Energy transfers

Power loss (as heat) in a component is proportional to the square of the current through it and to its resistance — this is why the National Grid transmits electricity at a high potential difference and low current: for the same power transferred, a lower current means much less power is wasted heating the cables.

Everyday electrical appliances transfer energy electrically from batteries or the mains to other, more useful stores — for example, a motor transfers electrical energy mainly to a kinetic energy store, and a kettle transfers electrical energy mainly to a thermal energy store.

• •Step-up transformers increase the potential difference (and correspondingly decrease the current) between the power station and the transmission cables.
• •Step-down transformers decrease the potential difference again, from the National Grid to a safer level for consumers.

Since power = potential difference × current (P = VI), transmitting power at a very high potential difference keeps the current low for the same power delivered. Because power lost as heat in the cables depends on the square of the current (P = I²R), a lower current dramatically reduces the energy wasted heating the cables over long transmission distances.