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Homework Help: Quasistatic compression

  1. Feb 13, 2012 #1
    Have already received excellent help in understanding this but might need a bit more.

    Suppose we have a gas inside a cylinder with a piston in it. Now my teacher said that to compress the gas quasistatically we would need to press in the piston with a speed, that is slow compared to the speed of sound, which makes sense as this is the speed at which pressure differences travel.
    Now my question is: Don't we also need to press in the piston with a force only infinitely larger than the force from the internal pressure? I mean if we have a gas in a cylinder with a piston of area 0.01m^3 and pressure of 100Pa such that the internal force on the piston initially is 1N. Then we couldn't really achieve a quasistatic compression by exerting 100N on the piston could we? But is this maybe accounted for in saying that the speed should be sufficiently slow?
    Overall I think this way of thinking of a proces is quite weird? Do they occur often in nature? Because it would seem that to compress something quasistatically you would the whole time have to keep an eye on not exerting a too large force..
  2. jcsd
  3. Feb 13, 2012 #2
    I'm not really sure what you are asking... the reason for quasi static compression is to minimise the increase in heat which goes along with compressing a gas - and an increase in heat means an increase in entropy and will be an irreversible process. So by changing the pressure infinitely slowly then the process may remain reversible - it you to carry it out over and over again.

    I'm not sure if quasi static compression occurs in nature - I'd be inclined not to agree because of the entropy principle of the universe but like I said it's more for the reversible nature of the process.

    Hope this helps
  4. Feb 13, 2012 #3
    But suppose we have a cylinder with a pressure of perhaps 1Pa and an area of 0.01m^2. The force is then 0.01N. Now say we slam that piston 100N. That compression wouldn't be quasistatic right? I mean, if we think of quastistatic doesn't it mean, that we start at equilibrium, then press down just infinitisimally more to reach a new equilibrium after dx and so on..
  5. Feb 13, 2012 #4

    Are you saying that isn't it difficult to compress quasi statically then yes... it is virtually impossible because to compress quasistatically as you have to compress infinitely slowly. How it is generally done is compress slowly, allow the gas to cool and return to equilibrium and then compress a tiny bit more..... and repeat
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