Download PHYSICS – Motor and Generators Section I

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PHYSICS – Motor and Generators
Section I- Motors use the effect of forces on current-carrying conductors in
magnetic fields
Perform a first-hand investigation to demonstrate the motor effect
Aim: To demonstrate the motor effect.
Hypothesis: N/A
Materials: powerful bar magnets, conducting wire, DC power source, switch, resistor, ammeter
(optional)
Method:
1. Set up a circuit with the DC power source, wire, switch, resistor and ammeter.
2. Place a section of the circuit between the north and south poles of two bar magnets.
3. Turn on the power source and close the switch.
4. Observe the movement of the wire, in particular its direction relative to the magnetic field
and current.
5. Repeat steps 3 and 4 but change the direction of the magnetic field as well as the strength
of the current.
Results: The wire was observed to move while a current flowed through it in the presence of a
magnetic field. The direction of the movement was predicted by the right-hand palm rule (explained
later).
Conclusion: The motor effect was successfully demonstrated. Information regarding the motor effect
is given throughout the rest of the section.
Identify that the motor effect is due to the force acting on a current-carrying conductor in a magnetic
field
As demonstrated in the practical task above, the motor effect is due to the force acting on a currentcarrying conductor in the presence of a magnetic field. The precise relationship can be given by the
right-hand palm rule, where the thumb points in direction of (conventional) current, the fingers in
the direction of the magnetic field (north to south) and the palm indicates direction of force.
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Discuss the effect, on the magnitude of the force on a current-carrying conductor, of variations in:
- the strength of the magnetic field in which it is located
- the magnitude of the current in the conductor
- 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
The variables B (magnetic field strength), i (current) and l (length) all relate to the force on the wire
linearly. That is, a constant increase in one results in a constant increase in the force. This should be
pretty obvious. The only variable that is not linear is the angle (angles are almost always related with
𝜋
sines and cosines). The force is greatest when the wire is perpendicular to the magnetic field (𝜃 = 2 )
and equal to zero when the wire is parallel (𝜃 = 0). Mathematically:
𝐹 = 𝐵𝑖𝑙𝑠𝑖𝑛𝜃
Solve problems and analyse information about the force on current-carrying conductors in magnetic
fields using 𝐹 = 𝐵𝑖𝑙𝑠𝑖𝑛𝜃
These calculations should not really be hard, ESPECIALLY IF YOU DO (4U) MATHS. Personally I’ve
never seen one ask you to calculate the angle. Make sure your calculator is set to degrees (since HSC
physics loves using degrees instead of radians), and remember that the length is in metres, not
centimetres. Here is an example:
Calculate the force exerted on a wire of length 5cm if it is placed at 30o in a magnetic field of
strength 0.01 Teslas and a current of 10 amps is flowing through.
𝐹 = 𝐵𝑖𝑙𝑠𝑖𝑛𝜃
𝐹 = 0.01 × 10 × 0.05 × sin 30o
𝐹 = 0.0025 𝑁
Remember to round off correctly in the real exam. I didn’t in this example.
Describe qualitatively and quantitatively the force between long parallel current-carrying conductors
𝐹
𝐼1 𝐼2
=𝑘
𝑙
𝑑
AND
Solve problems using
𝐹
𝐼1 𝐼2
=𝑘
𝑙
𝑑
The force per unit length on two parallel current-carrying conductors is related to the magnitude of
the currents and the distance between them. Currents flowing in the same direction attract and
currents flowing in opposite directions repel. Mathematically, their relationship is described by the
equation:
𝐹
𝐼1 𝐼2
=𝑘
𝑙
𝑑
Here k is the magnetic force constant 2.0 x 10-7 NA-2. The magnetic force constant is provided on the
data sheet. Calculations involving this equation should not be difficult. Remember lengths and
distances are in metres.
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Define torque as the turning moment of a force
Torque is a vector that describes the “force” on an object around a pivot point. Technically it is not
force as in Newtons, and is sometimes described as the “turning moment/strength” of a force.
Conceptually, torque explains why sitting on the end of a see-saw makes you “heavier” and why we
use spanners to tighten/remove screws instead of our hands. Notice how a longer pivot length
increases torque. Mathematically:
𝜏 = 𝐹𝑝
The small symbol of the Greek letter tau is used for torque. F represents force in Newtons and p is
the perpendicular distance. The length must be perpendicular to the direction of the applied force
otherwise another form of this equation is needed (not part of HSC).
