OK. So do we quantify this separately, e.g. as "liquid pressure"?
Well I would say so. If you want to know the total force acting on something, you add up all the different forces (and their vector directions). I would think pressure little different.
Not sure what you mean by force. Maybe the lack of attraction between the particles?
What I mean is, for there to be pressure on the walls of the container, there has to be a force that pushes against them. What is the force in this case? We can say that it is collisions between the moving gas particles and the container walls, but this begs the question - what is causing gas molecules to collide against the walls? Why do gasses expand outward?
Maybe it helps to consider that for an ideal gas, the energy does not depend on the volume. It only depends on the temperature. When you take a certain volume of ideal gas and cram it into a smaller volume at the same temperature, the internal energy does not change
. Yet the pressure (average force) goes up. Why? This would almost seem to be contradictory - if the energy of the gas doesn't increase as the volume decrease, how can the pressure increase? What factor is missing that causes this "force"?
(Note that the energy of a REAL gas IS dependent on volume. If you compress a real gas, the internal energy DOES change. WHY?)