Section 1 of 5Energy Exchange by Radiation
Every object both absorbs and emits thermal radiation, and its temperature depends on the balance between the two.
Absorption
- Absorption An object takes in radiation from its surroundings, which increases its internal energy.
Emission
- Emission Every object emits thermal radiation, and a hotter object emits more energy per second than a cooler one.
Net transfer
- Net transfer The net energy transfer depends on whether the object absorbs more energy each second than it emits.
Balance
- Balance When the energy absorbed each second equals the energy emitted each second, the temperature stays constant.
Section 2 of 5Reaching a Steady Temperature
An object in surroundings that are warmer than it is will warm up until its temperature becomes steady.
1
An object placed in warm surroundings absorbs more radiation than it emits, so its temperature rises.
2
As its temperature rises, the object emits radiation at an increasing rate.
3
Eventually the rate at which it emits energy matches the rate at which it absorbs energy.
4
The object then stays at a steady temperature, which is known as thermal equilibrium.
Section 3 of 5Properties of Surfaces
The colour and texture of a surface change how well it absorbs and emits radiation.
Properties of Surfaces
Dark, matt surface
- Dark, matt surface A good absorber and a good emitter of thermal radiation, so it heats up and cools down quickly.
Light, shiny surface
- Light, shiny surface A poor absorber and a poor emitter, so it reflects most radiation and changes temperature slowly.
Black body
- Black body An idealised object that absorbs all the radiation that falls on it and emits the most radiation at a given temperature.
| Surface | Absorbs radiation | Emits radiation |
|---|---|---|
| Dark, matt | Good absorber | Good emitter |
| Light, shiny | Poor absorber | Poor emitter |
| Black body (ideal) | Absorbs all radiation that hits it | Emits the most radiation at a given temperature |
Section 4 of 5Everyday Examples
Temperature balances explain many familiar observations about heating and cooling.
Everyday Examples
Thermos flask
- Thermos flask Shiny surfaces reduce both absorption and emission of radiation, so the flask slows heat transfer.
Planet Earth
- Planet Earth Earth's temperature stays roughly steady when the energy it absorbs from the Sun equals the energy it radiates back into space.
Light clothing
- Light clothing Light colours reflect more sunlight and absorb less energy, which helps keep a wearer cooler in the sun.
Section 5 of 5Exam Checklist & Tips
6.4.2 – Key Points for the Exam
- Thermal equilibrium A state in which an object's temperature stays constant because absorption and emission are equal.
- Absorption The taking in of radiation by a surface, which transfers energy to the object.
- Emission The release of radiation by an object, which transfers energy away from it.
- Thermal radiation Electromagnetic radiation emitted by an object because of its temperature.
- Black body An idealised object that absorbs all radiation incident on it.
Common Mistakes
- Saying an object stops emitting radiation once it reaches a steady temperature, when it still emits radiation at a constant rate.
- Describing a steady temperature as meaning no energy is transferred, when energy is still transferred in and out at equal rates.
- Saying dark surfaces are poor absorbers and good reflectors of radiation.
- Stating that only hot objects emit radiation, when every object emits thermal radiation.