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Why speed of the molecules is bigger the less they weigh?

  1. Feb 7, 2012 #1
    In statistical physics we have for an ideal gas about the average kinetic energy for its molecules:


    Now in my book this is derived using the ideal gas law as an experimental fact, but that does not really help you get a deeper understanding, does it? I'm assuming that this can be derived from statistical mechanics.

    I wonna ask the following?
    What is the intuition behind, that the speed of the molecules is bigger the less they weigh? This follows from the fact that every molecule regardless of mass, apparantly on average have the same kinetic energy.
  2. jcsd
  3. Feb 8, 2012 #2
    Kinetic energy is equal to half the mass times the velocity squared. Therefore, to have the same kinetic energy with something less massive, you need a greater velocity.

    That is if I understood you properly.
  4. Feb 8, 2012 #3
    well I was more interested in the deeper reason behind why <Ekin> is always 3/2kT, but maybe I should just wait with that till statistical mechanics.
  5. Feb 8, 2012 #4

    Philip Wood

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    Gold Member

    Sketch of an argument.
    (1) You can show by a dynamics argument (See Jeans: Kinetic theory of gases) that a gas will exchange energy in collisions with its container walls, unless mean KE of gas molecules is the same as that of wall molecules. (2) But macroscopically it's temperature difference that controls heat transfer. (3) So two gases with the same mean KE have the same temperature. (4) But this doesn't show that mean KE is proportional to temperature. (5) Nothing can show this until we have defined a temperature scale. (6) the fundamental scale is the thermodynamic scale (of which the kelvin scale is the practical expression) which is defined in terms of heat taken in and given out in a Carnot cycle. (7) By taking an ideal gas through a Carnot cycle (in a thought-experiment), we can show that the kelvin temperature of the gas is proportional to PV. (8) Kinetic theory shows that PV is proportional to the mean KE of the molecules.

    For a lot of elementary purposes, we can down the argument by defining an ideal gas scale of temperature such that T on this scale is proportional to the mean KE of the molecules. But at some stage, if you take Physics further, you're going to need to know how to establish the identity between the ideal gas scale and the thermodynamic scale.
    Last edited: Feb 8, 2012
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