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Force due to pressure gradient

What causes the force on gases/liquids when there is a pressure gradient/why do gases always go from a high pressure to a low pressure?
I used to believe that it the gas diffusing (moving out randomly in all directions to fill a volume evenly), but that didn't explain the more obvious situations such as explosive decompression. I have searched the internet but cannot find any good explanations.
If any more information/clarification is needed to answer, please just ask. Thank you!

chainfire Mon 30/06/08 01:31

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gen2
Mon 30/06/08
08:39

Pressure is the force that drives the molecules outwards. The pressure difference was created by the input of energy to the system. In the case of an explosion, that was chemical energy. In other situations it could be kinetic energy such as supplied through a pump (electrical energy).

The energy that was required to create the pressure difference will be stored as potential energy and this will be converted to kinetic energy when the system pressure equalises.

If you are asking what happens on a molecular/atomic level, then there is a difference depending upon whether the material can be compressed or not and also what medium it is being released into.

jake-the-peg
Mon 30/06/08
09:20

And underneath all of that making it happen is the second law of thermodynamics.

Possibly the most important scientific law there is

mibn2cweus
Wed 02/07/08
05:59

There are four attributes of gases that seek equilibrium; temperature, pressure, volume and density.
Changing one of these parameters inversely alters the combined product of the other three proportionately even when one or two of the other variables (such as the volume of a containment) is fixed.

animations with explanations

It is the average velocity of the individual molecules/atoms in proportion to their density (number/volume) that determines pressure. When two volumes of gas come together and mix they quickly move toward equilibrium because of the high velocities of the individual particles and the subsequent rate of the collisions whereby energy is distributed between them.

Observe the action of the balls of Newton's cradle when a ball at one end is pulled back and provided the potential energy of gravity. When the ball is released it accelerates towards the next ball in line. Upon striking the second ball the second ball almost instantaneously passes it motion on to the third and so forth until the last ball is struck. The last ball is free to travel some distance until its energy is paid back by its displacement in gravity. To relate this to the interaction of molecules imagine the balls are slightly separated when at rest. The interior balls do not move very far to transfer their energy.

The point I am trying to illustrate here is that the individual molecules of gas are moving rapidly and when the volume of gas is no longer contained you then witness the velocity of molecules that are no longer confined undergo a rapid expansion.

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