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Do water molecules at the bottom of a swimming pool move faster and closer together?

  1. Jan 23, 2010 #1
    Since the water pressure at the bottom of the swimming pool is very high, are the water molecules there bouncing faster , or are they closer together?
  2. jcsd
  3. Jan 24, 2010 #2


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    Re: Do water molecules at the bottom of a swimming pool move faster and closer togeth

    As you know, water has very little compressibility.

    The incompressibility of water is down to the incompressibility of the individual atoms. For atoms, the only way to become bigger is for electrons to transit to higher orbitals, and the only way to become smaller is for electrons to drop down to lower orbitals. When all low orbitals are occupied the atom cannot compress further.

    The most extreme state in which compressed matter is still atoms is in extinguished stars. Less and less heat is generated in the core, and subsequently gravity can contract the extinguished star to smaller and smaller volume. The last thing opposing gravity is called 'electron degeneracy pressure'. Quantummechanically it's not possible for two electrons to be in the same quantum state, this is called the 'Pauli exclusion principle'. Electron degeneracy pressure keeps gravity from contracting the extinguished star further.

    If the star is so massive that electron degeneracy pressure is not enough then the gravitational contraction changes the composition of the extinguished star. It becomes a neutron star. In a neutron star further contraction is opposed by neutron degeneracy pressure.

    Getting back to water and a gas:
    Pressure exerted by a compressed gas is different from pressure exerted by a compressed fluid. In the case of a gas the pressure exerted on the walls of the vessel does arise as the sum of many collisions with the walls. The harder the collisions, or the bigger the number of collisions, the higher the pressure.

    I'm slightly guessing here, but I think that in the case of a fluid the molecules are hugging the walls of the vessel anyway. I think the fluid molecules are elastically vibrating in roughly one spot, rather than flying in straight lines (in between collisions) as gas molecules do.

    The velocity of the vibrating motion is correlated with temperature, and the water at the bottom of a pool isn't warmer. Also the number of collisions cannot be much higher than at the surface. It follows that the incompressiblity of water is mainly due to electron degeneracy pressure.

    On the bottom of our oceans the pressure is huge by human standards, but it's still only a tiny, tiny fraction of the total capacity of electron degenaracy pressure.
  4. Jan 24, 2010 #3
    Re: Do water molecules at the bottom of a swimming pool move faster and closer togeth

    Thanks , it certainly helps. This question suddenly arose out of my mind when I was tutoring my student on pressure in fluids and Archimedes principle, when my student suddenly took the microscopic view of atoms and molecules of a fluid.
  5. Jan 25, 2010 #4


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    Re: Do water molecules at the bottom of a swimming pool move faster and closer togeth

    I would have thought that the molecules in water are close but still free to move about. Perhaps the ratio of molecule size versus the space between molecules is large enough that even a small change in density corresponds to a significant decrease in space between molecules and a significant increase in the rate of collisions between molecules, and in turn the rate of collisions would explain the higher pressure without a higher temperature.
    Last edited: Jan 25, 2010
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