Describe the forces experienced by a current-carrying loop in a magnetic field and describe the net
result of the forces
AND
Solve problems using
𝜏 = 𝑛𝐵𝑖𝐴 𝑐𝑜𝑠 𝜃
In a simple coil such as the one above, application of the right-hand palm rule shows that only the
length of conductor on the left and right experience a force. In the above diagram, the right hand
length experiences a force downwards and the left hand length experiences a force upwards. The
net result is a clockwise rotational motion.
Mathematically:
𝐹 = 𝐵𝑖𝐿 , for the force on each length of wire
𝑏
𝜏 = 2 (𝐹 × ) = 𝐹𝑏
2
𝜏 = 𝐵𝑖𝐿𝑏
𝜏 = 𝐵𝑖𝐴 , where A is the area (Lb)
𝜏 = 𝑛𝐵𝑖𝐴 cos 𝜃 , for the general case
The reason cosine is used here instead of sine is because the angle measured here is different from
the F = Bilsinθ one. That angle is rotated in the plane of the coil whereas this is the angle between
the planes. Calculations involving this equation shouldn’t be difficult.
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Describe the main features of a DC electric motor and the role of each feature
Magnets
Magnets form part of the stator (usually) and are arranged around the coil. Usually curved magnets
are used to provide constant torque rather than the straight ones seen in diagrams. The magnets
can be either permanent or electromagnetic. The role of the magnets is to provide a (radial)
magnetic field surrounding the current-carrying coil so that it experiences a force.
Coil and Armature
Simple diagrams of motors show only one coil as part of the rotor. However, all working DC motors
have many coils of conducting wire wrapped longitudinally around an armature, which is a core of
laminated soft iron. As part of the rotor, the coil provides a structure which current can flow
through, and the shape of the armature translates this into circular motion.
Split-Ring Commutator
Applying the right-hand palm rule after the coil has turned the first 90o shows that the force is now
in the opposite direction (anticlockwise in the diagram above). Left uncorrected, the coil would
merely oscillate around this vertical position and no net rotation would occur. This is corrected with
a split-ring commutator, which is a small metal cylinder with two slits parallel to its length. The use
of a split-ring commutator reverses the direction of current every half-cycle, making the coil rotate in
one direction continuously. This is a noticeable feature of DC motors (AC motors do not use splitring commutators).
Conducting Brushes
Conducting brushes are pieces of conductor that make contact with the two halves of the
commutator, providing current and linking the coils to the rest of the electric circuit. The brushes are
usually graphite, which has the added advantage of being a lubricant. Springs hold the brushes
tightly against the commutator to minimise sparking (which damages the motor).
Axle
The axle is a bar of metal (usually hardened steel) that passes through the middle of the armature.
The axle provides structural support as the centre of motion for the rotor to rotate around. Work
done by the motor is transferred via the axle (for example to the wheels of a vehicle).
Identify that the required magnetic fields in DC motors can be produced either by current-carrying
coils or permanent magnets
As learnt previously, an electric current produces a magnetic field, and when arranged in a coil
(solenoid), produces a magnetic field similar to that of a bar magnet. Thus, the required magnetic
field in a DC motor can either be from permanent magnets or from a current-carrying coil.
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Identify data sources, gather and process information to qualitatively describe the applications of the
motor effect in the galvanometer and the loudspeaker
The Galvanometer
A galvanometer is a very sensitive ammeter that detects small levels of
current. The main features of a galvanometer, depicted on the right, are a
carrying coil, magnets and a restoring spring. When current flows through the
circuit and through the galvanometer, the motor effect is induced, producing
a force that rotates the needle to one direction. However instead of rotating
as a motor would, a spring limits the movement of the needle, producing a
reading of current (the force exerted by a spring is proportional to the
amount it is rotated by, giving a linear scale). Some galvanometers have the
needle zeroed in the centre.
