Electromagnetism
5 marks · NormalA power station uses large generators and transformers to supply electrical energy to homes.
Explain why the electricity generated by the power station must pass through step-up transformers before long-distance transmission.
A step-up transformer has 120 turns on its primary coil and 2,880 turns on its secondary. If the input voltage is 11,000 V a.c., calculate the output voltage.
Describe one danger associated with transmitting electricity at high voltages.
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 loss during transmission (by reducing current). · must show reference to reduced current or reduced energy loss
(b) 264,000 V · must show correct transformer ratio and calculation
(c) High voltages are dangerous and can cause fatal electric shocks.
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Mentions reduction of current or energy loss as the reason for step-up transformers.
M1 (b) · 1 mark · Uses transformer equation: Vs/Vp = ns/np, and substitutes values.
A2 (b) · 1 mark · Correctly calculates the output voltage (264,000 V).
A3 (c) · 1 mark · Identifies a relevant danger (e.g., risk of fatal electric shocks, arcing).
A4 (a) · 1 mark · Explains that less current means less heat is lost in the wires (I^2R).
Worked solution
a) Step-up transformers increase the voltage, which lowers the current for the same power. Lower current means less energy is lost as heat in the transmission lines, making transmission more efficient.
Vs = (2880/120) × 11,000 = 24 × 11,000 = 264,000 V
c) High voltages are very dangerous and can cause severe or fatal electric shocks if insulation fails or if touch occurs.