotate.
Thus in large-scale AC generators, the stator consists of coils mounted on an iron core, and
the rotor consists of a DC-supplied electromagnet that spins with a frequency of 50 Hz, as
shown below.
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An alternating current is produced in the coils due to the changing flux through the plane of
each coil
Power station generators have three sets of coils mounted at angles of 120° to produce AC
with three different voltage signals that are out of phase by 120°, as shown below
Compare the structure and function of a generator to an electric motor
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The structure of both generators and electric motors are similar. Both possess a stator that
provides a magnetic field, and a rotor that carries electrical current
The main difference between motors and generators is that a motor converts electrical
energy into mechanical energy, whilst a generator converts mechanical energy into electrical
energy
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In a DC motor, the terminals are connected to a power source, and the axle is connected to a
load. But in a DC generator, the terminals are connected to a circuit, and the axle is
connected to an energy source
An AC induction motor is different from an AC generator as its rotor coils are not connected
to an external circuit, and its field is always supplied by electromagnets
Describe the differences between AC and DC generators
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The main difference between AC and DC generators is how they are connected to the
external circuit
An AC generator is connected via a slip-ring commutator, which produces an output with
current going in both directions
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On the other hand, a DC generator is connected via a split-ring commutator, which produces
an output with current going in one direction only
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Note that the internal circuit produces AC in both cases, but the split-ring commutator
changes the direction of output so the external circuit receives an absolute-value graph of
current
Gather secondary information to discuss advantages/disadvantages of AC and DC
generators and relate these to their use
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DISADVANTAGES OF DC GENERATORS
A DC generator requires a split ring commutator for its function. This inevitably complicates
the design of the generator. This results in more expensive construction as well as more cost
and effort for maintenance
The gap present in the split ring commutator results in sparks being produced during the
generation of electricity if metal becomes lodged in the insulation. The gap also wears down
the carbon brushes more quickly, so the brushes need to be replaced more often than in AC
generators, which use smooth slip rings.
The output of DC generators (DC electricity) loses more energy than that of AC generators
during transmission (see below)
ADVANTAGES OF AC GENERATORS
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DC generators can produce a smooth currents by having multiple commutators, and
superimposing the out-of-phase peaks
Some devices, such as battery rechargers, electromagnets, and cathode ray tubes, rely solely
on DC currents for their function. Although AC current can be converted to DC current using
electronic devices, it is more convenient and cost-effective to produce DC directly using a DC
generator
For a given voltage, DC current is generally more powerful than AC, so DC is preferred in
heavy-duty tools such as a drill. Under this circumstance, DC generators are advantageous
DISAVANTAGES OF AC GENERATORS
Refer to the advantages of DC generators
The phase of AC generators in different regions must be synchronised for electrical
production, which increases the cost of their use
The output of AC generators is more dangerous than DC generators, as alternating current is
more likely to cause heart fibrillation
AC is susceptible to energy losses through electromagnetic radiation or magnetic induction
ADVANTAGES OF DC GENERATORS
Refer to the disadvantages of DC generators
Three-phase AC currents are made possible, which can be used for powering induction
motors (see section 5 below)
AC voltage is easily increased or decreased using transformers (see transformers below),
thus AC generators are preferred for large-scale electrical distribution
AC can easily be converted to DC using solid state diodes, thus DC can be easily produced by
AC generators
Discuss the energy losses that occur as energy is fed through transmission lines
from the generator to the consumer
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The two main types of energy losses that occur as energy is fed through transmission lines
from the generator to the consumer are resistive heating in transmission lines, and eddy
currents in transformers. Energy losses in transformers will be discussed in the transformers
section.
Power stations are usually situated large distances from cities (where most of the consumers
are located) for environmental and pollution reasons
The large distances lead to power losses in transmission to the consumer due to the
resistance of the material used in transmission lines
The power loss can be calculated by Joule’s law:
where:
o P = Power [W]
o I = Current [A]
o R = Resistance [Ω]
Thus to minimise power losses in transmission wires, low currents are supplied, and the
resistance of the material needs to be minimal
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The current through transmission wires can be achieved using transformers (see section 4),
which only operate using AC electricity. Thus AC electricity is used in power distribution
networks.
According to P=VI, a low current will result in a high voltage for the same power output. This
requires high poles or towers and large insulators, which are expensive to build and
maintain. Trees must also be kept well clear from high voltage transmission lines to reduce
the possibility of a short to earth, which can result in environmental concerns.
