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Thread: Kinetic molecular theory - time between wall collisions

  1. #1 Kinetic molecular theory - time between wall collisions 
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    In every explanation I have read about kinetic molecular theory, the time between collisions the molecule makes with the wall, is based on the molecule traveling the length of the container. How can the collisions between molecules be ignored?

    Thank you.
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  2. #2  
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    That is how an ideal gas works. At the temperatures and pressures on earth's surface most gases are close to ideal.
    Calculate the volume of a nitrogen molecule or helium atom. The vanderwalls radius can help you.
    Look up avogrados number.
    Now calculate the volume of the molecules in that g mole. Look at the volume of this gas at STP.
    What percentage of the volume do the molecules or atoms take?

    Now do the same calculation with a liquid.
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  3. #3  
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    Quote Originally Posted by pikpobedy View Post
    That is how an ideal gas works. ...
    Yes, when the idealisation includes pointlike atoms/molecules with zero interaction cross-section, there are no collisions, except with the container's walls.

    Things are different for real gases, of course. The mean free path between collisions is, to first order, an inverse function of pressure. And so the assumption that the gas particles go from wall to wall without collision isn't correct for real gases. For air, a typical mean free path for, say, nitrogen, is roughly of the order of 100nm at atmospheric pressure, if memory serves, so the assumption of infinite mfp is only well satisfied for quite tiny containers (or at quite low pressures).
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  4. #4  
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    The time for traveling the length of the container is thus what? There is no net drift in one direction or another.

    I am curious as to how the OP goes about finding an answer to his question.



    My background is extractive metallurgy thermodynamics, processes and heat transfer. Physical chemistry was a prerequisite.
    The use of PV=nRT was common.
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  5. #5  
    KJW
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    I think the OP is asking why the collisions between molecules have little effect on the wall-to-wall travel-time used in the calculation of pressure in the kinetic theory of gases. A clue to this is to consider the one-dimensional elastic collision between two equal objects, or even to consider the behaviour of a Newton's cradle. If one treats the colliding objects as if they are indistinguishable, then the kinematics of the collision will be such that there was no collision and that the objects passed through one another.
    A tensor equation that is valid in any coordinate system is valid in every coordinate system.
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