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

Pressure and density of compressible fluids.

  1. May 19, 2014 #1

    Nathanael

    User Avatar
    Homework Helper

    If a fluid is compressible, will the density be directly proportional to the pressure?

    (I'm sure there's a limit where an increase in pressure stops producing a change in density, but I'm talking about for more "normal" (smaller) amounts of pressure.)
     
  2. jcsd
  3. May 19, 2014 #2
    Are you assuming constant temperature as well?
     
  4. May 19, 2014 #3

    Nathanael

    User Avatar
    Homework Helper

    Temperature would effect the energy, right?

    And pressure is essentially the density of energy, (Newton-Meters per Cubic Meter)

    So wouldn't a change in temperature only effect the pressure? And so it doesn't really need to be considered when considering the relationship between pressure and density, right?

    Or no?
     
  5. May 19, 2014 #4
    The word "proportional" has a certain exactness to it, and nothing is exact. The most one can say is that the density increases with increasing pressure. Over limited regions of pressure, the density change is approximately proportional to the pressure change.

    Chet
     
  6. May 19, 2014 #5
    Well, if temperature affects pressure and pressure in turn affects density then the conclusion is that temperature would also affect density.

    Or no?
     
  7. May 19, 2014 #6
    I think Nathanael, in his original post, was referring to the effect of pressure on density at constant temperature. At least that was my interpretation.

    Nathanael, is that what you meant?

    Chet
     
  8. May 20, 2014 #7

    Nathanael

    User Avatar
    Homework Helper

    Sorry, I wasn't thinking about my last post. I was under the unexamined impression that a change in pressure caused by temperature would affect the density (simply because it affects pressure) but now that I'm thinking about it, I realize that a change in pressure caused by temperature will have no effect on density (right?).

    In a sense I was talking about constant temperature, (in the sense that temperature was not a part of my picture) but honestly I didn't think about temperature.

    (I never learned or thought of temperature affecting pressure, even though it's a common-sense kind of idea.)

    My original question was just asking is if the increase in density was (approximately) linear, or if it curved upward, or curved downward, (or what I really mean is does it "bend" downward) or what?

    That answers the question. Thanks.
     
  9. May 20, 2014 #8

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    Temperature absolutely and directly affects the pressure and density. For example, many gases behave as ideal gases at most ordinary conditions and so very closely follow the ideal gas law, which can be expressed as

    $$p = \rho R T$$

    where ##p## is pressure, ##\rho## is density, ##R## is the specific gas constant, and ##T## is temperature.
     
  10. May 22, 2014 #9

    boneh3ad

    User Avatar
    Science Advisor
    Gold Member

    Liquids are generally nearly incompressible so usually this isn't a concern. You can likely find a similar equation of state for liquids like water but I don't know it off the top of my head and it won't be terribly useful.

    Real gases generally follow a similar pattern to ideal gases but with various corrections and variations on things like exponents depending in the gas in question and the phenomenon causing it to behave non-ideally.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Pressure and density of compressible fluids.
  1. Fluid Density (Replies: 3)

  2. Pressure of fluid (Replies: 8)

Loading...