The Loudspeaker
A loudspeaker usually contains a three-pronged
magnet, with the middle projection being the opposite
of the other two. A coil of conducting wire is wrapped
around the middle, and a variable current applied (not
regular DC). The interaction between the currentcarrying coil and the permanent magnetic structure
via the motor effect produces varying forces on the
magnet, which is transferred to a cardboard (usually)
cone. This cone vibrates in accordance to the
amplitude and frequency of the current, producing
sound waves.
current-
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Section II- The relative motion between a conductor and a 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
Michael Faraday is accredited with the discovery of electromagnetic induction, the phenomenon
whereby relative movement between a magnetic field and a conductor generates a current. After
Han Oersted showed that a current generated a magnetic field, many attempted to show the
converse- that a magnetic field could generate current. Many did not succeed because they failed to
realise that it was a moving magnetic field (or conductor) that was required.
Faraday’s initial experiments involved an “induction ring”, a ring of iron with two separate coils of
insulated copper wire wrapped around it. Faraday noticed that when he connected one coil to a DC
source, there was momentary current in the other coil. He realised that the initial activation of a
magnetic field was the cause of the current in the other coil. From there Faraday performed many
more experiments regarding the newly discovered phenomenon of electromagnetic induction.
Perform an investigation to model the generation of an electric current by moving a magnet in a coil
or a coil near a magnet
AND
Plan, choose equipment or resources for, and perform a first-hand investigation to predict and verify
when:
- the distance between the coil and magnet is varied
- the strength of the magnet is varied
- the relative motion between the coil and magnet is varied
Aim: To generate an electric current via electromagnetic induction and investigate factors that may
affect the strength of the induced current.
Hypothesis: N/A
Materials: cardboard tube, length of conducting wire, bar magnets, centre-reading galvanometer
Method:
1. Wrap the conducting wire around the cardboard tube to form a coil of about fifty turns.
2. Connect the ends of the wire to the centre-reading galvanometer.
3. Move a bar magnet through the solenoid in one direction.
4. Observe the reading on the galvanometer.
5. Repeat steps 3 and 4 but with varying distances, magnetic strength and relative motion.
Results: When the magnet entered the solenoid a current was induced. When it moved inside the
solenoid, no current was induced. When it left the solenoid, negative current was induced. A
stronger magnet generated a stronger current, greater distance between the solenoid and magnet
generated less current, and a greater relative velocity generated a greater current.
Conclusion: An electric current was successfully generated via electromagnetic induction, and
factors affecting this current were investigated. It was shown that relative motion between a
magnet and conductor was required to generate current and that various factors could affect the
strength of the current.
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⃗ as magnetic flux density
Define magnetic field strength 𝐵
AND
Describe the concept of magnetic flux in terms of magnetic flux density and surface area
As with the other forces such as gravity, the magnetic (technically electromagnetic) force can be
described using imaginary field lines. The strength of a magnetic field in an area is defined to be the
number of arbitrarily designated field lines (flux) passing through a flat surface. Qualitatively, the
more field lines, the greater the magnetic field strength. Thus the magnetic field strength can be
determined as the number of field lines per unit area, i.e. magnetic flux density. Mathematically:
𝜙
𝐴
𝜙 = 𝐵𝐴
𝐵=
The magnetic flux of an area (i.e. the total number of field lines) is given by the product of the flux
density (lines per unit area) and the surface area.
[NOTE]- Magnetic field strength is actually a vector (as can be seen with the vector notation). The
“lines through an area” definition gives only the magnitude not the direction, which is what the
above formula shows.
Describe generated potential difference as the rate of change of magnetic flux through a circuit
Previously it was shown that a changing magnetic field was required to induce a current. More
specifically, the magnitude of the induced emf (electromotive force) is proportional to the rate of
change of magnetic flux. Faraday’s Law describes this relationship:
ℇ𝑖 ∝
𝑑𝜙
𝑑𝑡
Lenz’s Law (explained later) gives the constant of proportionality:
ℇ𝑖 = −𝑛
𝑑𝜙
𝑑𝑡
Account for Lenz’s Law in terms of conservation of energy and relate it to the production of back emf
in motors
Lenz’s Law states that the direction of the induced emf is such that the current produced creates a
magnetic field opposing the change that produced the emf. Simply put, the magnetic field created
opposes the motion of the original magnet. For example, moving a north pole into a solenoid will
induce a current such that the new magnetic field produces a north pole closest to the magnet (the
repulsion opposes the motion). The direction of the current can be found with the right-hand grip
rule (thumb in direction of north pole, fingers curl in direction of current).
