Infra-Red Radiation Noadswood Science, 2011

Infra-Red Radiation  To understand what affects energy transfer by radiation Tuesday, May 12, 2015

Heat Transfer  There are three distinct methods of heat transfer: - Conduction Convection Radiation  If there is a difference in temperature between two places heat will flow between them

Earth & The Sun  Earth is warmed by heat energy from the Sun – how does it get here?!  There are no particles between the Earth and the Sun, so the heat cannot travel by conduction or convection  Instead the heat travels to Earth via infrared waves – similar to light waves (with a longer wavelength), travelling at 300’000kmsec -1 (like any wave they can be reflected and absorbed)

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

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

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 absorbers because they reflect most of the radiation away

Keeping Hot Experiment  Your task is to carry out an experiment to see which conditions are best to keep something hot: - Take 4 boiling tubes and fill each with the same volume of hot water Wrap one in black paper, one in white paper, one in silver foil and leave one with no wrapping Record their temperatures and start the timer  Which will lose the most heat in 10 minutes? Write a prediction, explaining why you think this…

Results  Are there any anomalous results?  Can you think of any explanations for any anomalous results? Material Temperature loss after 10 minutes ( ◦C) Av Black paper White paper Foil No wrapping

Evaluation  Write down your findings – which material is the best emitter / absorber?  Matt black surfaces are the best emitters and absorbers of radiation  Shiny surfaces are the worst emitters and absorbers of radiation (as they reflect most of the radiation away)

Infra-Red Imaging  Thermal images can be taken of objects showing where they emit most of their heat – the hotter the object, the more thermal radiation it emits: - Ostrich – thermal imagingLion – thermal imaging

Infra-Red Imaging

Leslie Cube  A Leslie cube is a device that has sides made from different materials – it is filled with hot water and the energy emitted from each side is measured  Explain which side will record the highest temperature using the terms absorber and emitter…

Leslie Cube  The side which records the highest temperature is the matt black side, as this not only absorbs the heat from the water inside best, but then emits it to the surroundings best too, hence heating the thermometer…

True Or False?

Anagrams

Quick Quiz

Exam Questions  Answer these typical exam questions (include the word particle in your answers): - Why does the handle of a metal tea spoon get hot? Why are wooden spoons used to stir saucepans? Why is the loft often the hottest room in the house? Why do black car seats get hotter than white car seats

Exam Questions Why does the handle of a metal tea spoon get hot?  If there is a difference in temperature heat will flow from the hot region to cold – in solids conduction occurs when vibrating particles pass on their extra kinetic energy to neighbouring particles from the hot region (in the cup) to the colder region (the top of the spoon by your hand)

Exam Questions Why are wooden spoons used to stir saucepans?  Wooden spoons are not very good thermal conductors (insulators) so heat transfer does not take place through them well (as unlike a metal they do not have any free electrons so heat transfer only takes place via the vibration of the atoms (much less effective))

Exam Questions Why is the loft often the hottest room in the house?  Convection occurs when fluids are heated (gases or liquids) – more energetic particles which have been heated become less dense (take up more space as their particles vibrate more) and rise up taking their energy with them. This energy is then transferred to the surroundings (the roof) and then sink down again (where they are re-heated) continuing the convection current

Exam Questions Why do black car seats get hotter than white car seats  Radiant energy (infra-red) is given off by a hot object and absorbed by another object – matt black surfaces are the best at absorbing radiation, whilst white car seats are the worst (they reflect it)