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Heat and movement

  1. Jun 18, 2005 #1
    hello, my first post here, i'm training as a physics teacher and my physics is quite rusty, it's been a good few years since university, i was thinking about heat (of air for example) being the kinetic energy of the particles and then i thought if that was the case wouldn't a mass of air with a certain temperature increase in temperature if it started to 'move' for whatever reason (wind, a fan...) as the total kinetic energy of that air would increase.

    Does this have anything to do with the frame of reference you use to calculate the air particles' total Kinetic energy, do you use some kind of internal frame of reference?

    Or have i completely confused myself?

    this is bugging me, hope someone can help
  2. jcsd
  3. Jun 18, 2005 #2

    Doc Al

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    Staff: Mentor

    Exactly. You'd use a frame in which the momentum is zero.
  4. Jun 19, 2005 #3
    Actually it is not all that straight.Yes, the heat energy is due to increase in the K.E of the molecules present in the air. Actually when the wind speed is normal , the molecules are randomly striking the walla of the room , remember the kinetic energy is still present , and when wind/speed increases, the motion becoms more random , the more random and tedious motion , more the probability that the momentum's carried by th emolecules keeo on cancelling on striking the walls and against other equally sized molecules.Therefore we feel comfortable under fan.
  5. Jun 19, 2005 #4

    Doc Al

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    Staff: Mentor

    Just to be clear, the answer to that is no. Temperature is a measure of the average KE of the molecules in a frame in which the momentum is zero. Otherwise, temperature would be frame dependent.

    Of course, the macroscopic KE of the air imparted by the fan soon becomes random KE and an increase in temperature, but that's only after the moving air is slowed down by collision with other air molecules in the room and with walls, etc. (We feel comfortable under a fan because the moving air helps evaporate moisture on the skin, thus cooling us off. But having a fan on will increase the temperature of the room.)
  6. Jun 19, 2005 #5
    thanks doc al, i suspected it could be this something like this, so you could kind of say temperature is measure of the mayhem going on around you when you're sat in the middle of things
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