Lenz’s Law is a necessary consequence of the Law of Conservation of Energy. Because we are
generating a current by moving the magnet (i.e. generating energy), kinetic energy of the magnet
must be lost to compensate. If this were not so, and the induced magnetic field aided the motion,
we would have increased kinetic energy and electrical energy. This is in violation of the Law of
Conservation of Energy, and thus the induced magnetic field must oppose the motion, converting
kinetic energy into electric energy.
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Further application of Lenz’s law shows that it applies to electric currents as well as magnetic fields.
A current-carrying conductor moving in a magnetic field will induce an emf that opposes the current
that created the motion. This is known as back emf, with important consequences in electric motors.
Explain that in electric motors, back emf opposes the supply emf
In electric motors, a current is produced using an external emf (from the power source). The current
in the coil generates a magnetic field which interacts with the existing magnetic field from the stator.
This interaction produces the rotational motion of the coil. Once it is moving, however, Lenz’s Law
takes effect. A moving conductor in the presence of a magnetic field will induce a current, the back
emf, which opposes the original current (the supply emf). The result is that the net current is
reduced, with an upper limit to the current in the coil. When the supply emf is equal to the back
emf, the motor reaches a steady speed and is self-regulating.
Although back emf may sound disadvantageous (after all you are losing current), it has one main
significant feature making it desirable. This is that it provides resistance to the coil, preventing a
short-circuit. Large amounts of current flowing through the coil can burn out the motor, and as such
the back emf provides a self-regulating level of resistance. Motors are most at risk during start-up
and if they are jammed, because there is no motion and thus no back emf, but the supply emf is still
present. To minimise the risk, a starting resistance is sometimes added during activation, which is
taken away after the back emf is significant.
Explain the production of eddy currents in terms of Lenz’s Law
Previously, Lenz’s Law was only applied to wire conductors. However, it is equally applicable to solid
sheets or blocks of conductors, where they induced currents are termed eddy currents.
The diagram above shows the formation of a simple eddy current. As a sheet of metal moves in or
out of a magnetic field, currents are induced (determined using the right-hand palm rule) which
often rotate in a loop. These eddy currents are formed in such a way that they generate a magnetic
field to oppose the motion of the conducting metal.
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Gather, analyse and present information to explain how induction is used in cooktops in electric
ranges
Induction cooktops use coils beneath a glass-ceramic surface to generate heat for cooking food.
Alternating current is supplied to the coils, generating a rapidly oscillating magnetic field which
induces eddy currents in the metal cookware above the coils. The eddy currents face resistance from
the metal, which converts the energy to heat. The heat form the metal is then transferred to the
food, cooking it. Induction heaters have the advantage in that almost all the energy goes into
heating the cookware and not the surface of the cooktop, giving them a higher degree of efficiency.
Gather secondary information to identify how eddy currents have been utilised in electromagnetic
braking
Electromagnetic braking involves using eddy currents to
slow the motion of a wheel made from a conducting metal
(e.g. aluminium). The figure to the right shows a simplified
diagram of an electromagnetic braking system. Magnets are
brought close to the rotating metal, and the relative motion
generates eddy currents. As per Lenz’s Law, these eddy
currents generate a magnetic field that is set up to oppose
the motion of the original magnetic field. The interaction of
the opposing magnetic fields creates a resistive force that
quickly slows down the motion of the wheel, bringing it to a
halt. Electromagnetic braking has the advantage in that the resistive force is proportional to the
rotational velocity of the wheel, making it smoother than traditional braking systems such as disc
brakes.
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Section III- Generators are used to provide large-scale power production
Describe the main components of a generator
The components of a generator are virtually the same as a motor. An extensive description of the
parts is given in the motor dot-point above.
Magnets
Generators have an array of magnets (permanent or electromagnetic) around the coil to provide the
magnetic field. Usually they do not move and form part of the stator. In AC generators they may be
part of the rotor.
Coil and Armature
The rotor contains a central armature and many coils wrapped around it. Rotating the coils in the
magnetic field induces the current. In some AC generators they form the stator.