Power losses can also be reduced by using wires of minimal resistance. The resistance of a
conductor of resistivity ρ, area A, and length l is given by
Thus transmission lines are typically made of aluminium as it has low resistivity
Resistance is also inversely proportional to area, so the thicker the conductor, the lower the
heat loss. But thick wires are also heavier, so require larger structures to support them,
which are more expensive. This is why aluminium is used in transmission lines as opposed to
copper (which has a lower resistivity), as aluminium is a lighter metal than copper, which
allows for thicker wires and reduces construction costs.
Recent experiments with superconducting material transmitting DC show some promise for
reducing energy losses from high voltage transmission lines in the future. DC does not
induce eddy currents in support structures, so energy losses would be further minimised
Analyse secondary information on the competition between Westinghouse and
Edison to supply electricity to cities
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In the late 19th century, Edison favoured generating and supplying DC electricity, whilst
Westinghouse promoted the use of AC electricity
Edison had a vested interest in DC, as electricity was primarily used for household lighting,
and he had patents on much of the technology involved
The drawback of a DC system was that it had to be distributed at the voltage used in
households, as transformers do not work with DC. Transmission therefore required large
currents, which limited the distance DC could be transmitted to several kilometres. This
required many power stations throughout cities, and was impractical for rural areas.
Westinghouse purchased many of the patents for AC motors and generators. AC was
economically advantageous due to the smaller power losses and the ability to place largerscale generators further apart.
Edison attempted to discredit the use of AC and promote it as unsafe. This included using an
AC generator to publicly electrocute animals, including an elephant. He also provided the
New York state prison system with AC generators for use on electric chairs, which Edison
called being ‘Westinghoused’.
The economic advantages of AC power distribution ultimately led to Westinghouse’s system
being favoured in large electricity generation projects, notably the hydroelectric plant at
Niagara Falls. Due to its many advantages, AC electricity was eventually adopted as the
standard worldwide.
Gather and analyse information to identify how transmission lines are:
•
insulated from supporting structures
•
protected from lightning strikes
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INSULATION FROM SUPPORTING STRUCTURES
In dry air, sparks can jump a distance of 1cm for every 10kV of potential difference.
Therefore a transmission line carrying a 330kV line will spark to a metal tower if it comes
within a distance of 33cm, and even larger distances in humid weather.
Typically, the wires are held more than a metre from supporting structures by large
insulators, one of which is shown below
The insulator consists of a series of connected ceramic discs, and at higher voltages, the
chains are longer
The insulators are designed to shed water and prevent dust from building up, as either
moisture or dust can made a conductive path across the surface of the insulator
The disc-like shape of the segments increases the distance that a current has pass over the
surface of the insulator, and so decreases the risk.
PROTECTION FROM LIGHTNING STRIKES
Transmission lines and towers are susceptible to lightning strikes, as they are built high
above the ground
The transmission towers are designed to minimise the risks of lightning strikes by acting as a
good conductor to the ground. The towers are well-earthed, with a large surface area of
metal buried in the ground, and the towers are widely-spaced to ensure minimal damage to
other towers if one is struck
The uppermost transmission wires are earth wires, which mean they carry no current. It
protects transmission lines from sudden surges in voltage by conducting any current due to
lightning strikes to Earth, as lightning is most likely to strike the uppermost wires.
Assess the effects of the development of AC generators on society and the
environment
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The development of AC generators in the 20th century enabled the widespread availability of
electricity, which have had a highly significant impact on society and the environment
POSITIVE IMPACTS ON SOCIETY
The ability to transmit electricity efficiently over long distances has allowed more people in
large cities to live further from the city centre
AC is cheaper to transmit than DC, so the development of AC generators has allowed for
cheaper electrical power distribution, and allowed electrical transmission to rural areas to
be practical
The use of electricity at home and in industry has reduced manual labour, allowing for
greater leisure time
Electricity has allowed for worldwide communication and many consumer products
considered necessary in modern life, such as refrigerators, medical equipment, and
computers
NEGATIVE IMPACTS ON SOCIETY
Electrical machinery has replaced many jobs, which has increased unemployment in some
areas of society
Sites of electrical power plants has led to the displacement of many people from their
homes, particularly for hydro-electric power plants
The proliferation of electronic devices has led to a more sedentary lifestyle, which has
inherent medical problems
POSTIIVE IMPACTS ON THE ENVIRONMENT
AC generators have allowed power plants and industry to move outside cities, which has
improved the urban environment and the health of city dwellers
The availability of electric power has reduced the need to burn wood or coal in houses,
significantly improving air quality in large cities
NEGATIVE IMPACTS ON SOCIETY
The construction of long-distance power lines often require the destruction of habitat, which
has also increased rates of erosion where vegetation has been removed
The burning of fossil fuels for electricity production has led to many negative environmental
impacts, such as acid rain, thermal pollution of marine ecosystems, release of particulate
matter, and release of greenhouse gases
Radioactive waste from nuclear power plants poses a threat to the environment if not
contained
Dams required for hydro-electric power plants have destroyed the habitat of many plants
and animals
Land has been degraded at mining sites for materials required in electrical generation
Plan, choose equipment or resources, and perform a first-hand investigation to
demonstrate the production of an alternating current
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METHOD
A 500-turn solenoid was connected to a galvanometer, and a bar magnet was repeatedly
moved in and out of the solenoid. The readings on the galvanometer were observed and
recorded.
