Thermal Radiation
- All bodies (objects), no matter what temperature, emit a spectrum of thermal radiation in the form of electromagnetic waves
- These electromagnetic waves usually lie in the infrared region of the spectrum
- Thermal radiation is defined as:
Heat transfer by means of electromagnetic radiation normally in the infrared region
- The hotter the object, the more infrared radiation it radiates in a given time
- This is because atoms and molecules above absolute zero are in constant motion
- Electric charges within the atoms in a material vibrate causing electromagnetic radiation to be emitted
- Therefore, the higher the temperature, the greater the thermal motion of the atoms and the greater the rate of emission of radiation
- Thermal radiation is the only method of thermal energy transfer that does not require matter in order to move or propagate
- Therefore, thermal radiation is the only way heat can travel through a vacuum
Thermal radiation demonstration on a Leslie cube
An image of a hot object, known as a Leslie cube, taken in both Infrared and visible light. The black surface emits more thermal radiation (infrared) than the shiny surface
- The amount of thermal radiation emitted by an object depends on a number of factors:
- The surface colour of the object (black = more radiation)
- The texture of the surface (shiny surfaces = more radiation)
- The surface area of the object (greater surface area = more area for radiation to be emitted from)
- Dark, dull objects are better at emitting and absorbing radiation
- Light, shiny objects are worse at emitting and absorbing radiation
Worked example
A hot meteorite hits the surface of the Moon.
Identify and discuss the principal means by which the meteorite can dissipate thermal energy once it has landed.
Answer:
Step 1: Identify the types of thermal energy transfer
- An object can lose energy through conduction, convection or radiation
- In this case, the hot meteorite will only be able to lose energy via conduction and radiation
Step 2: Explain these choices
- The meteorite can lose heat energy through conduction because it is in contact with the surface of the Moon
- The Moon does not have an atmosphere, so convection is not possible
- Infrared photons emitted by the meteorite are able to travel through a vacuum, so heat loss via radiation is possible
Exam Tip
If a question refers to the colour of something (black, white or shiny) then the answer will likely be about thermal radiation!
If a question involves a vacuum (empty space) remember that radiation is the only mechanism in which heat can be transferred, as conduction and convection require particles to transfer heat.