Split-Ring Commutator/Slip Rings
In DC motors, a split ring commutator is required to ensure that current flows in one direction only
by switching the circuit every 180o. In AC motors the current is required to change direction
constantly, so no split-ring commutator is required. Instead, slip rings are used.
Conducting Brushes
The conducting brushes carry current away from the coils. Like most other parts of the generator,
they serve the same purpose in a motor.
Plan, choose equipment or resources for, and perform a first-hand investigation to demonstrate the
production of an alternating current
Aim: To produce an alternating current.
Hypothesis: N/A
Materials: solenoid, bar magnet, centre-reading galvanometer
Method:
1. Connect the solenoid to the galvanometer.
2. Move the magnet in and out of the solenoid continuously.
3. Observe the reading on the galvanometer
Results: The reading on the galvanometer oscillated.
Conclusion: The oscillating current indicated the production of an alternating current.
You may have also been shown a larger AC generator that was operated with a handle and lit up a
small bulb.
Compare the structure and function of a generator to an electric motor
Structurally, simple generators and motors are very similar, and most motors can be used as
generators and vice versa. Applying a current to the structure of a motor/generator makes it rotate,
operating as a motor. Rotating the rotor manually generates an induced emf, making it operate as a
generator. The function of a motor is to convert electrical energy to kinetic energy, whereas the
function of an electric motor is to convert kinetic energy into electrical energy.
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Describe the differences between AC and DC generators
Structurally, the main difference between AC and DC motors is that DC motors have split-ring
commutators while AC motors have slip rings (there are also many differences in auxiliary
equipment, but this is not required at the HSC level).
DC generators, as their name suggests, are designed to produce a DC output.
Previously it was shown that the induced current was proportional to the rate of change of flux:
𝑑𝜙
ℇ𝑖 = −𝑛
𝑑𝑡
Observing the diagrams above, it can be seen that the flux is zero when the rotor is horizontal and at
a maximum when the rotor is vertical. The induced emf is proportional to the rate of change of flux,
and this is at a maximum when the rotor is moving about the horizontal position, and zero when the
rotor is moving about the vertical position (opposite to the flux). The relationship between the
position of the rotor and the size of the current is given by the absolute value of a sinusoidal graph
(as shown). This is because the motion is periodic, and the split-ring commutator prevents a negative
current. To produce a more constant current, radial magnets and multiple coils at an angle to each
other are used.
AC motors provide an AC output. Essentially they operate the same as a DC motor but without the
split-ring commutator, and thus their current output graph looks like a regular sine/cosine wave.
[NOTE]- Mathematically, flux is given as a sine curve, since it is really equivalent to the vertical height
of the coil in two-dimensional space (vertical component of circular motion is given by sine). Because
the induced current is proportional to the derivative, it is given by a negative cosine curve. When a
sine curve is at zero, the corresponding cosine curve is at a maximum and vice-versa.
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Gather secondary information to discuss advantages/ disadvantages of AC and DC generators and
relate these to their use
One advantage of AC generators over DC generators is that there is less wear and tear in the rotor.
DC generators use split-ring commutators which, lead to constant striking of the carbon brushes
against the leading edges of the commutator. The damage accumulates quickly, and thus DC motors
require constant maintenance due to the split-ring commutator. In contrast, AC motors use slip
rings which have a smooth, continuous surface. This reduces damage and removes the possibility of
sparking. Overall, AC motors require less maintenance and are more reliable.
In addition, DC motors must induce current in the rotor. For large currents, the rotor must be
correspondingly heavier, requiring larger support structures. The high currents also create an
opportunity for electrical arcing when the brushes and commutator separate, reducing efficiency
and generating radio noise. Because AC generators do not require a split ring commutator, the
current can be induced in the stator, with the magnets forming the rotor instead. This makes AC
more suited to high-current applications.
One advantage of a DC generator is that multiple coils can be arranged as part of the rotor at an
angle to each other. This produces multiple currents out of phase, which add up to form a relatively
constant current. This is not possible with an AC generator without additional equipment. Thus DC
generators can easily supply a steady DC current for smaller devices.
