Everyday appliances are designed to bring about useful energy transfers. A kettle or electric heater transfers energy from the mains to the thermal store of the water or the room. A drill, a fan or a washing machine uses an electric motor to transfer energy from the mains to the kinetic store of the moving parts. A battery-powered torch transfers energy from the chemical store of its battery to light and heat.
The energy an appliance transfers depends on its power and on how long it is switched on: energy transferred = power × time, or E = Pt. Energy is in joules (J), power in watts (W) and time in seconds (s). A 100 W lamp left on for 30 s transfers 100 × 30 = 3000 J. Two appliances on for the same time: the one with the higher power rating transfers more energy.
Work is done when charge flow moves through a circuit. The energy transferred is energy transferred = charge flow × potential difference, or E = QV, with charge flow Q in coulombs (C) and potential difference V in volts (V). When 5 C of charge flows through a heater and the potential difference across it is 12 V, the energy transferred is 5 × 12 = 60 J.