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HSC Physics
Motors and Generators
Week 5
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HSC Physics
Motors and Generators
DUX
1
Week 5 – Theory

Discuss the energy losses that occur as energy is fed through transmission lines from the
generator to the consumer
Transmission lines
Power plants are usually built close to primary sources of energy such as coal mines, oil wells,
hydroelectric dams etc, which are very far away from cities where most consumers are situated. The
generated electricity must be transmitted over long distances, and as a result, significant amounts of
energy are lost as heat.
Calculating the power-loss
The transmission lines that carry the electricity have resistance and over large distances, this leads to
fairly significant losses in energy. The equation for power loss is:
𝑃𝑙𝑜𝑠𝑠 = 𝑉𝐼
From Ohm’s law, we know:
𝑉𝑑𝑟𝑜𝑝 = 𝐼𝑅
Combining the two, we get:
𝑷𝒍𝒐𝒔𝒔 = 𝑰𝟐 𝑹
This is the rate of power-loss (we will be using this equation the most). The actual energy lost is
𝑝𝑜𝑤𝑒𝑟 × 𝑡𝑖𝑚𝑒, so we get:
𝐸𝑙𝑜𝑠𝑠 = 𝑃𝑡 = 𝐼 2 𝑅𝑡
Power loss therefore is proportional to the square of the current. Since power is conserved (P is
constant), and 𝑃 = 𝑉𝐼, the higher the voltage, the lower the current. If we minimise the current, we
minimise the energy loss (because 𝑃𝑙𝑜𝑠𝑠 = 𝐼 2 𝑅).
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HSC Physics
Motors and Generators
DUX
2
In order to minimise current, electricity is boosted up to high voltages, by using step-up transformers
(we will learn this later). Energy losses can also be minimised by using conductors with low resistivity
(aluminium is used) but minimising current is by far the most effective way at minimising power loss
over long distances.
Worked example
A power station generates 2.0 MW of power and transmits this power to a city 60 km away
through transmission lines with resistance of 3 Ω per km. For the transmission voltages of 30
000 V, calculate the following:
(a)
(b)
(c)
(d)
the current in the transmission lines
the power loss in the transmission lines
the power left after the losses
the percentage of power lost
Solution
(a) I = Pgenerated/V = 2000 000 W ÷ 30 000 V = 66⅔ A
(b) Ploss = I2R = (66⅔)2 × (3 × 60) = 800 000 W
(c) Pleft = Pgenerated - Ploss = 2000 000 – 800 000 W = 1200 000 W
(d) % lost = 800 000 ÷ 2000 000 × 100% = 40 %
Transformers
To change electrical power from the generated voltage to high voltage, step-up transformers must
be used. Step-down transformers are also necessary to reduce the high voltage to domestic
voltages. In transforming the power, energy losses can result from eddy currents generating heat
due to the resistance of the iron core inside transformers. (This will be discussed in detail at a later
dot-point)
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HSC Physics
Motors and Generators
DUX

3
Gather secondary information to discuss advantages/disadvantages of AC and DC
generators and relate these to their use
DC generators
Advantages
 Can deliver a relatively constant voltage
under different electrical loads (since
phases can be added up)
 No need for current rectifiers, since DC
output is already rectified by split-ring
commutator
 Wires carrying DC do not need
electromagnetic radiation shielding,
since DC does not emit radiowaves
(cheaper wires)
 DC does not cause heart attacks and is
less dangerous than AC. DC can only
cause tissue burns, which is less likely to
kill
Disadvantages
 Currents are not easily stepped-up or
down, making transmission of DC over
long distances very costly (huge power
losses)
 Commutator and brushes wear down
faster, and there is more friction in
commutator (more power loss)
o As a result, DC generators have
shorter lifespan, less reliable
 Existence of solid-state rectification
methods (e.g. using diodes / transistors
etc) means AC generators can produce
DC just as well
Due to the above disadvantages, DC generators are limited to specialised uses only where the need
for DC voltage is significant. These include:



