Electrical Power & Mains Electricity

Why current heats wires

• When current flows through a metal wire, the drifting electrons repeatedly collide with the vibrating positive ions in the lattice
• Each collision transfers some of the electron's kinetic energy to the ion, making the ion vibrate more vigorously
• That extra vibration shows up as a rise in temperature of the wire, because kinetic energy in the electrons has been transferred to internal (thermal) energy in the metal
• Some of this thermal energy is then dissipated to the surroundings by heating (and at higher temperatures by infrared radiation as well)

Useful applications of the heating effect

• The heating effect is the basis of every domestic appliance that turns electricity into warmth:
  • electric kettle: current through a high-resistance heating element brings the water to the boil
  • toaster: a coil of resistance wire glows red-hot under each slice of bread
  • electric oven and grill: a hidden resistive element heats the oven cavity
  • electric hob: a resistive element under the ceramic surface heats the cookware
  • bar heater / electric radiator: a long coil of resistance wire heats the room air
  • filament lamp: heats the metal filament until it glows white-hot, producing light (along with a lot of waste heat)
• In wiring that is not meant to heat up, such as extension leads, mains cables, and transformer windings, the heating effect is a waste of energy, and good design uses thick, low-resistance copper conductors to minimise it