Heating effect of current

Feel a wire or a bulb after it has been on for a while: it is warm. The source supplies energy as it pushes charge round the circuit; in a resistor, that energy turns into heat. This is the heating effect of current.

The heat produced:

H = V I t, and since V = IR, H = I²Rt (Joule's law of heating).

Heat grows with the square of the current (double the current, four times the heat), with the resistance, and with the time.

Example: an electric iron of 20 Ω takes 5 A for 30 s. H = 5² × 20 × 30 = 15,000 J.

Sometimes the heat is wasted, warming wires and computers. But many devices use it:

• irons, toasters, kettles, ovens and room heaters;
• the bulb: its thin tungsten filament (melting point 3380 °C) gets so hot it glows. Bulbs are filled with nitrogen and argon, unreactive gases, to make the filament last longer.

"Why does a heater's element glow red, while its cord stays cool?" The same current flows through both, but the cord is copper with very low resistance, so it produces little heat. The element is an alloy with high resistance, so it gets very hot.

The fuse protects a circuit. It is a short wire of a metal or alloy with a suitable melting point (such as aluminium, copper, iron or lead), placed in series. If too large a current flows, the fuse wire heats up, melts and breaks the circuit before the appliances or wiring are damaged. It is usually inside a porcelain cartridge. Choose a rating a little above the normal current: a 1 kW iron on 220 V draws about 4.5 A, so a 5 A fuse is used. Home fuses come in ratings such as 1, 2, 3, 5 and 10 A.