Electroplating
Aluminium refining
Chlorine production
AC generators
Advantages
 Voltages can be stepped up, allowing AC
to be transmitted over long distances
with minimal power losses.
 AC generators have less moving parts,
so they have longer lifespan, cheaper to
service and are more reliable
o AC generators can be designed
to work as a stationary coil and
rotating magnet, which is more
efficient, reliable, less moving
parts overall and easier to
maintain
 AC can be used to power induction
motors (we will learn this later),
something DC can’t do
Disadvantages
 AC is dangerous as it causes fatal heart
attacks and muscle convulsions which
prevent breathing, and muscle
contractions which prevent movement
 Transmission wires carring AC needs EM
insulation to block radiowaves and other
electromagnetic emissions
 A certain frequency must be maintained
and monitored
 AC produces the “skin effect” whereby
current only flows near the surface of
the conductor, which increases its
effective resistance
 AC needs rectification before it can be
used for most household electronics
The advantages, however, outweigh the disadvantages and for this reason, AC generators are widely
used for large-scale commercial generation of electricity. The main advantage is that AC voltages can
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HSC Physics
Motors and Generators
DUX
4
be stepped-up by transformers with minimal loss, and therefore be transmitted over large distances
with minimal power loss, at very high voltages.

Assess the effects of the development of AC generators on society and then environment
Effects on society are generally positive

Positive: Widespread access to cheap electricity
The development of AC generators has allowed large amounts of electrical energy to be transmitted
over long distances with minimal losses in energy and at a practical cost. This has allowed the
consumers affordable and widely available access to electricity.

Positive: More free-time in everyday life
Appliances such as washing machines, fridges, vacuum cleaners etc rely on such electricity, and their
contribution to the consumer’s free time and convenience is an indirect result of the development of
AC generators.

Positive: Entertainment, communication and education
Computers, televisions and radios are also powered by this electricity; their contribution to the
consumer’s entertainment, communication and education is another indirect result of this
development. Different appliances can be operated using the same source of electricity, which is
more convenient and simple for consumers.

Positive: Positive health effects, lower pollution
The construction of power plants away from industrial, commercial and residential areas, decreases
the air pollution experienced by the society and reduces negative health effects.

Negative: Critical dependence on electricity
The development of AC generators has rendered society to be highly dependent on electricity. A
black out or system failure could cause a sudden halt to industrial production and the whole
economy. Lives are endangered if power is cut, because hospitals require electricity for many of
their systems. Other emergency services such as the fire department will not be able to function if
electricity is cut.

Negative: Unskilled labour suffers
The wide-spread use of electrical appliances, an indirect result of AC generators, has replaced most
unskilled labour. This results in unemployment and a lower quality lifestyle for a portion of society.
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HSC Physics
Motors and Generators
DUX
5
Effects on the Environment are generally negative

Negative: Construction of mines, power plants hurts the environment
Due to the increased availability and affordability of AC electricity, the demand has also risen
dramatically. This leads to the construction of more AC generators and power plants, which have a
significantly detrimental effect on the environment. For example, generators powered by water
turbines require the construction of large dams. Large areas of land must be cleared for dam
construction, which interfere with the growth of plants and the movement of animals, destroying
natural habitats and disrupting breeding cycles. Increased demand for fossil fuels requires more
mines to be built, which requires clearing of land, blasting mountains etc.

Negative: Burning of more fossil fuels – acid rain, global warming, depletion of resources
The burning of fossil fuels to generate power is another negative effect. This produces large amounts
of air pollution in the form of carbon dioxide (a greenhouse gas that contributes to global warming),
carbon monoxide (a toxic gas) and other gases that contribute to acid rain.

Negative: Nuclear power plants produce radioactive wastes
The development of nuclear power plants to produce steam has led to radioactive waste products
that need to be stored safely for a long time and can have detrimental effects on the environment
and the society.

