Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Why does air rush in from colder to hotter space and not vice versa?

  1. Feb 19, 2013 #1
    When you open a window of a warm room, the air from the colder outside rushes in (you can see the curtains bend into the room, for example).

    But shouldn't it be the opposite? The higher kinetic energy of the hotter air molecules inside the room should create more pressure than the colder air molecules outside, and so when a window is open the air should move from higher pressure to lower pressure, i.e. from inside to outside.

    And if the explanation is that the hot air does indeed move to the outside, and the cold air rushes in to fill the created vacuum, why do we feel only the cold air coming in and why is the net force pushing in the inside the room direction?
     
  2. jcsd
  3. Feb 19, 2013 #2

    davenn

    User Avatar
    Science Advisor
    Gold Member

    Re: Why does air rush in from colder to hotter space and not vice vers

    hi there james
    welcome to PF

    how is the room being cooled ? ducted A/Con?
    if so then it will be pumping air into the rooms and producing a higher pressure inside the building/room than the pressure outside.
    the temperature difference is pretty much irrelevent

    Dave
     
  4. Feb 19, 2013 #3

    DrClaude

    User Avatar

    Staff: Mentor

    Re: Why does air rush in from colder to hotter space and not vice vers

    Isn't the wind the dominant factor here?
     
  5. Feb 19, 2013 #4

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Re: Why does air rush in from colder to hotter space and not vice vers

    See my diagram. I believe this is an illusion due to the fact that the curtain is fixed at the top.

    When the window is opened the cold air outside will descend between the curtain and wall/window pushing the curtain into the room slightly. The hot air inside at ceiling level would flow over the top of the curtain and out of the window.

    The top of the curtain is fixed, where as the bottom is free to move.
     

    Attached Files:

  6. Feb 19, 2013 #5

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Re: Why does air rush in from colder to hotter space and not vice vers

    PS: If the room is otherwise sealed the hot and cold flow rates must be the same or the pressure in the room would change.
     
  7. Feb 19, 2013 #6

    davenn

    User Avatar
    Science Advisor
    Gold Member

    Re: Why does air rush in from colder to hotter space and not vice vers

    Duh...

    I just realised I totally misread the question
    Wish the edit feature lasted a little longer :(

    Dave
     
  8. Feb 21, 2013 #7
    Re: Why does air rush in from colder to hotter space and not vice vers

    i think the only reason the cold air comes in when you open a window is because of the pressure from the wind. if there was absolutely no wind then gravity would take over and the warm air in the room would escape at the top of the window (due to convection) and the cold outside air would be sucked in at the bottom of the window, until the temperatures equalized..
     
  9. Feb 21, 2013 #8

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    Re: Why does air rush in from colder to hotter space and not vice vers

    The cold air would come in even without wind. It more dense than the warm air so there would be a vertical stratification between the two densities, which would be unstable. The slightest perturbation (for example, from opening the window) would cause the situation to break down and the cold air would flow into the room from the bottom portion of the window while warmer air would then get pushed out the top assuming only one window was open, much like CWatters drew. The presence of wind or any sort of over- or under-pressure in the house would complicate matters a bit, but the overall idea is the same.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Why does air rush in from colder to hotter space and not vice versa?
Loading...