Pressure on Liquids: Where Does the Force Come From?

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    Liquid Pressure
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Discussion Overview

The discussion centers on the nature of pressure in liquids, specifically exploring the sources of the force that liquids exert when under pressure. Participants examine the roles of molecular interactions, electrostatic forces, and the implications of compression on molecular and atomic structures.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • Laura questions the source of the force that liquids exert under pressure, suggesting it may be due to electron clouds and their interactions.
  • One participant asserts that the electric force is the dominant force at the molecular scale, responsible for the repulsion between molecules and their resistance to compression.
  • Another participant elaborates that while liquids and solids are tightly packed, compression alters molecular shapes and leads to significant resistance due to coulomb repulsion within atoms.
  • There is a query about whether a theoretically perfectly rigid object could experience pressure, with a response indicating that such an object cannot exist in reality, as all materials must exert equal and opposite forces to applied pressure.
  • A participant notes that while liquids exert pressure from kinetic energy, they are nearly incompressible, suggesting that most pressure arises from electrostatic repulsion rather than kinetic energy.
  • Laura raises further questions about the nature of molecular hardness and the consequences of pushing electron clouds together, speculating on the emergence of new quantum states.
  • Another participant mentions that applying sufficient pressure can lead to phase transitions in substances, which can result in complex and unpredictable states.
  • Laura draws an analogy with bar magnets to illustrate the resistance of molecules to compression and suggests that understanding these interactions may require knowledge of quantum electrodynamics (QED).

Areas of Agreement / Disagreement

Participants express differing views on the sources of pressure in liquids, with some emphasizing electrostatic repulsion and others discussing kinetic energy. The discussion remains unresolved regarding the exact mechanisms and implications of these forces.

Contextual Notes

The discussion includes assumptions about molecular behavior under pressure and the nature of rigidity in materials, which are not fully explored or defined. The implications of phase transitions and the complexity of resulting states are acknowledged but not resolved.

lark
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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
 
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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.
 
lark said:
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
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
 
would a theoretically perfectly rigid object be able to experience a pressure, since there is no deformation?
 
jasc15 said:
would a theoretically perfectly rigid object be able to experience a pressure, since there is no deformation?
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
 
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
 
lark said:
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?
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.
 
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].
 
Hootenanny said:
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.
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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