Electricity generated at a power station is transmitted across the country through the National Grid. Explain why electricity is transmitted at very high voltages, and describe the role of transformers in making the National Grid both efficient and safe for domestic use.

Electricity is transmitted at very high voltage because power loss in the transmission cables is proportional to the square of the current (P=I^2R). By increasing the voltage, the current required to deliver the same power is reduced, so the I^2R losses are much smaller and the transmission is more efficient.

A step‑up transformer is used at the power station to raise the generated voltage from 25 kV to 400 kV. The higher voltage means a lower current in the long‑distance transmission lines, reducing heating losses and allowing a smaller, cheaper cable to be used.

Near the end of the transmission lines, a step‑down transformer reduces the high voltage back to a lower, safer level for domestic supply. The transformer works only with AC because the alternating current creates a continuously changing magnetic field in the primary coil, which induces the secondary current.

Thus transformers make the National Grid efficient (by allowing high‑voltage, low‑current transmission) and safe (by stepping the voltage down for home use).