1. Radiation
D. Crowley, 2008

2. Radiation To know that heat can travel via radiation
Thursday, February 21, 2019Thursday, February 21, 2019
Radiation
To know that heat can travel via radiation

3. Space
Does heat travel via conduction or convection in space?

4. Space Heat does not travel via conduction or convection in space!
Conduction relies on vibrating particles passing on their energy to their neighbours
Convection is the movement of heated particles to cooler regions
In space there are no particles, so conduction and convection cannot take place!
Instead, thermal energy (heat) is transferred via radiation

5. Model
Think back to the conduction, convection and radiation model (with the tennis balls)
You are taking the place of the particle, and the tennis ball is the energy you have
In space you cannot pass the tennis ball from person to person like in conduction (there are no particles in space)
You cannot walk around from one area to the other like in convection (particles do not travel from the Sun to Earth)
Instead the tennis ball can be thrown like in radiation, where the energy is emitted from a hot object (Sun) to a colder one (Earth)

6. Radiation
In radiation, radiant energy (infra-red) is given off by a hot object and absorbed by another object
All hot objects radiate heat to the surroundings via infra-red waves – this heat radiation does not need particles, so it means heat can travel through a vacuum

7. Radiation Experiment
You will be wrapping test tubes in 3 different materials – foil, black paper and white paper
You will then see which looses its heat quickest

8. Radiation Experiment
What materials do you think will emit most radiation - explain why you think this
Which material do you think will absorb most radiation - explain why you think this
Which material do you think will reflect most radiation - explain why you think this

9. Thermal Radiation All objects emit (give out) some thermal radiation
Some surfaces are better at emitting thermal radiation than others…
Matt black surfaces are the best emitters of radiation
Shiny surfaces are the worst emitters of radiation

10. Thermal Radiation
Infrared waves heat objects that absorb (take in) them
Some surfaces are better at absorbing thermal radiation than others – good emitters are also good absorbers
Matt black surfaces are the best absorbers of radiation
Shiny surfaces are the worst emitters because they reflect most of the radiation away

11. Colour
Why are houses painted white in hot countries / cricket players wear whites?
Why is wearing black in summer a bad idea?
Why do marathon runners get wrapped in a foil cloak at the end of a race?

12. Colour
Black and dull surfaces emit (give out) and absorb radiation well – in summer you do not want to absorb too much heat radiation
White and shiny surfaces do not emit radiation well and reflect radiation instead of absorbing it – useful in hot countries and when you want to keep cool
Marathon runners are wrapped in foil blankets at the end of the race as the shiny surface is a poor emitter of radiation and so prevents them losing too much precious body heat - this heat is reflected back to the runner
An object with a matt (dull) surface will absorb and emit infra-red radiation at a faster rate than an object with a shiny surface
An object with a dark surface will absorb and emit infra-red radiation at a faster rate than an object with a light surface

13. Temperature
Imagine a hot piece of metal which has a matt black surface and a shiny white surface - a thermometer is placed at the same distance from both surfaces
Why are the thermometers showing different readings?
The thermometer next to the matt black surface shows a higher temperature because it emits radiation at a faster rate
Heat leaves the metal more quickly through the matt black surface than the shiny white surface