State — 3 marks
A power station generates electricity at high voltage and transmits it across the country using power lines. A transformer at a substation steps down the voltage to a safer level for use in homes and businesses.
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(a) State what type of transformer is used to step down the voltage from the power lines to a safer level for homes.
[1 mark]
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(b) State the name of the component in a transformer that allows the magnetic field to change from the primary coil to the secondary coil.
[1 mark]
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(c) State one reason why transformers are used in the National Grid to reduce energy losses when transmitting electricity over long distances.
[1 mark]
Show mark scheme
- (a) Step-down transformer / Stepping down transformer / Reducing transformer
- (b) Iron core / Soft iron core / Ferromagnetic core
- (c) High voltage reduces current in the cables, reducing heat loss / I²R losses are minimised / Lower current means less energy wasted as heat in the resistance of the wires
Explain — 4 marks
A power station generates electricity at 25 kV and uses a step-up transformer to increase the voltage to 400 kV for transmission across the National Grid. At a substation near a town, a step-down transformer reduces the voltage to 230 V for domestic use. The step-up transformer has 80 turns on the primary coil.
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(a) Explain why a step-up transformer is used at the power station rather than transmitting electricity at 25 kV.
[2 marks]
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(b) Calculate the number of turns on the secondary coil of the step-up transformer. Show your working.
[1 mark]
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(c) Explain why energy losses during transmission would be significantly greater if the voltage had not been stepped up to 400 kV.
[1 mark]
Show mark scheme
- (a) High voltage reduces current in the transmission cables (1 mark)
- (a) Lower current reduces power loss (I²R loss) in the cables / makes transmission more efficient (1 mark)
- (b) Using Vp/Vs = Np/Ns: 25/400 = 80/Ns, therefore Ns = 1280 turns (1 mark)
- (c) Power loss is proportional to I² (or current squared), so lower voltage means higher current which causes much greater power loss in the resistance of the cables (1 mark)
Evaluate — 5 marks
A school laboratory uses a step-down transformer to reduce the mains voltage (230 V) to a safe voltage of 12 V for powering a low-voltage circuit board. The transformer has 2300 turns on the primary coil and 120 turns on the secondary coil. A technician notices that the secondary coil is getting warm during operation.
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(a) Calculate the expected secondary voltage using the turns ratio. Show your working.
[2 marks]
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(b) The measured secondary voltage is 11.5 V instead of the calculated value. Suggest one reason why the measured voltage is lower than expected.
[1 mark]
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(c) Evaluate whether this transformer is suitable for the application, considering both the voltage output and the fact that the secondary coil is getting warm. Justify your answer.
[2 marks]
Show mark scheme
- (a) Uses formula Vs/Vp = Ns/Np or Vs = Vp × (Ns/Np) [1 mark]
- (a) Correctly calculates Vs = 230 × (120/2300) = 12 V or equivalent working shown [1 mark]
- (b) Mentions energy loss/heat loss in the coils due to resistance of the wire [1 mark] OR voltage drop across internal resistance [1 mark] OR some energy dissipated as heat [1 mark]
- (c) The voltage is acceptable/suitable because 11.5 V is close enough to 12 V for the circuit board to operate (or similar justification) [1 mark]
- (c) The transformer has a disadvantage because the heating of the secondary coil indicates energy loss/inefficiency, which could damage the transformer if it overheats, or wastes energy [1 mark]
Calculate — 2 marks
A mobile phone charger contains a transformer that steps down the mains voltage of 230 V to 5 V suitable for charging a phone. The primary coil has 460 turns.
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(a) The transformer has 460 turns on the primary coil and a turns ratio (primary:secondary) of 46:1. Calculate the number of turns on the secondary coil.
[1 mark]
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(b) Calculate the current in the secondary coil when the current in the primary coil is 0.10 A. Assume the transformer is 100% efficient.
[1 mark]
Show mark scheme
- (a) 10 (turns)
- (b) 4.6 (A) / 4.60 (A)
Explain — 3 marks
A student plugs their mobile phone into a charger. The charger contains a transformer that changes the 230 V mains supply to the 5 V needed by the phone. The transformer has two coils of wire wrapped around an iron core.
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(a) The primary coil of the transformer is connected to an alternating current (a.c.) supply. Explain why an a.c. supply must be used rather than a direct current (d.c.) supply.
[1 mark]
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(b) Explain how the transformer produces a potential difference across the secondary coil.
[2 marks]
Show mark scheme
- (a) a.c. creates a changing magnetic field (but d.c. does not)
- (b) the alternating current in the primary coil produces a changing magnetic field
- (b) the changing magnetic field induces a potential difference across the secondary coil
GCSE Perfect