Pressure in liquids

[Delta²]

What causes the pressure in Liquids(or gases)? Is it deep down of electromagnetic nature? By that i dont mean that there are macroscopic electromagnetic fields from a liquid under pressure but at the microscopic level the pressure is caused by electromagnetic force between the atoms/molecules of the liquid and of the surrounding media.

[Jack the Stri]

Van der Waals forces (http://en.wikipedia.org/wiki/Van_der_Waals_force) probably account for the majority of the repulsion (& thus outward pressure) when molecules are forced together.

[ogion]

Your question is quite interesting.

One direction to proceed to understand what systems have zero pressure?

Another point to note - apropos Jack's comment - is that most atoms in a gas/liquid are neutral. So it cant really be electromagnetic in origin. If you have a gas of neutrinos - which dont have any electric charge at all - they will still have pressure.

Hope this spurs you on a bit.