Transformers and the National Grid

Question (5 marks): Explain why electricity is transmitted across the National Grid at very high voltages (e.g. 400 000 V) rather than at domestic voltage (230 V). Refer to the role of step-up and step-down transformers.

Mark scheme (5 marks — any 5 from):

• Power loss in cables = I²R (1)
• Higher voltage means lower current for the same power transmitted (P = IV) (1)
• Lower current dramatically reduces heating losses in cables (1)
• Step-up transformer increases voltage at the power station (1)
• Step-down transformer reduces voltage to safe levels for homes (230 V) (1)
• Increases overall efficiency / reduces wasted energy as heat (1)