Analyse secondary information on the competition between Westinghouse and Edison to supply
electricity to cities
In the late 19th century, the competition between DC (developed and advocated by Thomas Edison)
and AC (developed by Nikola Tesla and advocated by George Westinghouse) was such that it is
labelled as the “War of Currents”. Initially Edison had the advantage as his system of DC was well
developed and he held a monopoly on electricity distribution. DC worked well as it was simple, but it
had to be produced at high current for use in homes and factories, incurring huge energy losses
(energy loss is proportional to the square of the current). This meant that many generators had to be
built, and placed all around the city, connected with many unsightly power lines.
However, Westinghouse realised the potential of AC in that it the voltage/current could easily be
altered using a transformer. This gave it a huge advantage over DC, although it was less established
than Edison’s DC system. The ability of AC to be transformed meant it could be converted to high
voltage for transportation and then reduced for used in homes and factories. This meant generating
plants could be built far away from the busy cities and fewer and smaller power lines were required.
The great economic advantages of AC allowed it to compete with Edison’s DC as the primary source
of power for America.
Competition was rarely fair, with many attempts by Edison to destroy Westinghouse’s efforts to
establish an AC electrical supply system. Edison began a smear campaign against AC, claiming that it
was dangerous due to the high voltages involved (which is true). He strongly advocated for the use
of AC as the city’s chosen current for execution, believing that this would prove his point (he tried to
have the term “electrocuted” known as “Westinghoused”). However despite his efforts, AC had
several important victories, including the contract for the Niagara Falls project and the Chicago
World Fair. Eventually AC was chosen over DC due to its many advantages, and we still use AC
predominantly today.
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Discuss the energy losses that occur as energy is fed through transmission lines from the generator to
the consumer
Although transmission lines for electricity distribution are designed to be low resistance,
nevertheless the long distances involved increases the total resistive force of the wire.
Mathematically, the power loss in a wire is given by:
𝑃=
𝐼2
𝑅
Since the power loss is proportional to the square of the current, electricity is transported at the
highest reasonable voltage, which results in low current. This minimises the power loss in energy
cables, and is the reason AC is used over DC (because AC can easily be transformed to different
voltages). A suitable choice of conductor, usually copper or aluminium is also used to minimise
energy losses.
Further energy losses can be attributed to the ionisation of air by the electron flow, which is
minimised by insulation, and inductive energy losses from the formation of eddy currents in
transformers, which are minimised by using laminations (explained later).
Gather and analyse information to identify how transmission lines are:
- insulated from supporting structures
- protected from lightning strikes
It is essential that transmission lines be sufficiently insulated to prevent arcing and short-circuits,
both of which would damage structures, result in massive power losses and be a health hazard to
citizens. Insulation from supporting structures is achieved with saucer-shaped ceramic or reinforced
glass. These are hung downwards and the power lines wrapped around, preventing electricity from
jumping to nearby structures and protecting them from rain and hail.
Protection from lightning strikes is achieved by stringing a protective wire high above the normal
transmission lines. This line attracts lightning strikes, and the surge of electricity is earthed to
prevent damage to the pylon.
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Assess the effects of the development of AC generators on society and the environment
Prior to the establishment of electricity as our main source of energy, cities were extremely dirty,
crowded areas. This was because factories and power plants have to be built close to where they
were required, meaning cities were dotted with many power plants. Not only was this unsightly, but
the pollution produced by the many coal plants in the city made city life extremely unhealthy. The
development of AC, which can be transported easily over large distances, is responsible for the cities
we see today. In modern times, electricity plants can be situated far away in the countryside, where
they are not seen, and far fewer of them are needed. This has led to an improvement in living
conditions and a subsequent increase in life-expectancy.
Furthermore, the development of electricity as a power source has enabled the technological
revolution to occur, resulting in the high-tech society we live in today. Almost every aspect of our
lives relates to electronic devices in some way or other, and computers and digital media are slowly
finding applications everywhere. The explosion in computer use in recent decades has also greatly
increased society’s technological capabilities, and this would only be possible with the invention of
electricity. More evidence showing society’s dependence on electricity can be seen with the
economic losses incurred by blackouts and the presence of back-up generators in hospitals.
However, the development of electricity has come at a cost to the natural environment as the
demand for energy skyrockets. The need for energy is causing many natural areas to be cleared for
power production, and rivers may be dammed for hydroelectric plants. The pollution given off by
these plants also negatively impacts on the environment, although it is carefully monitored to
minimise serious harm.
