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Why do fluids expand with heat? explaining covection currents

  1. May 23, 2009 #1
    I'm guessing it has something to do with the fluid gaining thermal energy from the heat, the heat makes the molecules repel each other further so the fluids volume expands?

    if not why do fluids expand with heat?

    -
    the rest is mandatory, after expanding the volume increases, decreasing the mass density (as density is inversely proportional to volume)
    and thus rises from the rule of boyent fluids, the less dense fluid has a lower pressure, so rises above the stronger pressure fluid to maintain equilibrium
    CONVECTION XD
     
  2. jcsd
  3. May 23, 2009 #2

    mathman

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    The molecules move around faster when the fluid is heated. Therefore they need more room.
     
  4. May 23, 2009 #3
    Water is densest at 4 degrees C, about 4 deg C above freezing.
     
  5. May 23, 2009 #4
    i don't think that's very good explanation, does it cover everything? why would the heat energy vibrating the molecules faster have much to do with the expanding of the fluid?

    sorry i just need a better explanation instead of "Therefore they need more room." it has to do with something more molecular instead of just them moving faster, with it moving faster ther'd be more collisions not a expansion
     
  6. May 23, 2009 #5

    russ_watters

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    This isn't just about fluids: solids behave the same way. Solid, liquid, gas - the idea of thermal expansion is the same: whatever force holds them together, the kinetic energy of the individual molecules opposes it and tries to separate the molecules. The more kinetic energy, the more they get separated. That's all there is to it.

    For a macroscopic visual representation, consider a spring-mass system of any type. Set it in motion with a push - it has a certain wavelength/displacement based on the kinetic energy you just gave it. Then set it in motion with a harder push: now it has a bigger displacement. For a lot of behaviors, many solids (metals, in particular), can be very accurately modeled as collections of many spring-mass systems stuck together.
     
  7. May 23, 2009 #6

    russ_watters

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    That's true, but it is a special case that not relevant to this thread. Please don't confuse the OP with special exceptions.
     
  8. May 24, 2009 #7
    That would explain expansion in a solid, i want to know how gases expand :\
    would it have anything to do with the mean path between gas molecules?
     
  9. May 24, 2009 #8
    It might help if you think of it in terms of pressure.A gas does not necessarily expand when heated for example if it was contained in a cylinder with a fixed piston.If a gas is heated the molecules gain energy move faster and the pressure increases.If the piston were free to move the increased pressure would result in the gas expanding.The same result would be obtained if there was localised heating in a free gas ,the hotter molecules would exert a higher pressure and force and push against any surrounding cooler molecules.And yes,the mean free path increases.
     
  10. May 24, 2009 #9
    Thank you very much :)
     
  11. May 24, 2009 #10

    russ_watters

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    Same principle: the molecules bang out their own personal space with their kinetic energy. The faster they are moving, the harder they push.
     
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