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!

Adiabatic Compression

  1. Jan 16, 2012 #1
    Hi, I have a little problem understanding adiabatic compression.

    Let me start with the definition of adiabatic process from wikipedia, "In thermodynamics, an adiabatic process or an isocaloric process is a thermodynamic process in which the net heat transfer to or from the working fluid is zero."

    My problem is, when we compress a certain gas in a closed container, we inject our kinetic energy to decrease the volume of the container, so shouldn't this means there is a net heat change, or a change in the total energy of the system?

    or this kind of injection of energy does not categorize under heat transfer, Im quite confused.

    I give my greatest thanks in advance! :)
     
  2. jcsd
  3. Jan 16, 2012 #2

    Doc Al

    User Avatar

    Staff: Mentor

    When you do work on the gas, you definitely add energy. But that's not 'heat'. Heat is the flow of energy due to temperature difference.

    Read this: What is Heat?
     
  4. Jan 16, 2012 #3
    Thank you for the link, it helped me a lot!

    One more small thing, why is it that the rotation of a diatomic molecule along the atom-atom bond not counted as one of the degree of freedom?
     
  5. Jan 16, 2012 #4

    Ken G

    User Avatar
    Gold Member

    That's actually a quantum mechanical effect. The kinetic energy for something undergoing cyclical motion (vibration or rotation) is characterized by mx2w2, where x is the spatial size scale and w is the frequency. But here's where a big difference between vibrations and rotations appears-- for vibrations, x is a variable, and can be as large as it needs to be to get ~kT of energy into the mode in question. But for rotations, x is fixed by the size scale of the rotating object, the "lever arm" of the appropriate rotation. So when x is extremely small, as in the case you mention, it would require huge w to get kT of energy into that mode, of order w=(kT/m)1/2x-1. However, huge w, coupled with the quantum mechanical minimum action h, means you won't excite that mode, since here hw >> kT, because (kT/m)1/2x-1>> kT/h whenever x << h/(mkT)1/2. So we only exite modes like that when T is very high, and it generally isn't that high in the applications you have in mind.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Adiabatic Compression
  1. Adiabatic compression (Replies: 3)

Loading...