Electromagnetic Induction
6 marks · NormalMains electricity is transmitted over large distances from power stations to towns and cities using the National Grid system. Step-up and step-down transformers are used at different stages.
Explain why step-up transformers are used before electrical energy is transmitted on the National Grid.
A step-up transformer has 500 turns on its primary coil and 15,000 turns on its secondary coil. The input voltage is 2,500 V a.c. Calculate the output voltage.
Before the electricity reaches homes, it passes through a step-down transformer. State the purpose of the step-down transformer, and give a typical output voltage for domestic use.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) To reduce energy (power) lost as heat in transmission cables by lowering the current and increasing the voltage.
(b) 75,000 V
(c) To reduce the voltage to a safe level for houses (typical value: 230 V).
Common mistakes: mixing up primary and secondary turns
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · States energy is lost as heat/power in cables.
B2 (a) · 1 mark · Explains that increasing voltage reduces current and thus reduces losses.
M1 (b) · 1 mark · Uses transformer equation setup correctly.
A1 (b) · 1 mark · Correct calculation: 75,000 V (accept rounding/tolerance).
B1 (c) · 1 mark · States purpose is to reduce voltage for domestic use.
B2 (c) · 1 mark · Gives correct typical output: 230 V (accept range 220–240 V).
Worked solution
a) Step-up transformers are used to increase the voltage and decrease the current before transmission, which reduces energy lost as heat in the transmission lines (because power loss is proportional to the square of the current).
b) Transformer equation: Given
c) The step-down transformer reduces the very high transmission voltage to a safe level for domestic use. A typical value is 230 V.