No specific safety applies to this experiment apart from basic laboratory safety
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RESULTS
The reading on the galvanometer oscillated, reaching both positive and negative values. The
maximum current occurred when the magnet was moved quicker in and out of the coil
ACCURACY/RELIABILITY/VALIDITY
No quantitative data was obtained, so accuracy was not of concern
Reliability was improved by repeating the experiment and comparing results to others in the
class
A control reading was taken with the magnet stationary inside the coil, so the production of
an alternating current can be attributed to the moving magnet
The galvanometer and solenoid were correctly wired, so the equipment provided valid
results
All other variables were controlled (e.g. solenoid used, magnet used), thus the experiment
was valid
Possible problems with the experiment include using a galvanometer that isn’t sensitive
enough, loose wire connections, and slow movement of the magnet (the flux change would
be too slow for a significant current to be induced)
The experiment could have been improved by using an oscilloscope to measure induced
current
4. Transformers allow generated voltage to be either increased or decreased
before it is used
Describe the purpose of transformers in electrical circuits
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Transformers are devices that increase or decrease AC voltages in electrical circuits
The domestic supply voltage in Australia is 230V single-phase AC, but some devices require
lower voltages (such as amplifiers in radios) or higher voltages (such as the cathode ray
tubes in CRT televisions) to operate. Thus transformers are placed in the circuit between the
AC supply and the component to reduce or increase the supplied voltage.
Below is a diagram of a common type of transformer
Transformers generally consist of two coils of insulated wire with a different number of
turns called primary and secondary coils. These coils can be wound together on the same
laminated iron core so maximum magnetic flux from the primary coil passes through the
secondary coil
The changing current in the primary coil creates a changing magnetic flux, which passes
through the secondary coil. The changing flux induces an emf in the secondary coil of the
same frequency, but not of the same voltage or current (see below)
Transformers only work on AC, as a changing magnetic flux is required to induce a current in
the secondary coil. Using DC in the primary coil would produce a constant flux, thus no emf
would be induced in the secondary coil.
Compare step-up and step-down transformers
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A step-up transformer is a transformer that increases voltage, whilst a step-down
transformer decreases voltage
Below is a table summarising the differences between step-up and step-down transformers.
Coil ratios will be discussed in the next section
Type
Step-up transformer
Step-down transformer
Structure
Consists of two inductively coupled coils wound on a laminated iron core
Coil ratios
More turns in the secondary coil
Fewer turns in the secondary coil
Voltage output
Current output
Role in power
distribution
Domestic use
than the primary coil
Higher output voltage than input
voltage
Lower output current than input
current
Used at power stations to
increase voltage and reduce
current for long-distance
transmission
Used in television sets to increase
the voltage to operate the picture
tube
than the primary coil
Lower output voltage than input
voltage
Higher output current than input
current
Used at substations and in towns to
reduce transmission line voltage for
domestic and industrial use
Used in computers, radios, and CD
players to reduce household
electricity to very low voltages for
electronic components
Identify the relationship between the ratio of the number of turns in the primary
and secondary oils and the ratio of primary to secondary voltage
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By applying Faraday’s Law of Electromagnetic Induction, the magnitude of the voltage
output of the secondary coil is given by
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Similarly, the input primary voltage is related to the change in magnetic flux by the same
equation
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As mentioned above, transformers are designed so that almost all of the magnetic flux
produced in the primary coil threads through the secondary coil. Assuming 100% efficiency,
the terms ΔΦB/Δt in both of the above equations are equal, thus dividing the second
equation by the first equation gives
Solve problems and analyse information about transformers using:
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Be careful to distinguish between the primary and secondary coil in questions
Remember that whilst ratios do not have units, voltage does, so include relevant units in
calculations
Explain why voltage transformations are related to conservation of energy
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The Principle of Conservation of Energy states that energy cannot be created nor destroyed,
but only transformed from one form to another
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Thus the power output of an ideal transformer equals the power input. In most
transformers, some energy is lost due to eddy currents in the iron core, so power output is
in fact less than power input
Recall that power in electrical circuits can be given by P=VI. If there is no power loss, Pp=Ps,
so VpIp=VsVs. Substituting into the transformer equation gives the following:
The above expression shows that current and voltage in a transformer are inversely
proportional. Thus in a step-up transformer, where voltage is increased, the output current
is less than the input current. Also, in a step-down transformer, the output current is greater
than the input current, so ideally no power is lost.
