Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Is pressure distinct from temperature in gas laws?

  1. Feb 19, 2009 #1
    Is pressure distinct from temperature in gas laws? How do we separate these attributes?

    Take Charles law for example, now unless I've read this example from a textbook (textbook Silberberg Chemistry 3rd edition ) incorrect, it describes an experiment to determine the relationship between the volume and temperature. It says for this experiment pressure is constant so I assume it should be unchanging. But then in my textbook it shows a graph alongside this experiment showing the pressure changing / rising with the temperature and volume, which makes sense to me but not to their pre-condition of a constant pressure.

    It makes sense to me because a rising temperature is going to cause increased kinetic energy and movement of the molecules, which means faster more powerful collisions with the container walls so more pressure.

    So the question remains can one keep the pressure constant, and change the temperature? Has anyone observed that; one can change whilst the other remains unchanged. e.g. That the pressure can change whilst the temperature remains the same etc...

    What are the forum's thoughts?
     
    Last edited: Feb 19, 2009
  2. jcsd
  3. Feb 19, 2009 #2

    mgb_phys

    User Avatar
    Science Advisor
    Homework Helper

    The overall gas law is
    PV = nR T ( n is the amount of gas and R is a constant)
    So this means.
    You can increase pressure, decrease volume and keep the same temperature
    or increase pressure, increase temperature and keep the same volume
    or increase volume, increase temperature and keep the same pressure

    (assuming you keep the same amount of gas)
     
  4. Feb 19, 2009 #3

    Doc Al

    User Avatar

    Staff: Mentor

    OK.
    OK.
    Maybe... if the volume stays the same. Pressure depends on how many collisions per area per time, not just the kinetic energy of the molecules.

    Sure. Consider the ideal gas law.
     
  5. Feb 19, 2009 #4
    I am still unsure but you do insure yourself there, (post contains irony...). I appreciate quick responses, but I have read reliable material and am familar with the basic thoeries as presented by textbooks and don't need them restated unless they sufficiently show where I'm going wrong.
    I reckon you were too hasty to overlook my reasoning on a molecular level,
    1/ Take another look at the contadiction I've highlighted, quote; "It makes sense to me because a rising temperature is going to cause increased kinetic energy and movement of the molecules, which means faster more powerful collisions with the container walls so more pressure".
    2/ The conflict is also in the experiment, how is this resolved?
    I'm challenging that your definitions may be incorrect and that reasoning on a molecular level, suggests that increasing pressure, increases temperature, increases volume no?
     
  6. Feb 19, 2009 #5
    Oh I was forgetting that these are the Ideal Gas Laws, but I'm pretty sure having read the refined ones that they don't address these conflicts. However I may have to re-search...
     
  7. Feb 19, 2009 #6

    jtbell

    User Avatar

    Staff: Mentor

    The pressure depends not only on the kinetic energy of the molecules (actually it's more closely related to the momentum of the molecules), but also on the number of wall-molecule collisions per second.

    If the volume is bigger, each individual gas molecule has to travel further between collisions with the walls, so each molecule collides less often with the walls.
     
  8. Feb 19, 2009 #7

    rcgldr

    User Avatar
    Homework Helper

    From wiki: The relationship of kinetic energy, mass, and velocity is given by the formula Ek = 1⁄2 m v2. Accordingly, particles with one unit of mass moving at one unit of velocity have precisely the same kinetic energy—and precisely the same temperature—as those with four times the mass but half the velocity.

    http://en.wikipedia.org/wiki/Thermodynamic_temperature

    Also from wiki, pressure ... temperature .. kinetic energy
    Pressure = 1/3 ρ vrms2

    where ρ is the density of the gas.

    http://en.wikipedia.org/wiki/Kinetic_theory#Pressure
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook