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Does decreasing pressure of fluid in a fixed volume lower temperature?

  1. Jun 18, 2013 #1
    Sorry if the formulation is imprecise, I'm a biology major trying to study some physics. The question is:
    So far, my reasoning is:
    I make an assumption in Step (2) that decreasing the pressure will decrease the temperature. But I'm not sure what the justification is. I only know of the ideal gas law, and I'm not sure if the behavior is the same for incompressible fluids. Maybe incompressibility means fluid is similar to ideal gas behavior, or maybe it holds in general and the incompressibility assumption can be removed? Or maybe it's dependent on the fluid?

    Thanks for any clarification!
  2. jcsd
  3. Jun 18, 2013 #2


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    The bottom line is it's a bad question. The answer depends on what the fluid is (or more mathematically, the equation of state), so it will be have differently I it is a gas or liquid. Further, it would depend on the flow properties of the fluid as well since gases behave differently at different pressures and velocities.

    The ideal gas law is the equation of state for an ideal gas and it only really applies to compressible gas flows.
  4. Jun 26, 2013 #3
    My reasoning is completely different from yours.

    The answer cannot be D because A, B, and C cover all possible situations regarding temperature, therefore 1 of them must be true.

    We are told nothing about the nature of the fluid or the reason it is flowing at different rates through the pipe, Only that it flows at different rates. Given that the authors of the question felt this was the only information necessary I believe the answer they are after is B. Increased velocity of the fluid results in increased friction between the fluid and the pipe resulting in higher temperature.
  5. Jun 27, 2013 #4


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    That isn't necessarily the case even then, though. Fluids are not like solids. Viscous dissipation of kinetic energy into thermal energy has a very weak effect on the temperature of the fluid except at high speeds compared to the effects of conduction and convection from the wall. In other words, without any information on the heat transfer boundary condition at the wall, it is impossible to make the conclusion you made.
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