Practise The National Grid. 14 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.
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
(c)Answer: 1250 A
500 000 000 = 400 000 × I
I = 500 000 000 ÷ 400 000
I = 1250 (A)
Question 3Hard8 marks
A power station has an output power of 600 MW. The electricity is transmitted through cables that have a total resistance of 5.0 Ω.
(a) Calculate the current in the cables when the transmission potential difference is 400 kV.[2]
(b) Calculate the power dissipated in the cables.[2]
(c) Calculate the percentage of the output power of the power station that is dissipated in the cables.[2]
(d) Explain why a much greater percentage of the power would be dissipated if the same power were transmitted at a lower potential difference.[2]
Show the answer and mark scheme
(a)Answer: 1500 A
600 000 000 = 400 000 × I
I = 1500 (A)
(b)Answer: 11 250 000 W (1.125 × 107 W) W
P = 15002 × 5.0
P = 11 250 000 (W)
(c)Answer: 1.9%
11.25 ÷ 600 × 100
1.9 (%)
(d)Answer: For the same power, a lower pd means a larger current; the power dissipated depends on the current squared (P = I²R), so the losses increase greatly.
for the same power, a lower potential difference means a larger current (P = VI)
power dissipated is proportional to current squared (P = I2R), so it increases greatly