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Brownian motion

  1. May 21, 2008 #1
    Do the molecules of a gas move randomly when they aren't colliding, or is it only when they have elastic collisions with other molecules? I know it's a retarded question, I don't know much about physics.

    Also, does the temperature of a body depend on the internal energy of it, or just kinetic energy? If you increase the potential energy are you increasing the temp?
  2. jcsd
  3. May 21, 2008 #2
    Hey, I am not quite sure what your getting at with the first question, but for the temperature of a body would normaly down to the thermal energey of an object, I believe kinetic energy would increase the temperature, but that would be down to the friction acting upon it, as friction generates heat. Increasing potential energy will increase the acceleration on the object, meaning giving it a greater force of gravitation on the object, which I do not believe would increase the temperature of the body.
  4. May 21, 2008 #3
    Hi Forest,
    The random motion is due entirely to elastic collisions. In certain situations (namely plasmas), you can have very complex motion without collisions - but still in no way random.
    Without the collisions, they'll just fly straight.

    The temperature of a body, especially a gas, is essentially synonymous with the kinetic energy of its particles. In a simplified ideal situation, the internal energies of the particles do not effect the temperature. In practical, real situations - thats not entirely true. When the molecules or particles interact with each-other or external bodies, often their internal energy will be transfered of otherwise converted - letting those internal energies effect the temperature.
  5. May 21, 2008 #4
    Do the molecules of a gas move randomly when they aren't colliding?
    no. if there are no collisions each particle will act like a billiard ball, with angles of incidence equal to angles of reflection.

    does the temperature of a body depend on the internal energy of it?
    yes, since U=TdS-PdV, which means the internal energy is equal to temperature times a small change in entropy minus pressure times a small change in volume.

    If you increase the potential energy are you increasing the temp?
    i dont think so. if we increased the mass of earth i dont think temps would change. if we compressed a spring, the mass on the end would not get hotter.

    imagine you have a partitioned system at thermal equalibrium. is there any potential we can add to one side that would cause heat transfer as a direct cause and not due to the effects of masses or charges moving?
    i would imagine decreasing the PE of one side would influence particles from the other side to find a lower energy until at equlibrium, meaning decreasing the potential would increase the probability of positive influx. this arguabley means that the high PE side is hotter than the low PE side, but i think this is a mixed up analogy (based on "hot stuff flows into cold stuff").

    even the definition of temperature is a bit confusing... something like "the thing that remains constant as energy changes with entropy" or T=dE/dS. I think we have to consider the center of mass frame to determine temperature, otherwise thermal motion gets confused with the motion

    if we consider 8 inert atoms in a zero kelvin solid lattice and increase the mass of the planet they are on, will this increase the temperature? there may be an increase in motion to find the new lowest energy states and the system will oscilate, becomeing dampened with time, until it is at rest again... unless it oscilates continually, thereby increasing its temp for a long time.

    but i still dont think potential energy increases temp... just a gut feeling though
  6. May 21, 2008 #5


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    Interestingly, it can be shown via thermodynamics that stretching a spring elastically (which increases its potential energy) causes either an increase or decrease in temperature, depending on whether the spring material has a negative or positive coefficient of thermal expansion, respectively. You can experiment with this by stretching a thick rubber band, waiting a few seconds so that it equilibrates to room temperature, releasing it, and putting it to your lips (our handy built-in temperature sensor!). It should have cooled noticeably.
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