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I How does a pressure/temperature difference cause a force?

  1. Jun 4, 2017 #1
    Hello. I am inquiring about how a pressure difference between a hot room (roughly room temperature) and a cold room (roughly the temperature of a standard refrigerator) can cause a force inside the cold room when exposed to its hotter counterpart. More specifically, I would like to apply this concept to an area where I work.

    I work at a grocery store and we have a large cooler that we keep produce items in. Said room is roughly 30 to 40 meters in length between one entrance (two, big flap doors) and another exit (a big, metal door that seems fitting for a butcher's shop). So, it goes from an area with room temperature to the cooler around the temperature of a refrigerator to another area with room temperature. The big metal door is always left ajar slightly, so it is not closed fully. I enter the cooler from the side with the big flap doors and almost immediately notice the big, metal door on the opposite side swing open, almost fully, to then later go back to its starting position. This same effect happens whether it is from me entering the cooler or exiting it, both using the side with the big flap doors (I never use the metal door myself).

    How does the pressure difference and sudden exposure to the hot room cause the door on the opposite side to open? It feels like it would be too far of a distance to be a gust of wind from me entering the room, and I don't know why the difference in pressure on one side 40 meters away would affect the door on the opposite end so quickly.

    I tried researching this part of fluid dynamics without fully reading an entire chapter on it, but am coming up short. I would greatly appreciate either a direct and in-depth answer to my question, or a referenced source to this exact topic. If possible, an explanation on a molecular level is more helpful for my comprehension. Thank you in advance for your time.
  2. jcsd
  3. Jun 4, 2017 #2
    Pressure at the molecular level is the result of collisions. The particles in question have speeds of around 500m/s which is much faster than a gust of wind. There is also a large number of particles that collisions are common and the new pressure is registered rather quickly.
  4. Jun 4, 2017 #3


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    Pressure disturbances propagate at the speed of sound.
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