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6 May, 2017
The National Grid
Objectives
Be able to explain the use of the National Grid.
HSW: AF2: Make balanced judgments about scientific developments by
evaluating the economic, moral, social or cultural implications.
Used before in:
Will use again in:
PLTS: Independent enquirers: support conclusions, using reasoned arguments and evidence.
Used before in:
Will use again in:
Keywords
Transformers, Power, Voltage, Current, Energy,
National Grid, Megawatt, Kilowatt, Joule, Energy.
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Outcomes
All students should be able to:
• Draw and label a diagram of the National
Grid.
Most students should be able to:
• Describe why electricity is transferred at
very high voltages .
Some students should be able to:
• Use power calculations to explain the use
of transformers in the National Grid.
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What is the National Grid?
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Demo Time!!
• Watch the demo of the National Grid.
• Can you answer these questions?
• When the transformers were not
connected what happened to the bulb at
the far end of the model?
• What do the transformers do?
• Why does the voltage have to be so big
in the wires?
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1 Copy and complete using the key words below:
 The National Grid takes …………… from
power stations to businesses and homes at
extremely large voltages.
 It does this to limit the ………….. lost by heat
from the wires.
 The bigger the electric ………… in a wire, the
more heat is lost.
 To make the voltages bigger, step up
……………….. are used. To make
voltages…………, step down transformers
are used.
transformers
smaller electricity
energy
current
2 Copy Figure 2 on Page 260 – label the parts and voltages.
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What is a transformer?
The voltage of an alternating current can be changed using
a device called a transformer.
A transformer contains two
coils that are wound around
a soft iron core.
iron core
The alternating current in
the primary (input) coil
produces an alternating
magnetic field.
This alternating magnetic
field induces an
alternating current in the
secondary (output) coil.
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primary
coil
secondary
coil
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Where are transformers used?
A television needs a very high
voltage to operate. It contains
a step-up transformer,
which increases the voltage
of the electricity supplied to
the television.
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This outdoor transformer
decreases the voltage of
the electricity carried by
the national grid. It is an
example of a step-down
transformer.
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What is the formula for electrical power?
In electrical devices, power can be calculated using the
formula:
power = current x voltage
P = IxV
What are the units of power, current and voltage?
 Power is measured in watts (W) or kilowatts (kW).
 Current is measured in amps (A).
 Voltage is measured in volts (V).
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Can I use a formula triangle?
A formula triangle helps you to rearrange a formula.
The formula triangle for P = IV is shown below.
Cover up whatever quantity you are trying to find to leave
the calculation required.
So to find current (I),
cover up I…


x
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…which gives
the formula…
P
I =
V
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How is power calculated?
A power station produces
electricity of 2.5 kV 20, 000 A.
What is the Power of the Power
station?
P = IV
= 20 000 A x 2500 V
= 500 MW
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Power Calculations
500 MW
Power of 500 MW
remains constant
but Voltage increases
to 400 000V.
• What is the current running in the wires of the
National Grid?
• I = P/V
• I = 500 000 000 W/ 400 000V
• I = 1250 A (A lot less than the 20 000A from the
Power Station.)
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Power – The Big Q: Why have a National Grid?
All
Finish Figure 2 on Page 260 – label the parts and voltages.
Most
Explain why the National Grid uses high voltages in the
overhead cables.
Some
Given the power output of the power station you have
drawn is 500MW, what current does it produce? And
therefore, from the Voltage that you have labelled in the
diagram what is the current through the National Grid?
Describe what the transformer has done and why this is
useful.
SO: from today’s lesson and previous lessons on Electricity
Generation answer this: Why have a National Grid?
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Plenary
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Plenary
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