Negative: Thermal pollution
Steam turbines encourage thermal pollution in which hot waste water is returned to natural
waterways, warming up the water. This encourages algae growth, kills sensitive aquatic life (basically
nearly all fish) and disrupts breeding cycles of all organisms.

Positive: Society has become more environmentally conscious
The availability of essentially limitless power has allowed society to become more environmentally
conscious; investing revenue into reduction, prevention and reversal of damage done to the
environment. The cost of energy becomes less of a concern and environmental matters take priority.
Assessment
Overall, the development of AC generators has had very positive effects on society but significantly
detrimental effects on the environment.
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
HSC Physics
Motors and Generators
6
Plan, choose equipment or resources for, and perform a first-hand investigation to
demonstrate the production of an alternating current
This dot-point requires students to be able to recall an experiment they did in school which models
the generation of AC. The most simple way to achieve this is by using a solenoid (another word for a
coil) and permanent magnet, using a milliammeter to measure the current and complete the circuit.
Method
A solenoid was connected to a milliammeter (necessary to measure the tiny currents produced). A
bar magnet was repeatedly moved into and out of the solenoid. Observations of the milliammeter
were made and recorded.
Results and analysis
The pointer in the milliammeter oscillated between positive and negative values. The movement of
the pointer indicated the production of a potential difference and alternating current in the circuit.
An alternating current was produced because the movement of the magnet into and out of the
solenoid created a change in magnetic flux. This induced a potential difference in the solenoid. The
current produced was alternating because of the movement of the magnet in alternating directions.
Alternative experiment
There is perhaps an easier (though less hands-on) method of generating an AC current. The coil in an
AC motor can be physically rotated to form an AC generator. The generated alternating current can
be observed if the motor is connected to an oscilloscope.
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
HSC Physics
Motors and Generators
7
Analyse secondary information on competition between Westinghouse and Edison to
supply electricity to cities
War of Currents
Edison and Westinghouse competed against each other for a contract to supply electricity to
American cities. This historical event occurred during the late 1880s and was called the “War of
Currents”.
Thomas Edison
Edison developed a business to set up a direct current system of distributing electricity and heavily
promoted DC. He planned to build numerous power stations along a long canal, each supplying
electricity to individual factories. The advantages of his system were that DC was well established in
society (appliances worked on DC), DC was effective over short distances and there was no energy
loss through EMR. The disadvantages of Edison’s system included the high expense of constructing
many power stations within cities and the increased air pollution.
George Westinghouse
Westinghouse developed an alternating current system of distributing electricity across large
distances to the city via transmission lines. The advantages of his system included the ability for the
voltage to be stepped up and down to minimise energy loss, the larger distance between power
plants and cities leading to less pollution, and a general increase in accessibility and affordability of
electricity. The main disadvantage of Westinghouse’s AC system was that AC power is more likely to
kill or injure in the event of an electrical accident. In fact, Edison attempted to make AC unpopular in
society through public electrocutions of animals and the use of the electric chair for capital
punishment.
Westinghouse won the bid to distribute electricity to cities using AC generators, transformers and
high voltage transmission lines. The rise of AC distribution began when Westinghouse harnessed the
water power from Niagara Falls to produce AC electricity for companies in New York City.
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
HSC Physics
Motors and Generators
8
Gather and analyse information to identify how transmission lines are: insulated from
supporting structures, and protected from lightning strikes
Insulation from supporting structures
Why insulate: If the current in the transmission cables came in contact with the support pylon, the
current will be earthed, and the energy will be totally dissipated. Therefore correct insulation from
supporting pylon is important.
Ceramic plates: Insulation is provided by ceramic plates (commonly either glass or porcelain)
successively stacked upon each other. These materials are chosen because they are strong, have
extremely high resistance and do not degrade due to environmental exposure.
Without the plates, as it rains, water droplets can form a conducting bridge as it trickles down from
the cable onto the pylon steel, carrying dirt and other potentially conductive materials with it. Plates
are stacked on top of each other, firstly preventing rainwater from forming a connecting trickle
(water is forced to drop as individual droplets). Secondly, these plates act as an umbrella for plates
beneath it, preventing buildup of dirt and other potentially conductive materials under the plates
beneath. Dust and grime only collects upon the top plate, and the outer rims of the others. Hence,
no dust effectively reaches the spaces within the plates to form a current bridge. Effectively, these
plates increase the distance the current must jump (arc) in order to reach the pylon.
The greater the voltage being carried, the more plates used to separate the wires from the
supporting structures (to increase the distance the current must travel to reach the pylon).
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HSC Physics
Motors and Generators
9
Protection from lightning strikes
High voltage transmission lines are highly susceptible to lightning strikes as they are usually the
tallest conductors in the vicinity. Lightning can cause dangerous current spikes if the lightning was
able to contribute to the grid. These spikes can damage the supply system and sensitive electronics
such as computers and televisions (ever had a modem die after a thunderstorm?). The damage
would inconvenience our electrically dependent society and be highly expensive to repair.
To protect transmission lines from lightning strikes, a pair of wires (earthed conductors) are
connected to the supporting towers and suspended above the transmission lines. These wires, being
the highest in altitude, are more likely to be struck by lightning than the transmission lines. When
this happens, the current is harmlessly earthed through the metallic support towers.
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HSC Physics
Motors and Generators
DUX
10
Week 5 – Homework

