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Which will warm first?

  1. Jan 16, 2010 #1
    Which will warm first???

    Consider a cold cool drink bottle taken from freezer and exposed to atmosphere....as everyone knows, water would form on the bottle due to condensation of atm. air.....now i want the cool drink to be colder for longer time,for that should the condensed water on bottle surface be periodically wiped as they form or should condensation be allowed ie not wiping the condensed water????
     
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  3. Jan 16, 2010 #2

    russ_watters

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    Re: Which will warm first???

    The condensation should be allowed to remain as it will form a buffer between the bottle and the air, inhibiting heat transfer. The difference will be very, very small though.
     
  4. Jan 16, 2010 #3
    Re: Which will warm first???

    but condensation is more when the surroundin atmosphere is humid....n thats y a cold bottle remains cool for longer time in air conditioned room and hence condensation is less which means if we are able to prevent condensation,then the bottle might remain cool for longer time aint it so???
     
  5. Jan 16, 2010 #4
    Re: Which will warm first???

    and also, heat transfer coefficient of water is more than that of air which means heat transfer is more with condensed water than wit air,though the temp difference is greater for bottle-air combination than that of bottle-water combination....isnt so???
     
  6. Jan 16, 2010 #5
    Re: Which will warm first??

    I dont think it will make much of a difference in keeping it colder by wiping it or not.
    Although, you could look at the heat transfer. Atmospheric air at temperature T that is stagnant. I think the condensation layer will act as a resistance the same as the plastic (or glass) and will slow down the heat transfer from the air to the fluid inside, hence better to not wipe it to keep it colder longer. Your heat transfer coefficient of the condensation layer should different than the initial stagnant air.

    Interesting point though on how your overall heat transfer coefficient can change due to air properties changing with temperature
     
  7. Jan 17, 2010 #6

    russ_watters

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    Re: Which will warm first???

    That doesn't have anything to do with the question you asked in your first post.
    When there is condensation on the bottle, the heat is still coming from the air, so no, the heat transfer is less because the heat has further to go before getting to the bottle.
     
  8. Jan 17, 2010 #7
    Re: Which will warm first???

    Yes...then heat transfer between air-bottle is less than bottle-condensed water combo which means air-bottle combo ie case where condensed water is removed will stay cool longer aint it so???
     
  9. Jan 17, 2010 #8
    Re: Which will warm first???

    Don't wipe as stated before. All you are doing is exposing a nice, dry, cool surface to the air again and that just invites more condensation, which will just warm it up more.
     
  10. Jan 17, 2010 #9

    russ_watters

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    Re: Which will warm first???

    Again, heat transfer is not less for the dry bottle. In general, adding *anything* into a heat transfer path will decrease the heat transfer rate, even if the heat transfer coefficient of that "anything" is better than the other parts of the path.

    Try it with resistors in electricity: If you have a 3ohm and a 6ohm resistor in series with each other and you add a 1 ohm resistor between them, have you increased or decreased the total resistance?
     
  11. Jan 17, 2010 #10
    Re: Which will warm first???

    "Again, heat transfer is not less for the dry bottle. In general, adding *anything* into a heat transfer path will decrease the heat transfer rate, even if the heat transfer coefficient of that "anything" is better than the other parts of the path."

    if wat u said is correct then there should be no fins!!!!!!fins enhance ht...n they should hav good ht coefficint with surrounding....
     
  12. Jan 17, 2010 #11
    Re: Which will warm first???

    Keeping to Russ's resistor analogy; using fins to dissipate heat would be equivalent to adding more resistors in parallel. It increases the area through which the heat travels and the rate of flow of heat is proportional to this. It is inversely proportional to the length of the conductor.
    By the way, the reason why condensation in a humid room causes the drink to warm up more quickly, is that the condensation releases a large amount of latent heat, over 2000kJ per kg of water, an effect which is far more significant than the thermal conductivity if the water layer. (Which, as others have said, would tend to reduce the rate of conduction for the reason I stated above)
     
  13. Jan 18, 2010 #12
    Re: Which will warm first???

    @Stonebride so wat u mean to say is condensation warms the bottle faster rite??so the bottle where condensation is not allowed to happen(ie the condensed water being wiped) would stay cooler for longer time period huh???pls help out..
     
  14. Jan 18, 2010 #13

    HallsofIvy

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    Re: Which will warm first???

    No, that's exactly the opposite of what Stonebridge said! (And it is "Stonebridge", not "Stonebride"). Yes, condensation warms the bottle. But condensation occurs at the surface of the bottle. If you leave the condensate on the on the bottle, that prevents further condensation. If you wipe it off, you allow more condensation, warming the bottle more. Every response here has said that NOT wiping the condensation of the bottle will be keep it cooler yet you managed to come to the exact opposite conclusion!
     
  15. Jan 18, 2010 #14

    russ_watters

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    Re: Which will warm first???

    That's different. Now you've added surface area.
     
  16. Jan 18, 2010 #15
    Re: Which will warm first???

    Others have answered before me, but no, you are wrong.
    Wiping away the condensation is not equivalent to "not allowing it to happen". How can you wipe it away unless it has already happened? Then when you have wiped it, you just get more.
    You are (purposely I suspect) mixing up two separate issues.
    1. The cooling in a dry room compared to a humid room. (latent heat effect and not part of your original question)
    2. The cooling in a humid room and the effect of the thickness of the condensation layer. (thermal conductivity effect and the original point you made)

    I think we have answered your original question satisfactorily.
     
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