Liquid under pressure

  1. When a liquid is put under pressure, it doesn't get compressed very much.
    So where is the extra force that the liquid molecules push back with,
    coming from?
    Is it electron clouds getting squished together and pushing back with
    electrostatic repulsion - or Pauli exclusion force - or what?
    thanks
    Laura
     
  2. jcsd
  3. The electric force is the dominant force in a molecular scale, so it's responsible for the repulsion between molecules and the liquid's resistance to compression.
     
  4. Andrew Mason

    Andrew Mason 6,856
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    The molecules in most liquids and solids are packed together pretty tightly but not perfectly tightly because of molecular shape. Compression can change the physical shape of the molecules (which can lead to chemical changes depending on the molecule) and reduce that space. But after the intermolecular space is gone, you only have the intra-atomic space to reduce. The coulomb repulsion within the atom (ie between the nuclei and the electron clouds) provides a huge resistance to further compression.

    AM
     
  5. would a theoretically perfectly rigid object be able to experience a pressure, since there is no deformation?
     
  6. Andrew Mason

    Andrew Mason 6,856
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    No. Besides, there is no theoretically perfectly rigid object made out of matter. A rigid object must exert a force that is equal and opposite to the applied force. In order to do that, the molecules in the rigid body that are in contact with the object applying the force must be pushing back on the next layer of molecules beneath them in the rigid object with the same force/unit area as the object that is applying force. This goes on down through the rigid body. So every part of the rigid body must feel the applied force and push back with equal and opposite force per unit area (ie. pressure).

    AM
     
  7. A liquid under pressure would be exerting pressure from the kinetic energy of the molecules, the same as a gas does. But, a liquid is nearly
    incompressible. The pressure from the kinetic energy of molecules wouldn't be increasing when you put pressure on the liquid.

    So, it looks like the kinetic-energy pressure from the liquid is a
    small part of its overall pressure - most of the pressure in the liquid
    being from electrostatic repulsion between the molecules, or something like that. Why are molecules "hard" - resisting being jammed into each other? What happens when you try to push one electron cloud into another? I suppose maybe new quantum states would arise - high energy
    states?

    Laura
     
  8. Hootenanny

    Hootenanny 9,679
    Staff Emeritus
    Science Advisor
    Gold Member

    What happens when you try to put the south poles from two bar magnets together? If you want further understanding of 'how' this happens, your going to have to start reading up on QED.
     
  9. Chronos

    Chronos 9,976
    Science Advisor
    Gold Member

    If you put enough pressure on any substance it will undergo a phase transition. This can get pretty complicated and the resulting states are not easily predicted. For example, you can get exotic states like metallic hydrogen and deuterium - which are believed to be significant components of Saturn's core. re:

    http://www.sciencemag.org/cgi/content/summary/281/5380/1135
    Under Pressure, Deuterium Gets Into Quite a State

    At extreme pressures, fusion occurs in lighter elements [up to iron].
     
  10. If it were just a matter of magnetic attraction of the spins, you'd think
    the electron clouds would attract if oriented right.

    Laura
     
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