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Homework Help: Is my sisters science test wrong ?

  1. May 9, 2012 #1
    my sister asked for help with a science test she had as preperation for her gcse's
    in this test was a table asking about heat transfer in a vacuum flask some parts of the table were filled in as an example but the question was would heat be lost through

    A convection
    B conduction
    C radiaton
    for different parts of the flask
    acording to the examples in the table heat would be lost via the vacuum by convection and conduction

    i told my sister that this was BS as a perfect vacuum canot conduct or convex heat at all i know a thermos flask is not a perfect vacuum but the diagram of a flask was labled only vacum was i wrong to tell my sister this or was the example answers bs as i had told her ?
  2. jcsd
  3. May 9, 2012 #2
    you were wrong.
  4. May 9, 2012 #3
    "i was wrong "

    well i always assume thats possible dont supose you could explain why ?
  5. May 9, 2012 #4
    because it was contrary to the given solution.
  6. May 9, 2012 #5


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    Gold Member

    vacuum can't propagate fluids, so it's not convection and it can't transfer energy states through a material because there's no material, so it's not conduction.

    Radiation however, requires no medium.
  7. May 9, 2012 #6


    User Avatar
    Gold Member

    This does not mean the OP was wrong. The given solution is not always correct.
  8. May 9, 2012 #7
    True, but the way the op formulated the problem, we cannot be certain what the question was.
  9. May 9, 2012 #8
    the table on the test stated as example, in a vacumm flask heat was lost from the vacuum flask via convection and conduction which i said to my sister was wrong at least in a perfect vacuum but as the diagram atached was just labled vacuum it should be taken as such despite the difficulty of achiving a perfect vacuum

    but as im no expert on these things i thorght i better check hense my post
    it also said it could lose heat via radiation which i agreed with the student was then asked to tick the boxes as to how the stopper on the flask and the jacket of the flask would lose heat

    [convection] [conduction] [radiation]
    vacuum flask √ √ √
    Last edited: May 9, 2012
  10. May 9, 2012 #9
    please use proper punctuation.
  11. May 9, 2012 #10
    my atempt at a table hasnt formated properly its ok on the preview but not when i post but im sure you get the idea

    dickfore you seem to be trolling a bit there dont make me dig out my grammer nazi gif :-)
  12. May 9, 2012 #11
    Please use capital letters at the start of a sentence. Fgs
  13. May 9, 2012 #12
    Please post the question as it appeared exactly in the test, the avaliable options for an answer and the suggested solution. Then, we will arbitrate the correctness of the solution.

    Asking for proper punctuation is not trolling. In fact, it is a requirement in the rules of PF.
  14. May 9, 2012 #13
    If I read you correctly: "the question appeared in a test, available options for an answer and in suggested solutions"
    Is that what you mean?
    Last edited: May 9, 2012
  15. May 9, 2012 #14
    Question was as follows

    Labled picture of a vacuum flask with a stopper in it and a jacket around the flask like a cross section of a thermous flask.

    the student is asked to complete the table below
    "in what ways can the flask lose heat"
    all 3 are ticked for the vacuum flask preceded by the word example

    ________________[convection] [conduction] [radiation]
    vacuum flask| eg_____√__________√________√

    So i said to my sister that i thorght that the examples given are wrong, that the flask could not lose heat via convection or conduction.

    It now occurres to me that the flask cold lose heat via conduction in the walls of the vacuum flask. But that still leaves convection i dont belive that box should have been ticked in the example unless you assume that the vacuum is not perfect.
    If the vacuum was not perfect it should have been labled partial vacuum in the diagram.

    i hope that is clearer
  16. May 9, 2012 #15
    The intended nature of the vacuum flask is to preclude heat loss through conduction or convection. But let us suppose we did have a perfect vacuum in the region between the outer shell and the inner shell. Would there still be conduction? I would say yes, to the limited extent that if the fluid is in contact with the stopper (even if the stopper provides a perfect seal). And obviously if the stopper is in contact with the fluid it is going to be conduction-heated by the fluid over some time. Then the stopper could slowly leak the heat out through conduction into the surrounding air, or to the outer surface of the flask, which would itself transport the heat away by radiation or conduction. So to this extent I could agree that conduction is responsible for the flask losing heat (if by flask we mean the entire system). Convection also plays a role in this, since the above process will result in a temperature differential between the part of the fluid near the stopper and the part near the bottom, allowing warmer fluid to rise to the top and repeat the process. Whether this could be said to be responsible for the heat losing flask might be a matter of semantics; it is hard to discern the exact nature of the question from what you've posted, but based on what you've said it sounds like I would not agree that convection is responsible, since it is only conduction (and of course radiation) between the stopper and the surrounding medium that is ultimately responsible for the flask losing heat.
  17. May 9, 2012 #16
    Thats what i was thinking
    Apart from the diagram which was a standad cross section of a themous flask you have all the information in the question. After been given the examples you had to complete the table for the stopper and jacket.
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