Producing Electricity
Introduction

Electricity is produced in power stations. It is then distributed at high voltage using a network of overhead and underground cables. The voltage of the supply is stepped down at local substations and sent to individual houses.
Power stations
Power
Power is the rate of doing work, or the rate of energy transfer. The unit of power is the watt, W.
Power
stations convert
energy
Energy is the capacity to do work. The SI unit of energy is the joule (J).
energy
from one form to another.

Figure 1.   A coal-burning power station.
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Coal-burning power stations convert the chemical energy stored in coal into heat energy by burning it. This heat is used to heat up water in a heat exchanger. The hot steam produced is used to drive turbines, giving them
kinetic energy
Kinetic energy is the energy associated with motion. A fast moving object has more kinetic energy than an otherwise identical slow moving object.
kinetic energy
. The turbines are connected to large generators which produce
electrical energy
Electrical energy is used to power many appliances around the home. It can be supplied by batteries or from the mains supply and is easily converted to other forms of energy.
electrical energy
from the
kinetic energy
Kinetic energy is the energy associated with motion. A fast moving object has more kinetic energy than an otherwise identical slow moving object.
kinetic energy
.

These energy changes in a coal burning power station are summarized in Fig.2 below.

Figure 2.   Energy changes in a coal-burning power station.
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For every 100 J of chemical energy, how much energy is lost as heat?
  •  J
  • Click here to mark the question


Hydroelectric power stations provide electricity from a different source. They use the gravitational
potential energy
Potential energy is the energy associated with position or state. It is energy which can be converted into another form. A stretched elastic band has more potential energy than an unstretched one. A ball at the top of a cliff has more potential energy than one at the bottom of the same cliff.
potential energy
of water stored in mountain reservoirs.

Flowing water is used to drive turbines. The turbines rotate and generate electricity.

Figure 3.   Energy changes in an hydroelectric power station.
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What type of energy do the turbines have when they are rotating?
  • energy
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Transmission networks
Many years ago, different areas all had their own power stations. The voltage and the
frequency
In general, the frequency of an event describes how often it occurs. When talking about waves, the frequency is a measure of how many waves go past a fixed point in a given time.
frequency
of the signals varied from place to place. Nowadays, power stations within one country are usually connected together on a transmission and distribution network. In Britain, this network is called the
National Grid
The National Grid is the distribution network of electric pylons and cables that is used to transmit electricity across the country, wherever it is needed.
National Grid
.

Figure 4.   Transmission networks distribute electricity throughout a country.
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In general, a transmission network consists of thousands of kilometres of cables which supply electricity to all the houses and businesses in the country. During winter, the power supplied by the network can be huge, more than enough for everyone to turn on 10 light bulbs each.

Transmitting electricity
Electricity can be transmitted from power stations to homes at any voltage. However, there are advantages to transmitting at high voltages. To see this, consider a power station which produces 400 kW (400,000 W) of
electrical power
Electrical power is a measure of how quickly a device consumes electrical energy. This is often referred to as the power rating of the device. Electrical power can by calculated using the equation: P = I V
electrical power
. This electricity is sent along a transmission cable at a voltage of around 100 kV (100,000 V). The current along the cable can be found by rearranging the equation for electrical power below (click on the I).

The current in the cable is found by dividing the power by the voltage.
We can now substitute in values. The power, P is 400,000 W and the voltage V is 100,000 V
     The current in the cable is 4 A.



When this current flows through the cable, some of the electrical power is converted into heat and cannot be used.

In Fig.5 below you can investigate how the power lost through heat depends on the voltage of the transmission cable. Move the slider to set the transmission voltage to 100 kV. You should find that the power lost as heat is equal to 1.6 kW. All the rest of the power is received by the step-down
transformer
Transformers are electrical devices that can change the voltage of a signal. They only work with alternating current.
transformer
and converted for use in homes.

How much power is received by the step-down transformer?
  •  kW
  • Click here to mark the question
Figure 5.   Investigating power loss in electric cables.
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Now reduce the transmission voltage to 10kV. The power produced by the power station remains the same, but the current in the wire changes.

What is the current in the cable now?
  •  A
  • Click here to mark the question


Observe how much power in the cable is lost as heat.

How much power is now lost as heat?
  •  kW
  • Click here to mark the question


Vary the transmission voltage again and look for a pattern between transmission voltage and heat lost in the cable.

When is more power lost as heat?
  • Click here to mark the question


The reason why electricity is transmitted at voltages above 120 kV is thus to do with conserving energy. However, high voltages are very dangerous. Local substations have large step-down transformers in them to reduce the voltage of the electricity to between 110 V and 250 V depending on the country (e.g. 230 V in the EU; 120 V in the USA). This makes it safer to use in houses. However, even 110 V is enough to kill a person, so always be very careful near the domestic electricity supply.

Would transformers work if electricity was transmitted as
direct current
Direct current (often abbreviated to d.c.) is when the flow of electric current is in a constant direction. Batteries are a source of direct current.
direct current
?
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When people first started to use electricity, no one was sure which would be more useful,
a.c.
The common abbreviation for alternating current is a.c. This is when the flow of electric current changes direction periodically. The mains is a source of alternating current.
a.c.
or
d.c.
The common abbreviation for direct current is d.c. This is when the flow of electric current is in a constant direction. Batteries are a source of direct current.
d.c.
Due to the fact that
alternating current
Alternating current (often abbreviated to a.c.) is when the flow of electric current changes direction periodically. The mains is a source of alternating current.
alternating current
can be efficiently transmitted at high voltages and stepped down using transformers, it became the standard for electric power companies worldwide.

Electricity and the environment
Electricity can be transmitted by overhead pylons or in underground cables.
Look at each of these reasons and decide whether it applies to pylons or to underground cables.
  • They are more likely to look unsightly.
    They are much more expensive to lay.
    They are easier to access and repair.
    They are more dangerous to planes and people flying kites.
  • Click here to mark the question


Electricity is more usually transmitted in overhead cables hung from pylons than in underground cables.

Select the correct words to complete the sentences below.

  • You are more likely to see electric pylons . Underground cables are used to carry electricity .
  • Click here to mark the question
Summary


Electricity is produced in power stations all over the country.

The electricity is transmitted wherever it is needed using a transmission and distribution network of electric pylons and underground cables.

Electricity is transmitted at high voltages to reduce the amount of power lost as heat in the cables.

Exercises
1. Select the correct words to complete the sentences below describing energy changes in the production of electricity.

  • When coal is burnt in a power station, energy of the coal is converted to energy of water and steam. The steam is used to drive turbines, giving them energy. The rotating turbines generate energy.

    Hydroelectric power stations use water stored high in the mountains. This water has lots of energy. The water flows down the mountainside and through large turbines, giving them energy. The turbines then generate energy.
  • Click here to mark the question
2. What is the name of the network of power stations and electric cables in Britain?
  • Click here to mark the question
Figure 6.  
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3. Match each element in Fig.6 above with its name.
  • A:
    B:
    C:
    D:
    E:
    F:
  • Click here to mark the question
4. Where is the voltage of electricity highest in Fig.6 above?
  • Click here to mark the question
5. What type of electricity, a.c. or d.c., is used by the transformers in Fig.6 above?
  • Click here to mark the question
6. What is the voltage of the electricity between E and F in Fig.6 above?
  • Click here to mark the question
7. Which of the statements below, describing what happens if the electricity transmitted through D (in Fig.6 above) is at a low voltage, are true?
  • A small amount of power is lost.
    Power is lost as heat.
    The current is high.
  • Click here to mark the question
Well done!
Try again!