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Radiation & Energy Transfer

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					Radiation & Energy Transfer




                              D. Crowley, 2008
                                              Friday, July 22, 2011



Radiation & Energy Transfer
   To investigate what affects the energy transferred by
    radiation
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)
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.
    Infrared waves heat objects that absorb (take in)
    them

   Some surfaces are better at emitting thermal
    radiation than others and some surfaces are better
    at absorbing thermal radiation than others – good
    emitters are also good absorbers
Experiment
   Your task is to design 2 experiments to investigate
    the following: -
     1.  Which colour material is the best absorber of
         thermal radiation
     2.  Which colour is the best emitter of thermal
         radiation

   Apparatus: black paper; silver foil; beakers; water;
    elastic bands; thermometer; kettle; stop-watch; ice

   Write a description of how you will carry out your
    experiment, what you are going to measure and
    what your predict your results will be (and why)!
Experiment 1 - Absorption
   To compare the absorption by different surfaces use
    2 identical beakers where one is wrapped in black
    paper and the other is wrapped in silver foil
   To both add an equal volume of water with some
    ice, and measure the start temperature
   Place the beakers in sunlight – use a thermometer
    to measure the temperature every 5 minutes,
    identifying which beaker warmed up fastest / the
    most
Experiment 2 - Radiation
   To compare the radiation of different surfaces use 2
    identical beakers where one is wrapped in black
    paper and the other is wrapped in silver foil
   To both add an equal volume of hot water and
    measure the start temperature (do not start the
    stop watch until these both reach a certain initial
    temperature, e.g. 70oC)
   Use a thermometer to measure the temperature
    every 5 minutes, identifying which beaker cooled the
    quickest / most
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)
     Infrared 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 imaging            Lion – thermal imaging
True Or False?

				
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