plasticiser
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Hi,
I tutor GCSE physics and I've got myself stuck on a question - it could just be me being stupid but I can't get my head around the mark scheme. I've inserted and attached it
Name:  heat transfer.PNG
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The mark scheme gives the answers as:
(i) absorber
(ii) reflector
(iii) emitter

I agree with the first two, but why won't they allow conductor for the third gap? Surely heat is being conducted away from the water and dissipated into the surroundings?
Any help would be much appreciated, thanks in advance!
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vybzkartel
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in experiment 1 the dull black surface is absorbing heat from the radiator and making the cold water hot

in experiment 2 you start off with boiling water so the dull black surface is absorbing heat and then emitting (?)

whereas the shiny surface is a poor absorber and emitter of radiation because it just reflects it


conduction convection and radiation are different methods of heat transfer maybe thats why they wont allow conductor
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Desk-Lamp
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The word conduction is normally used between solids/liquids. Think about it- If you're talking about energy moving from a solid to gas, what's the difference between conduction and convection?
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JordLndr
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He's a neat trick for anything like this.

Although Matt black surfaces are the best at absorbing heat, they are also the best at giving off heat (emitting).

Same with shiny reflective surfaces, they take in less heat and also give off less heat (Hence why a kettle is most commonly reflective)
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Physics Enemy
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(Original post by plasticiser)
Why won't they allow conductor for the third gap? Surely heat is being conducted away from the water and dissipated into the surroundings
Better radiation from can to air is why the black can has cooler water; colour has no effect on conduction from water to can. It's not one method of heat transfer.
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Physics Enemy
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(Original post by Desk-Lamp)
The word conduction is normally used between solids/liquids. Think about it- If you're talking about energy moving from a solid to gas, what's the difference between conduction and convection?
Heat radiates outward from can to air here, convection is movement/circulation of heat in liquids and gasses. Cans conduct equally well, but black can emits better.
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Joinedup
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I think the inclusion of the word surface is your big clue - putting a shiny surface on an object, even it it's a microscopically thin layer, can alter the emissivity dramatically - like the example of a mirror finish kettle given above.
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