2.12 The National Grid
9 exam questions worth 23 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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What is the job of a step-up transformer in the National Grid?
Choose one answer.
Correct Not quite: the answer is To increase the potential difference before transmission
Mark scheme
A: to increase the potential difference before transmission
Model answer
To increase the potential difference before transmission (A).
State what is meant by the National Grid.
Mark scheme
a network of cables and transformers (1); that transfers electrical energy from power stations to consumers (homes and businesses) (1)
Model answer
The National Grid is a network of cables and transformers that carries electrical energy from power stations to homes and businesses.
Explain why electricity is transmitted through the National Grid at a very high potential difference.
Mark scheme
for a given power, a higher potential difference means a lower current (P = V I) (1); the cables heat up less (the power wasted is I² R) (1); so less energy is wasted / the process is more efficient (1)
Model answer
For the same power, a higher potential difference means a smaller current. A smaller current causes less heating of the cables, because the wasted power is I² R. So less energy is wasted and the National Grid is more efficient.
Explain why a step-down transformer is used near homes.
Mark scheme
to reduce the potential difference to 230 V / mains potential difference (1); because high potential difference is dangerous / appliances are designed for 230 V (1)
Model answer
A step-down transformer reduces the potential difference to 230 V, which is safe to use in homes and which appliances are designed for.
A power cable has a resistance of 10 Ω. Calculate the power wasted when the current is 100 A, and when the current is halved to 50 A.
Mark scheme
P = I² R = 100² × 10 = 100 000 W (1); 50² × 10 = 25 000 W (1); halving the current reduces the power wasted to a quarter (1)
Model answer
At 100 A: P = 100² × 10 = 100 000 W. At 50 A: P = 50² × 10 = 25 000 W. Halving the current reduces the power wasted to a quarter.
A transformer is 100% efficient. The primary coil has a potential difference of 25 V and a current of 8 A. The secondary coil has a potential difference of 200 V. Calculate the current in the secondary coil. (Separate Science)
Mark scheme
Vp × Ip = Vs × Is (1); 25 × 8 = 200 × Is (1); Is = 1 A (1)
Model answer
Vp × Ip = Vs × Is, so 25 × 8 = 200 × Is. Is = 200 / 200 = 1 A.
A power station transmits 100 MW (1.0 × 10⁸ W) at a potential difference of 200 kV (2.0 × 10⁵ V) through cables of total resistance 1.0 Ω. Calculate the current in the cables and the power wasted.
Mark scheme
I = P ÷ V = 1.0 × 10⁸ ÷ 2.0 × 10⁵ (1); 500 A (1); P = I² R = 500² × 1.0 (1); 250 000 W (250 kW) (1)
Model answer
I = P / V = 1.0 × 10⁸ / 2.0 × 10⁵ = 500 A. Power wasted = I² R = 500² × 1.0 = 250 000 W (0.25 MW).
Give two advantages of using overhead cables and one advantage of using underground cables.
Mark scheme
overhead: any two from cheaper to install / easier to repair / cooled by the air (1 each); underground: hidden from view / do not spoil the landscape / less affected by weather (1)
Model answer
Overhead cables are cheaper to install and easier to repair. Underground cables do not spoil the view.
Explain why the National Grid uses alternating current.
Mark scheme
transformers only work with a.c. (1); and transformers are needed to step up and step down the potential difference (1)
Model answer
Transformers only work with alternating current, and the Grid needs transformers to step the potential difference up and down.
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