- Heat energy is transferred by conduction, convection and radiation.
- Conduction and convection involve particles, but radiation does not
- Dark, matt surfaces are good absorbers and good emitters of thermal radiation
- Light, shiny surfaces are poor absorbers and poor emitters of thermal radiation
- The rate at which something radiates heat depends on its surface area and the temperature difference in relation to its surroundings
P1a 1.1 Thermal radiation
- Thermal radiation is energy transfer by electromagnetic waves (infra-red waves)
- All objects emit thermal radiation, just at different rates
- The hotter the object, the more radiation is emitted
- Heat can travel through a vacuum (e.g. space) because it does not require particles to radiate
P1a 1.2 Surfaces and radiation
Dark, matt surfaces are:
- Good absorbers of thermal radiation
- Good emitters of thermal radiation
Light, shiny surfaces are:
- Poor absorbers of thermal radiation
- Poor emitters of thermal radiation
P1a 1.3 Conduction
Conduction in metals:
- Conduction in a metal is mainly due to free electrons
- When the metal is heated, the free electrons gain kinetic energy and move through the metal, transferring energy to other particles
- All metals are good conductors because they all have free electrons
- An example of a non-metal with free electrons is graphite
Conduction in non-metals:
- Liquids and gases are generally poor conductors
- Conduction in non-metals occurs when the particles being heated vibrate and pass energy along to the next particle which it is vibrating against
Materials which are poor conductors are known as insulators
P1a 1.4 Convection
Convection only occurs in liquids and gases
Process of convection:
- Liquid or gas is heated
- It becomes less dense
- The less dense, hot part of the liquid/gas rises and the denser, cooler part sinks
- This creates a “convection current” – a cycle of rising and sinking hot and cold particles
Convection can occur on a small scale in a saucepan or on a large scale in the air
P1a 1.5 Heat transfer by design
To maximise heat loss and reduce overheating (e.g. for a component of a machine) we can use:
- Good conductors
- Dull, black surfaces
- Things with a good flow of air surrounding them
- Things with large surface areas
We can reduce heat loss from a building by using:
- Aluminium foil behind radiators
- Cavity wall insulation
- Double glazing
- Loft insulation
small objects and objecs with a large surface area to volume ratio lose heat quickly
p1a 1 Summary question
1. What are the three types of heat transfer?
2. Which type of heat transfer occurs in solids?
3. How do convection currents occur in liquids?
4. Describe the process of heat transfer through a metal.
5. What happens to the density of a fluid when it is heated?
6. How does cavity wall insulation reduce heat loss?