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A question about condensation of water

  1. Nov 21, 2009 #1


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    Gold Member

    When I put a near 0°C water in a glass which is at room temperature (around 25-30°C), some water in air (humidity) condensate over the glass. That's what I believe.

    I understand this like, the water present in the air as gas has molecules with more energy (kinetic, but overall?) than liquid water. When these fast molecules hit the "cold" glass, they transmit a lot of their energy and hence they change their phase from gas to liquid. Again, that's what I believe. (point me out if I'm wrong)

    But why don't it happen even if the glass is at room temperature? In other words, why the water present in the air doesn't condensate on everything? Why the water that evaporates doesn't condensate on any object? I guess it does but much less than if the object is cold, which I can understand, but I can't understand the proportion. By this I mean, I can't understand why a glass at 0°C is much, much, much, much cooler than a glass at 20°C for the water vapor.

    Is there any mathematical formula I can look at?
    Thanks in advance... I'm bothered by that.
  2. jcsd
  3. Nov 22, 2009 #2


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    Staff: Mentor

    Water condenses on an object who'se temperature is less than the dew point of the ambient air. The dew point is the temperature at which the air can no longer hold the moisture in it and the moisture starts to condense. So the object cools the air adjacent to it down to the dew point, causing water to condense and the only place for the condensed water to go is to stick to the object.

    This phenomena can be analyzed on a psychrometric chart: http://www.fao.org/docrep/s1250e/S1250EEW.GIF
  4. Nov 22, 2009 #3


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    Thank you very much for the explanation and the link to the chart.
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