Conduction, Convection and Radiation — WJEC GCSE Science (Double Award) - Physics Revision Notes & Topical Questions

Thermal energy always moves from a region of higher temperature to a region of lower temperature.

Conduction is the transfer of thermal energy through a material without the material itself moving. It happens in all states of matter but is best in solids.

In a non-metal the hot end has particles vibrating with larger amplitude. They jostle their neighbours, which pass the vibrations on, so energy is handed along the solid from particle to particle.

Metals are much better conductors because they contain free (delocalised) electrons.

Gases are poor conductors because the particles are far apart and collide rarely. Materials that trap air, such as wool, foam and fibreglass, are good insulators.

Convection happens in liquids and gases (fluids). When part of a fluid is heated its particles move faster and spread apart, so that region becomes less dense. The warmer, less dense fluid rises. Cooler, denser fluid sinks to take its place and is heated in turn. A circulating pattern, called a convection current, carries thermal energy through the fluid.

Examples

• a hot air balloon rises because the air inside is less dense than the air outside;
  • a heater at the bottom of a tank warms all the water;
  • sea breezes occur when air over warm land rises and cooler sea air flows in.
• Convection cannot occur in solids, because the particles are fixed in position, and cannot occur in a vacuum.

Radiation is the transfer of energy by infrared electromagnetic waves. No particles are needed, so it works through a vacuum; this is how energy reaches Earth from the Sun. All objects emit infrared radiation, and the hotter an object is, the more it emits each second and the shorter the typical wavelength.

Surfaces differ in how well they emit and absorb:

This is why radiators on a car engine or a fridge back are often black, why solar heating panels are black, and why emergency blankets and house roofs in hot countries are shiny or white.

A thermos flask combines several ideasthe vacuum between the walls stops conduction and convection, silvered surfaces reduce radiation, and the plastic stopper (a poor conductor) reduces conduction and convection from the top.