1.
Discuss the energy losses that occur as energy is fed through transmission lines from the
generator to the consumer
Derive an equation to calculate the energy loss from transmission lines. Describe the
implications of your equation for the prevention of energy loss. [3 marks]
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2.
A power station is producing 80 kW of power and transmits this power to a city 40 km
away through transmission lines with a total resistance of 0.6 Ω. If the power is transmitted
at 240 V, calculate the current and voltage drop across the transmission lines. Calculate the
voltage available to consumers. Calculate the power left after losses. [2 marks]
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DUX

HSC Physics
Motors and Generators
11
Gather secondary information to discuss advantages/disadvantages of AC and DC
generators and relate these to their use
1.
Construct a table to briefly summarise the advantages and disadvantages of AC and DC
generators. [4 marks]
2.
Explain the main reason why AC generators are used instead of DC generators for largescale commercial generation of electricity. [2 marks]
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3.
Assess the compromise between safety and cost-efficiency of using AC for household
electricity. [2 marks]
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DUX

HSC Physics
Motors and Generators
12
Assess the effects of the development of AC generators on society and then environment
1.
Construct a table to briefly summarise the positive and negative effects of AC generators
on society and the environment. [4 marks]
2.
[2007 HSC] Assess the effects of the development of AC generators on today’s society and
the environment. [4 marks]
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DUX

1.

1.
HSC Physics
Motors and Generators
13
Plan, choose equipment or resources for, and perform a first-hand investigation to
demonstrate the production of an alternating current
Draw the output of an oscilloscope connected to an AC generator. [2 marks]
Analyse secondary information on competition between Westinghouse and Edison to
supply electricity to cities
Identify the main limitation in Edison’s system. Predict the effects of this limitation on the
distribution of electricity to cities. [3 marks]
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DUX
2.
HSC Physics
Motors and Generators
14
[2004 HSC] In the late nineteenth century Westinghouse and Edison were in competition to
supply electricity to cities. This competition led to Edison holding public demonstrations to
promote his system of DC generation over Westinghouse’s system of AC generation.
Propose arguments that Westinghouse could have used to convince authorities of the
advantages of his AC system of generation and distribution of electrical energy over
Edison’s DC supply. [6 marks]
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HSC Physics
Motors and Generators
DUX

1.
15
Gather and analyse information to identify how transmission lines are: insulated from
supporting structures, and protected from lightning strikes
[2004 HSC] The photograph below shows the details of the top section of a transmission
line support tower. Describe the role of each of the parts labelled A and B in the
photograph. [2 marks]
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End of homework
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