1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

How was the van der Waals equation derived?

  1. Jun 22, 2015 #1
    the equation (P+a(n/V)^2)(V-nb)=nRT was derived in this manner:

    The pressure of a real gas is affected by intermolecular forces and so the a(n/V)^2 term must be added to the measured pressure to obtain the ideal pressure where Pmeasured+a(n/V)^2=Pideal

    On the other hand when they explained the volume, they stated that the molecule had a finite size so we had to subtract the measured volume with the nb term to get the actual volume of the gas. So essentially Videal-nb=Vreal

    So this seems pretty weird to me.. For the P+a(n/V)^2 term we are substituting values to get Pideal while for the V-nb term we are substituting values to get Vreal

    Is there a reason for this?
     
  2. jcsd
  3. Jun 23, 2015 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Yes. It is often useful to work in terms of idealized situation when you want to know the physics more qualitatively but when you do experiments you need to be able to anticipate the real-world values. So it is unclear what your objection is?
     
  4. Jun 26, 2015 #3
    I'm not sure why the (P+a(n/V)^2) term represents the ideal pressure while the (V-nb) term represents the real volume, The equation is essentially PidealVreal=nRT which seems weird to me. Shouldn't both terms represent the ideal volume?
     
  5. Jun 26, 2015 #4

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Is the "real volume" not a modification of another variable called "ideal volume" already?
     
  6. Jun 27, 2015 #5
    Yes it is a modification of the ideal volume. Since we are trying to equate the van der Waals equation to the ideal gas equation (PV=nRT), shouldn't the V-nb in the equation just be V? So we have (Preal+a(n/V)^2)Videal=PidealVideal=nRT?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: How was the van der Waals equation derived?
Loading...