Real transformers produce heat, however, because the changing magnetic flux produces
eddy currents in the iron core. Thus in real transformers, output power is generally less than
input power .
Gather, analyse and use available evidence to discuss how difficulties of heating
caused by eddy currents in transformers may be overcome
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The coils of wire in transformers are generally wrapped around a soft iron core to
concentrate the magnetic field, allowing a maximum possible inductive coupling between
the primary and secondary coil
As transformers depend on a changing magnetic field in order to operate, the flux through
the iron core rapidly changes, which induces eddy currents in the iron. The eddy currents
cause heating in the iron cores due to the high resistance of iron to eddy currents, which
causes power to be lost in transformers
Excessive heating can also damage or destroy the transformer, so power losses in
transformers needs to be reduced.
One of the primary ways to reduce heating in transformers is to construct the cores out of
laminated iron
o Laminated iron consists of thin sheets of iron pressed together, but separated by
thin insulating layers
o By positioning the cross-section of insulating layers perpendicularly to the direction
of magnetic flux, the size of eddy currents can be greatly reduced, as the eddy
currents are limited to the thickness of one lamina
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Heating can also be reduced by constructing iron cores out of ferrites, which are complex of
iron and other metals. These materials are good transmitters of magnetic flux, but poor
conductors of electricity, so the magnitude of eddy currents are significantly reduced
Once the transformer becomes hot, it must be cooled to prevent overheating, which can be
achieved in a variety of ways:
o The transformer can be made of black material so that heat produced internally can
be efficiently radiated to the environment => most domestic transformer rectifier
units are made out of black material
o Pad-mounted transformers at ground level have ventilated cases to allow air to
remove heat by convection. They may also have an internal fan to assist air
circulation and remove heat faster.
Explain the role of transformers in electricity sub-stations
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Recall that electricity is transmitted at high voltages (up to 500kV) over long distances from
the power station to the sub-station to reduce power losses
The domestic electricity supply is typically 230V single-phase AC, whilst the industrial
electricity supply is usually 415V three-phase AC, thus the voltage needs to be reduced for
consumer use
The voltage of electricity is progressively reduced at sub-stations, where step-down
transformers are used to reduce voltage for the consumer. At each stage, the output voltage
is chosen to match the power demand and the distance over which supply is needed
Gather and analyse secondary information to discuss the need for transformers in
the transfer of electrical energy from a power station to its point of use
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Electricity is typically consumed in homes at 230V and in industry at 415V. If there were no
transformers to change voltage, electricity would have to be generated and distributed at
these same voltages.
In order to transmit electricity efficiently over long distances, a low current is required (recall
P=I2R), which in turn means that electricity must be transmitted at high voltages of up to
500kV
In addition, power stations run more efficiently if electricity is generated at higher voltages,
such as 23kV
Thus transformers are needed in the transfer of electrical energy from a power station to its
point of use, as each stage of transmission runs more efficiently and economically at certain
voltages and currents
ELECTRICAL POWER DISTRIBUTION
Electricity at the generator is typically produced at levels of 23kV
A step-up transformer is used to increase the voltage to higher voltages (generally 330kV),
which is transmitted to regional substations
At regional substations, step-down transformers are used to reduce voltage to 110kV or
66kV for regional distribution
Local substations use step-down transformers to reduce voltage to 33kV or 11kV for
distribution along local streets
Finally, pole transformers reduce voltage to 230V (domestic) or 415V
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Power substations usually perform three tasks:
o Step down the voltage using transformers
o Split the distribution voltage to go in different directions
o Enable, using circuit breakers and switches, the disconnection of the substation from
the transmission grid or sections of the distribution grid to be switched on and off
The diagram below summarising the use of transformers in power transmission
Discuss why some electrical appliances in the home that are connected to the mains
domestic power supply use a transformer
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Australian houses are provided with 230V AC electricity. Whilst many devices are designed
to operate efficiently at this voltage, some appliances contain components that operate best
at different voltages from the mains supply
For example, the component within a microwave oven that produces the microwaves
typically requires thousands of volts, whilst the control circuits and control panel on the
front only require small voltages. This means that a microwave oven would require both
step-up and step-down transformers to supply the power for its components.
Another device that requires a step-up transformers is the cathode ray tube in CRT
television, which require voltages of up to 25kV
Devices that require step-down transformers include electric keyboards, answering
machines, the amplifier in radios, and the batteries in cordless telephones and laptop
computers. Most electronic circuits are designed to operate at low DC voltages.
Discuss the impact of the development of transformers on society
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The development of transformers has allowed electrical energy to be efficiently transmitted
over great distances. This had had a range of impacts on society:
POSITIVE IMPACTS
Power stations can be placed far away from urban centres, otherwise many power stations
would be required in cities. This reduces pollution, reduces cost and improves the standard
of living of people living in cities.
Transformers enable power from one power station to be used in many different
applications, as the voltage can be changed as required. Without transformers, different
industries requiring different voltages would have to build generators to produce those
specific voltages, which would have extra cost and cause more urban pollution.
Transformers have enabled the construction of many electronic labour-saving and
entertainment devices, such as TVs, computers, mobile phones, electronic clocks, and
kitchen appliances.
See section three on the advantages of generators for more positive impacts
NEGATIVE IMPACTS
See section three on the disadvantages of generators for negative impacts
Perform an investigation to model the structure of a transformer to demonstrate
how secondary voltage is produced
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METHOD
A 300 turn solenoid was placed inside a 5000 turn solenoid of a larger radius. The smaller
solenoid was connected to an AC supply, whilst the larger solenoid was connected to both
ends of an adjustable spark gap. The AC supply was turned on, and any observations were
noted.
Draw the apparatus if possible when asked
SAFETY
The sparks produced can produce X-rays, which are harmful to cells if exposed for too long.
Stand at least 3m away from the apparatus whilst the AC supply is turned on to reduce
exposure (according to the inverse square law)
Ozone is also produced by the spark gap, which can cause respiratory damage if inhaled.
Conduct the experiment in a well-ventilated are to minimise this problem
RESULTS
When the AC supply was turned on, sparks would jump across the gap, which demonstrated
the induction of a current in the secondary coil
As the primary coil was connected to an AC supply, the flux produced by the coil was rapidly
changing, which in turn induced an emf in the secondary coil
The voltage produced by the secondary coil can be calculated using the transformer
equation
ACCURACY/RELIABILITY/VALIDITY
No quantitative data was recorded, so accuracy is irrelevant
The experiment was repeated, and sparks occurred each time, so the experiment was
reliable
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A control reading was taken with the AC supply turned off, which produced no sparks
Problems with the experimental design include the following:
o Not all the flux from the primary coil threaded through the secondary coil, which
could have been improved by using a soft iron core
o Some of the power from the primary coil was transferred to heat, which reduced the
efficiency of the transformer
5. Motors are used in industries and the home usually to convert electrical
energy into more useful forms of energy
Describe the main features of an AC electric motor
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There are two main types of AC electric motors: the universal motor, and the AC induction
motor
UNIVERSAL MOTOR
The universal motor is designed to operate on both single-phase AC and DC, and is the most
common type of motor in domestic appliances
Below is a schematic diagram of a universal motor
The stator consists of coils wound around a laminated iron core, which acts as an
electromagnet
The rotor consists of several coils wound onto the rotor armature, which are connected in
series with the electromagnet coils. The ends of the rotor coils connect to opposite
segments of a commutator.
The interaction between the current in the rotor coils and the external magnetic field
produces the torque that makes the rotor rotate
An external variable resistor controls the speed of the universal motor by varying the current
through the motor
Even though the direction of the current is changing when operating using AC, the rotor will
rotate in the same direction because the magnetic field flux of the stator is also changing
with the same frequency
The diagram shows the various components of a universal motor
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AC INDUCTION MOTOR
An induction motor operates on the principle of electromagnetic induction, as the changing
magnetic field set up in the stator induces a current in the rotor, which produces a torque
and causes the rotor to rotate
The simplest induction motor is the three-phase induction motor, though single-phase
induction motors are available
The main components of the three-phase induction motor are the stator and the squirrelcage rotor
Below is a diagram of the stator:
The stator consists of three pairs of electromagnet coils, each of which is connected to an
individual phase of the three-phase supply
The out-of-phase AC produces a rotating magnetic field that has constant magnitude, which
rotates at the same frequency as the mains supply (i.e. 50Hz)
Below is a diagram of the squirrel-cage rotor
Around the circumference of the rotor are a number of parallel conducting bars made of
either aluminium or copper, hence the name squirrel-cage rotor. These bars are joined at
the ends by an end ring that allows current to flow from one bar to another.
The rotor is encased in a laminated iron armature to intensify the magnetic field, and the
armature is mounted on a shaft that passes out through the end of the motor and is free to
rotate
The rotating stator field induces a current in the bars of the rotor, which experiences a
torque due to the magnetic field. The force is always in the same direction as the movement
of the magnetic field, so the rotor ‘chases’ the rotating magnetic field
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To work out the direction of the force, remember that a magnetic field moving to the right
relative to a stationary conductor has the same effect as a current-carrying conductor
moving to the left in a stationary magnetic field
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If the bars of the squirrel cage were to rotate at exactly the same rate as the magnetic field,
there would be no relative movement between the bars and the magnetic field, thus no
induced current and no torque
For the rotor to experience a force there must be relative movement or a ‘slip’, which is
caused by the retarding torque on the rotor when a load is attached. The difference
between the rotational speeds of the coil and the magnetic field is called the ‘slip speed’
When a large load is attached, the slip speed of the rotor is greater, thus the induced current
and magnetic torque are increased
The induction motor contains no brushes or commutator, which means easier manufacture,
no wear, no sparks, no ozone production, and none of the energy loss associated with them
Below is a table summarising the different types of motors and their uses
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Gather, process and analyse information to identify some of the energy transfers
and transformations involving the conversion of electrical energy into more useful
forms in the home and industry
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Electrical energy is transferred from the power station to the homes and industries through
transmission lines and transformers (see the relevant sections above for more details)
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Electrical energy is then transformed into more useful forms. Remember that not all
transformations are 100% efficient, and other forms of energy may also be produced as
waste energy
The table below summaries energy transformations in the home and industry
Form of energy
Home
Industry
Heat
Induction cooktops, home
Furnaces
heating, hot water system
Light
Light bulbs
Neon lights, laser printing
Sound
Speakers
Chemical
Recharging batteries
Producing batteries, electroplating
Kinetic
Blender, fan, drill
Industrial motors, transport
Perform an investigation to demonstrate the principle of an AC induction motor
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METHOD
The base of an aluminium drink can was attached to a long piece of fine thread. The thread
was attached to a clamp on a retort stand, and was allowed to hang vertically and freely
rotate. A bar magnet was then firmly attached to the end of a drill bit, and the drill bit was
secured in the drill chuck of a hand drill. The hand dill was rotated so that the magnet would
spin in one direction. The bar magnet was then removed, and rotated similarly.
Below is a diagram of the apparatus:
SAFETY: Make sure that all components are secured properly, as the rotation may cause
them to fly off, and cause damage to equipment and potential injury
RESULTS
The rotating magnet caused the aluminium disc to rotate in the same direction as the
magnet. The faster the magnet was rotated, the faster the disc rotated. The closer the
magnet was to the disc, the faster the disc rotated.
The disc did not rotate when the magnet was removed, which suggests that the disc rotated
due to electromagnetic interaction
ACCURACY/RELIABILITY/VALIDITY
No quantitative data was obtained, so accuracy is irrelevant
The experiment was repeated several times with similar observations, and similar results
were obtained by other members of the class
The rotating drill chuck without the magnet acted as the control, which confirmed that the
rotating magnet caused the disc to rotate
All variables were controlled, such as the type of disc used and the hand drill used
Potential problems with the experiment include:
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Impurities in the aluminium can may reduce the conductivity of the can, thus it may
be difficult to cause the disc to rotate
The tangling of the thread may reduce the ability of the disc to rotate
The hand drill may not provide the rotation speeds required to cause the disc to
rotate