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Raghnar

- 41

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So Pressure is a Thermodynamical intensive quantity, thus statistically defined. That teaches us the Barometric law and that to pressure gradient correspond to a force that is consistently defined with the density of matter and the gravitational pull in an equilibrium state.

So you consider that definition in the description of the air and atmospheric pressure, you do your math and considering that the Air does not contribute substantially to the mass of the Earth (or even approximating it constant) you find the usual exponential form of the barometric law.

Microscopically you have the weight that pull down the gas, the gas compress and reach an equilibrium with the force (pushing to external layers) exerted by the gradient of pressure. The pressure is then given by the density and temperature of the gas volume as a result of this process.

In this picture you don't have the pressure "compressing" the gas, but the weight. The pressure is the results of other thermodynamic quantities.

Now let's consider another matter, for example liquid.

How does the pressure of the water sums up to this?

By the same principle the gravity pull the water to the center of the earth, and the same principle determine a different density and so on and a different pressure gradient needed.

But if we consider a body made only of omogeneus matter (e.g. liquid or gas), its gravitational field, for the Gauss Theorem, will be proportional to

*r*

([itex]F \sim M_{int}/ r^2 = \rho r^3/r^2 = \rho r[/itex]), so also the pressure in principle should not increase dramatically going to the center.

How it is that the pressure is always considered increasing, and in many environment A LOT?

And what about a complex stellar environment?

The gravitational pull determine a density of stuff that generate a pressure gradient that counterbalance until you reach equilibrium?

And in exotic systems like neutron stars, where matter density change for tens and tens orders of magnitude in few km?

And in exploding environment like core-collapsing supernovae?

How pressure, that is defined in the familiar Barometric Law, as a counter-balancing force, is often described as the active force the determine the great densities of the neutron star or the explosing force of a core-collapsing nova?

What is pressure exactly? If its correlated to the mean free path in gases how this connect with other environments such as liquid, solid, nuclear matter, quark-gluonic matter...etc...? What are the boundaries of the representation of this thermodynamic quantity ideated to explain gas behavior and used to explain a wide variety of matter in a wide variety of environments?

I hope to receive some insight regarding this and if you can point out some reference text will be appreciated.