Overall, it can be concluded that the development of AC generators has had a profound effect on
society, allowing for the technological revolution we see today. It has also made cities far cleaner
places to be, improving life expectancy. However, this comes at the detriment of the environment,
which is negatively affected by the pollution given off by power plants.
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Section IV- Transformers allow generated voltage to be either increased or
decreased before it is used
Describe the purpose of transformers in electrical circuits
Transformers are used to change the voltage (and current) of a supply of electricity. This is done
because many appliances require lower voltages and transportation of electricity requires high
voltage to minimise energy losses. Transformers can only operate on AC, not DC.
Perform an investigation to model the structure of a transformer to demonstrate how secondary
voltage is produced
Aim: To model a transformer to demonstrate the production of a secondary voltage.
Hypothesis: N/A
Materials: Soft iron rod, two lengths of insulated copper wire, DC power source, voltmeter
Method:
1. Wrap one length of copper wire around the soft iron core 30 times.
2. Wrap another length of copper wire around the soft iron core 60 times.
3. Connect the coil of 30 turns to the DC power source.
4. Connect the coil of 60 turns to the voltmeter.
5. Turn on the power source and observe the reading.
Results: When the power was activated, a voltage was produced for an instant.
Conclusion: A secondary voltage was successfully produced in a model of a transformer. In theory
the secondary voltage should be twice that of the primary voltage.
Compare step-up and step-down transformers
Step-up transformer
Step-down transformer
More turns in the secondary than the primary
More turns in the primary than the secondary
Higher voltage in the secondary than the primary
Lower voltage in the secondary than the primary
Used to increase voltage for efficient
transmission
Used in sub-stations to lower voltage to more
useful levels
Used for devices such as phone chargers and
electronic displays
Used in TVs and other high-voltage appliances
Identify the relationship between the ratio of the number of turns in the primary and secondary coils
and the ratio of primary to secondary voltage
AND
Solve problems and analyse information about transformers using
𝑉𝑝
𝑉𝑠
=
𝑛𝑝
𝑛𝑠
The relationship is given by:
𝑉𝑝 𝑛𝑝
=
𝑉𝑠 𝑛𝑠
The voltage is proportional to the turns in both primary and secondary. This is assuming no energy
losses. Once again, calculations of this type should not be difficult.
Lok
Explain why voltage transformations are related to conservation of energy
Voltage transformations alter voltage, and by the Law of Conservation of Energy, this must result in
a change in current to compensate. Since time is constant (and thus can be neglected), the formula
for power/energy is:
𝑃 = 𝑉𝐼
To conserve energy, power must be conserved (power is the rate of change of energy). Thus if
voltage is increased, current is decreased and vice versa. Conceptually, it can be thought that the
increased number of turns “spreads out” the current, and thus voltage must increase to
compensate.
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
As mentioned many times before, a high voltage is required for minimising power losses over long
distances. However, household appliances do not operate at such high voltages (usually...), so
transformers are used to decrease voltage to useful values in populated areas. In NSW, the voltage
transformations go as follows:
20 kV– 500 kV (power plant supply to long-distance transmission voltage)
500 kV – 132 kV (at district transformer station)
132 kV – 66 kV (at local transformer station)
66 kV – 11 kV (at substation)
11 kV – 240 V (pole transformers for household use)
Notice how the voltages decrease as the supply approaches cities and suburbs. Some factories and
heavy-industry areas require higher voltages, which they take straight off transformer or
substations.
Explain the role of transformers in electricity sub-stations
Transformers in substations have the role of decreasing voltage from high-voltage transmission to
practical voltage for household use. Household appliances cannot operate at 500 kV, and such high
voltages would be extremely dangerous in suburbs and cities due to the sparking that could occur.
Discuss why some electrical appliances in the home that are connected to the mains domestic power
supply use a transformer
Many electrical appliances come with a transformer, which usually looks like a small black device.
Many of these transformers are built around the electrical plug, for example in mobile phone
chargers. Some devices (e.g. game consoles) require larger transformers, and they usually connect
the power port from the appliance to the socket. These transformers are required because some
appliances do not operate with the 240V supplied to homes. Smaller devices such as mobile phone
chargers operate on small voltages around 12V DC, and thus the charger has an in-built transformerrectifier that first reduces voltage then converts it to DC. Some appliances such as microwaves may
have multiple transformers. The magnetron requires high voltage while the electronic display
operates on low voltage, so two transformers are required.
