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

Homework Help: Ideal gas expansion/compression

  1. Nov 3, 2011 #1
    this may be a stupid question but i have to ask as my book doesn't specifically say, it'll help me figure out a problem in my homework. So, here it goes...

    When an ideal gas expands or is compressed, does the number of moles change or does it remain the same? if it remains the same, does that mean only the volume changes. For instance, in the case of compression of an ideal monatomic gas. Gas is compressed and the volume decreases by a factor of .5, pressure is increased by a factor of 4.5. Does the # of moles increase, decrease, or neither?

    Thank you in advance. Any and all help is much appreciated!
  2. jcsd
  3. Nov 4, 2011 #2
    In order to solve these problems, you need data about any three of the four variables (P, V, n, T)

    Your example only specifies two (P, V). Without a third specification, it's impossible to know if one or both mass and temperature have changed.

    I'd like to say "typically" ideal gas problems don't have mass flows associated with them; but that's not really true. There are plenty of open-system ideal gas problems. So proceed with the assumption that mass is constant at your own peril.

    You really do need more info. Ideal gases can be compressed and expanded isobarically (const. P), isothermally (const. T), isochorically (const. volume), or in a closed system (const. mass). The problem really should state it--read carefully--or it's incomplete and not well posed. Only mathematicians can solve a two-unknowns problem with one equation. ;-)

  4. Nov 5, 2011 #3


    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper

    If not specified otherwise, assume the number of moles of gas does not change. For the moles to change, either some more gas has to be added to the initial gas present, or some of the initial gas escapes.

    When the number of moles is constant, the other quantities (P, V, or T) can still change.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook