This topic accounts for approximately 10% of your exam marks.
stable
Medium
Stable10%
P = IV and energy calculations, plus mains electricity safety, appear in most series.
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