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Kinetic Energy and Scale

  1. Feb 11, 2016 #1
    On very small scales, an object's kinetic energy tells you its temperature, and on larger scales, an object's kinetic energy tells you its velocity. Where's the cut off? I mean, velocity is relativistic, but temperature... isn't... is it? Also, black body radiation decreases a body's temperature over time, but not its velocity. So what's the difference? Related question: How big can a molecule get before its kinetic energy stops telling you how hot it is and starts telling you how fast it is?
  2. jcsd
  3. Feb 11, 2016 #2
    KE=(mv2)/2. To my knowledge, this equation applies to particles in quantum mechanics as well as classical mechanics. I think the real difference is in the application. In thermodynamics, you have a large group of particles in a box, each with it's own kinetic energy. The temperature in the box is the average kinetic energy, or the average mass times velocity squared divided by two, of each particle. Yes, temperature is proportional to kinetic energy in some sense. Correct me if I'm wrong, but if I give an atom a little push (I'm not saying you can give an atom a push, but that there are ways an atom can move besides being heated up), the kinetic energy of the atom will have nothing to do with its temperature. Similarly, I don't think there is a maximum size for the temperature-kinetic energy relationship. I believe the process in which popcorn kernels (before popping) go crazy when heated up has something to do with this relationship.
  4. Feb 11, 2016 #3

    Vanadium 50

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    That's not true. (And it's hard to answer a question based on an incorrect premise)

    Kinetic energy is a function of collective motion; temperature is more a function of random motion.
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