Conduction
Conduction is the transfer of heat through a solid without the material itself moving. The particles at the hot end vibrate more strongly and pass this energy to their neighbours. In metals, free electrons also carry energy quickly, so metals are good conductors. Non-metals, liquids and gases (wood, plastic, air, wool) are poor conductors, called insulators.
| Material | Thermal conductivity k (W m⁻¹ K⁻¹) |
|---|---|
| Copper | 400 |
| Aluminium | 240 |
| Steel | 50 |
| Glass | 1.0 |
| Wood | 0.15 |
| Air | 0.026 |
Choose the thickness of the wall and the temperature difference across it. A wall of area 10 m² and conductivity k = 0.8 W/(m K) conducts heat. Find the heat lost every second.
Convection
Convection is the transfer of heat in a liquid or gas by the movement of the fluid itself. Heated fluid expands, becomes less dense and rises; cooler, denser fluid sinks to take its place. This sets up a convection current. It cannot happen in solids or in a vacuum.
| Example | What happens |
|---|---|
| Heating water in a pan | Hot water rises from the bottom and cool water sinks from the top |
| Sea breeze (day) | Land heats faster than sea; warm air rises over the land and cool air blows in from the sea |
| Land breeze (night) | Land cools faster; air flows from land to sea |
| A room heater near the floor | Warm air rises and circulates; ceilings fans or windows at the top let hot air out |
| Cooling in a refrigerator | The freezer is at the top so cold air sinks |
Radiation
Radiation is the transfer of heat by infrared electromagnetic waves. It needs no material, so it can cross a vacuum: this is how the Sun warms the Earth. All objects emit radiation, and hotter objects emit more. Dull black surfaces are the best absorbers and emitters; shiny white or silver surfaces are the best reflectors and the worst emitters.
| Application | Reason |
|---|---|
| Vacuum flask | A double-walled glass bottle with a vacuum between the walls (no conduction or convection) and silvered walls (reflect radiation), with a plastic stopper (insulator) |
| Solar water heater | A black pipe in a glass-covered panel absorbs radiation; the water inside warms and rises |
| White or light-coloured houses in hot climates | Reflect the Sun’s radiation |
| Car radiator painted black | Emits heat well |
| Cooking pots with black bases | Absorb heat from the fire quickly |
Key terms
- Conduction
- Transfer of heat through a material by particle vibrations and free electrons.
- Convection
- Transfer of heat by the movement of a heated fluid.
- Radiation
- Transfer of heat by infrared waves, needing no medium.
- Insulator
- A poor conductor of heat.
Summary
- Conduction: through solids, best in metals; convection: in fluids; radiation: by infrared waves in a vacuum.
- Black surfaces absorb and emit best; shiny surfaces reflect best.
- Vacuum flasks, solar heaters and insulation apply these ideas.
- Rate of conduction Q/t = kAΔT ÷ d.