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Questions on the kinetic molecular theory of gases

  1. Dec 25, 2011 #1

    1-"the intermolecular attractive forces between gas particles are very weak due to the large intermolecular distances separating them"
    Is that true?
    this is wriiten in my textbook and it is confusing me because i've always thought of it in a different way ,I thought the intermolecular attractive forces between gas molecules are very weak because they move in a very very large speed so they don't have enough time to attract each other,
    But I feel some contradiction between the way i think and my textbook so I want to know which of these two concepts are true or both of them are true?

    2-I've read before that an ideal gas doesnot exist ,so what is the importance of imaging such an ideal gas if it doesnot exist?Is it because many gases have very similar properties to the properties of that ideal guy in high temperatures and low pressures?I can't really imagine what is an ideal gas although I've read many times about it..Does an ideal gas remains ideal at high pressure and low temperature?

    3-what is the relation between the general gas laws(Charle,Boyle,Jolly) and the kinetic molecular theory of gases?The ideal gas follows both of them...let me rephrase this question,
    why should the gas have the following properties :
    A gas consists of a collection of small particles traveling in straight-line motion and obeying Newton's Laws.
    The molecules in a gas occupy no volume (that is, they are points).
    Collisions between molecules are perfectly elastic (that is, no energy is gained or lost during the collision).
    There are no attractive or repulsive forces between the molecules.
    The average kinetic energy of a molecule is 3kT/2. (T is the absolute temperature and k is the Boltzmann constant.)
    in order to follow exactely the general laws of gases?what is the relation between having this properties and following exactely the general gas laws?I hope you could understand my problem here..

    4-why does A mole of any gas at STP occupy the same volume?the answer to this question in my textbook is that 1- the no. of particles of one mole of any gas is constant
    2.the intermolecular spaces of all gases is constant at STP.
    That's not very bad but what about the size of the molecules?the sizes of molecules are different for different gases indeed SO,why does A mole of any gas at STP occupy the same volume?

    5-"At the same pressure and temperature,the intermolecular spaces between the molecules of the ideal gases are constant?because the intermolecular attractive forces between the molecules of the ideal gas is negligible,so the intermolecular spaces doesnot depend upon the mass of of the molecule or the type of the gas".

    That's stupid!what does that mean??what is the relation between the mass of the molecule and the intermolecular distances betweern molecules?

    6-the frequency of the molecule is calculated from the relation :
    frequency = V/2L
    how was this equation derived?I don't understand it

    7-what is the rest energy?

    Thanks in advance.
  2. jcsd
  3. Dec 26, 2011 #2
    You've got some great questions and thoughts in here
    The two explanations are concerned with somewhat different things. First, the speed of the particles has no effect on the strength of the force---the force is weak only because they are far apart. Second, the force has little effect because the force between two particles is significant for a short period of time---which is because of their speed.

    An ideal gas is a simplifying concept. You list below some of the fundamental principles behind the conception of an ideal gas, obviously they are approximations (e.g. particles are points without size). Because they are approximations, there is no such thing as a true ideal gas. The concept is useful because in a very wide range of situations, real gases (e.g. those in day-to-day life) behave very very similarly to how an ideal gas would behave.

    No idea what you're asking.

    As you list above, this model of gases assumes they are point particles---thus every molecule does have the same size. The volume a gas occupies is unrelated to the size of the particle (in this model---but in actuality, the size of the particle is entirely negligible).

    This is just a result of the previous point---if every mole of gas has the same volume, and the same number of particles, then it will have the same spacing between particles.
    The mass of the particle and the interparticle distance are unrelated---that is what they are saying.


    The energy something has at rest (i.e. without kinetic energy).
  4. Dec 26, 2011 #3
    Awwww Great answers indeed!!
    because they are point masses so they have no volume or mass ..Is that true?

    I'm asking where this (2L) in the equation has come from?why isn't the equation frequency = V/L instead?
    may be I can't understand the equation because I've a problem in understanding the kintetic molecular theory of gases https://www.physicsforums.com/showthread.php?t=562751
    hmmm potenial energy or something like that?

    Thanks very much for claryfying these important conceptions.
    best regards
  5. Dec 26, 2011 #4
    When two ideal gases - A and B are at same temperature and pressure , then the equal volume of the two gases will have same number of molecules. If Volume of gas A = Volume of gas B at same temperature and pressure , then molecules of gas A = molecules of gas B. That's Avogadro's law. As we know that all gases of same volume respond and expand equally with same rise in temperature and at same pressure and initial temperature , then mass of gas particle and intermolecular distance is unrelated as far as mole concept is concerned.

    Depends on node and anti-nodes of vibrating body. Anyways this question is not related to sound in physics. The original equation formulation is as follows :

    It was found out that f[itex]\propto[/itex]1/l , where l is length
    So as such f=kV/L
    Now if there is one anti-node (nodes in vibrating body having maximum displacement and 0 tension) then k=1/2 and f=V/2L. If there are two anti-nodes then f=V/L
    There equations are more applicable in sound chapter of physics. Not sure why are you using them here.

    Have I to tell about what are nodes or anti-nodes ? If you want to know then that's fine.

    Yes.. Potential energy is the energy by virtue of position and configuration and the body has to be at rest. When body come in motion , potential energy decreases and kinetic energy increases. Kinetic energy is energy by virtue of motion. If body is not in motion kinetic energy is 0. Rest energy is energy when a body is at rest. That's it ! Nothing to do about potential energy due to height. Just kinetic energy is zero when a body is at rest. :wink:

    Please have a look at your VM. Have you solved your problem about the relation between angle of dispersion and deviation ?
    Last edited: Dec 26, 2011
  6. Dec 26, 2011 #5
    They still have mass ('point masses'), but no volume---that is correct. Again, this is just a simplifying model.

    I have no idea what equation that is, what the symbols are, and what frequency you're referring to.

    All non-kinetic energy. The most basic form of rest-energy is the rest-mass energy (E = mc^2); also any chemical potential energy, etc etc.
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