AQA GCSE Combined Science Foundation (8464), Foundation tier · Physics › Electricity › Energy transfers
Practise The National Grid. 5 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
The National Grid is the system of cables and transformers that links power stations to consumers. You need to know what step-up and step-down transformers do, and to explain why transmitting at a high potential difference is an efficient way to transfer energy. Expect 2 to 4 mark 'explain' questions and some power calculations.
Key facts
National Grid = cables + transformers linking power stations to consumers
Step-up transformer: increases the pd (power station → transmission cables)
Step-down transformer: decreases the pd (transmission cables → homes, about 230 V)
P = V I: higher pd → lower current for the same power
Heating loss in cables: P = I2 R
Lower current → less heating → less energy wasted → more efficient
Notes
What the National Grid is
diagram
The National Grid is a system of cables and transformers that links power stations to consumers (homes, schools, offices and factories).
Electrical power is transferred from power stations to consumers using the National Grid.
Order: power station → step-up transformer → transmission cables → step-down transformer → consumers.
The National Grid: step up the pd for the cables, step down for homes.
Transformers
Step-up transformers increase the potential difference from the power station to the transmission cables, to a very high value (hundreds of thousands of volts).
Step-down transformers decrease the potential difference to a much lower value (about 230 V) for domestic use.
Such a high pd would be far too dangerous in homes, which is why it is stepped down before it reaches consumers.
Why a high pd is efficient
For a given power, P = V I. If the pd is increased, a smaller current is needed to transfer the same power.
The transmission cables have resistance, so a current in them heats them. The power wasted by heating is P = I2 R.
A smaller current means much less energy is transferred to the thermal energy store of the cables and surroundings. Less energy is wasted, so more reaches consumers and the transfer is more efficient.
How to answer each type of question
Name or describe the transformers
1 to 2 marksGrade 4
Transformer next to the power station: step-up, increases the pd.
Transformer next to the consumers: step-down, decreases the pd.
Say what happens to the pd, not just the name, if asked to describe.
Example. In the National Grid, transformer X is between the power station and the transmission cables. Transformer Y is between the transmission cables and the homes.
(a) Name the type of transformer X. (b) Describe what transformer Y does.
Show the model answerHide the model answer
(a) Step-up (transformer) (1) (b) It decreases the potential difference (to a much lower value, about 230 V, for use in homes) (1)
Don’t lose marks
Saying a high pd 'makes the electricity travel faster' or 'reduces the resistance'. It reduces the current.
Saying a step-up transformer increases the power. It increases the pd; the current in the cables is smaller.
Stopping at 'the current is lower' without saying what that does: less heating of the cables, so less energy wasted.
Mixing up where the step-up and step-down transformers go.
More tips
Memory tricks
Step UP the pd and the current goes DOWN: the grid trades a high pd for a low current.
'High volts, low amps, low losses.'
Order of the grid: power station → UP → cables → DOWN → homes.
Exam technique
For 'explain' questions, build the chain: high pd → low current → less heating in the cables → less energy wasted → efficient.
Use the word 'efficient' and say where the wasted energy goes (the thermal energy store of the cables and surroundings).
In calculations, convert MW to W (× 1 000 000) and kV to V (× 1000) before using P = V I.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Describe what the National Grid isA system of cables and transformers linking power stations to consumers.
Grade 4
State what a step-up transformer doesIt increases the potential difference from the power station to the transmission cables.
Grade 4
State what a step-down transformer doesIt decreases the potential difference to a much lower value for domestic use.
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy2 marks
(a) Why is an alternating potential difference, rather than a direct one, used for the National Grid? Tick (✓) one box.[1]
Transformers only work with an alternating potential difference
Alternating current is cheaper to generate
Direct current cannot travel long distances at all
Power stations can only produce alternating current
(b) State one advantage of transmitting electricity at a very high potential difference.[1]
Show the answer and mark scheme
(a)Answer: Transformers only work with an alternating potential difference
(b)Answer: It reduces the current needed for a given power, which reduces the energy dissipated (wasted) by heating in the cables.
reduces the current (for a given power), which reduces the energy dissipated / wasted by heating in the cables
Question 2Medium8 marks
Electrical power is transferred from power stations to consumers using the National Grid.
(a) Explain why electrical power is transmitted through the National Grid cables at a very high potential difference.[3]
(b) Explain why step-down transformers are used before the electricity reaches homes.[2]
(c) A power station transfers 500 MW through transmission cables at a potential difference of 400 kV. Calculate the current in the cables.[3]
Show the answer and mark scheme
(a)Answer: For a given power, a higher pd means a smaller current; a smaller current means less energy is dissipated by heating in the cables, so the transfer is more efficient.
for a given power, increasing the potential difference reduces the current
a smaller current means less energy is dissipated / transferred by heating in the cables
so the National Grid is more efficient / less energy is wasted
(b)Answer: To decrease the potential difference to a much lower value (230 V), because very high potential differences would be dangerous in homes.
to decrease the potential difference to a much lower value / 230 V
because a very high potential difference would be dangerous (for domestic use) / appliances are designed for 230 V