Radiation

1. Radiation D. Crowley, 2008

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

10. Thermal Radiation • Matt black surfaces are the best absorbers of radiation • Shiny surfaces are the worst emitters because they reflect most of the radiation away 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

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 • 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 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?