[NOTE]- The power plugs you see that are fat and prevent the use of sockets next to them are most
likely transformers (and rectifiers).
Lok
Discuss the impact of the development of transformers on society
The development of transformers has allowed for the efficient transfer of electrical energy over long
distances. This means that power plants can be situated far away from population centres, reducing
pollution and making cities less crowded. Because electricity can be sent over long distances, rural
communities have also benefitted greatly from the development of transformers, and industrial
factories no longer need to be situated close to a power plant. This has led to an increase in living
standards and health of the population as a whole. Transformers also allow for the use of devices
that operate on different voltages, such as microwave ovens and mobile phone chargers.
Overall, the development of the transformer has revolutionised modern society, allowing electricity
to be used as a necessity rather than a luxury. The rapid improvements in modern technology would
not have been possible without transformers.
Gather, analyse and use available evidence to discuss how difficulties of heating caused by eddy
currents in transformers may be overcome
As shown previously, changing magnetic flux through a conductor generates eddy currents that
create heat via friction. With the high voltages involved in transformers, it is essential that this heat
be minimised, otherwise the heat would reduce its efficiency and cause irreparable damage to the
transformer (eddy currents can generate enormous amounts of heat, illustrated by the induction
stove). Some ways in which the heat is minimised is listed:
 Laminations (insulation) perpendicular to the plane of the magnetic field in the metal. This
limits the size of the eddy currents formed, reducing energy loss by an order of magnitude.
 Cooling fins that extrude from the back of transformers, presenting a large cooling surface
area to the air.
 Fans that circulate air.
 A coolant system (usually oil) that removes heat from the transformer and radiates it away
into the air.
Lok
Section V- 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
A universal electric motor is essentially the same as a DC motor except for the slip-rings, which
augment the split-ring commutator. It can run on AC or DC. The main features are:
 An armature around which many coils are rotated. The force acts on the rotor, making it
spin.
 A field structure with magnets, either permanent or electro. This provides the magnetic
field.
 Slip rings which augment the split-ring commutator. AC power provides the current switch,
so a split-ring commutator is not required for an AC supply.
Of more interest is the AC induction motor, which is different to the motors studied previously. As
the name suggests, the AC induction motor utilises the principle of electromagnetic induction to
rotate a “squirrel cage”.
The cage is made of a conducting material, and is situated on the inside of the motor. Around the
outside is a circular array of many coils, which are activated sequentially in a rotating fashion. This
generates a rotating magnetic field, which induces eddy currents in the bars of the cage. These eddy
currents form their own magnetic fields which interact with the magnetic fields of the coils,
“dragging” the rotor around with the rotating external magnetic field.
Lok
Almost all AC motors is use today are induction motors due to their many advantages. Some are
listed below:
 Simplicity of design
 High efficiency and low maintenance required (because there is minimal contact and very
few moving parts)
 Ability to self-start (not always the case for AC motors)
 Low costs
Most induction motors are single-phase. Heavy-duty motors used in industry tend to be triple-phase.
Perform and investigation to demonstrate the principle of an AC induction motor
Aim: To demonstrate the principle of an AC induction motor.
Hypothesis: N/A
Materials: aluminium pie tray, plastic tub, water, bar magnet, string
Method:
1. Pour a layer water into the plastic tub.
2. Flatten out the bottom of the pie tray (to minimise friction) and float on the water.
3. Tie a string around the magnet and twist.
4. Hold the magnet over the pie tray with the string and release to spin the magnet.
5. Observe the motion of the pie tray
Results: The pie tray rotated in the same direction as the magnet.
Conclusion: The pie tray and magnet effectively demonstrated the principles of the AC induction
motor.
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
An energy transfer is energy moving from one body to another without changing form. An energy
transformation is energy converting from one form to another.
Energy transfers include:
 Electricity moving from the power plant to a house
 Electricity moving from the house supply to a mobile phone via a charger
Energy transformations include:
 Electrical energy changing to heat energy in a kettle or stove
 Electrical energy changing to light energy in a TV
 Electrical energy changing to kinetic energy in a washing machine
 Electrical energy changing to chemical energy in electroplating (for industry)