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Thread: Radiation heat and teapots

  1. #1 Radiation heat and teapots 
    Forum Sophomore
    Join Date
    Mar 2007
    I saw an experiment the other day where boiling water was put in a square tin can, which had one side shiny white, one side shiny black, one side matt black, and one side metallic. These sides were only different on the OUTSIDE. On the inside it was just plain tin.

    When an infra-red detector (i think that was what it was, it was a heat detector of some sort anyway) was placed on each side of the can, different readings where produced. The matt black had the highest, and the shiny white the lowest, and you can guess the others.

    I know about how particles bounce off harder from black surfaces than white, but these surfaces where only on the outside, not the inside, where the water was.

    In another example, why is it that heat is lost faster from unpolished teapots than polished ones, when the polishing is done on the outside?

    Can anyone explain this?

    I think i have an idea, but it is a bit far-fetched.

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  3. #2  
    I guess if it's a rougher surface it has more area to dissipate from, silver will reflect IR from either side, whether air or solid material.

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  4. #3  
    Moderator Moderator
    Join Date
    Jun 2005
    It's called "black-body radiation".
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  5. #4  
    M is offline
    Forum Junior
    Join Date
    Sep 2006
    Hazz, I detect some misunderstandings in the way you pose the question.

    Why don't you think the "outside" should matter? Radiation is heat emitted from those objects (on the outside) in form of electromagnetic waves (think of infrared). In the case of the teapot, each surface is provided essentially the same amount of heat on the inside, I assume they are also made of the same material, but their efficiency of emitting heat to the surrounding through radiation depends on their outside surfaces.

    You can really think of three effects deciding how quickly a tea pot looses heat (with a given liquid of certain initial temperature): a) how quickly is heat transferred from the liquid through the inside surface into the wall, b) how quickly is heat transferred within the wall from inside to outside surfaces, c) how quickly is heat transferred from the outside surface to the surrounding?

    All of this will depend on the shape and quality of the surfaces, as well as on the material of the walls. The problems you described focus on but one partial aspect of this: Radiation from the outside surfaces to the surrounding,... i.e. by definition, it's the outside surface that matters, here.

    Don't confuse an experiment trying to teach you one singled-out aspect of heat transfer, with a complete treatise on the subject.
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