Difference between pressure and heat on an atomic level?

[Gen1GT]

Whatis the difference between heat and pressure on an atomic level? It is my understanding that heat is the manifestation of the movement of atoms. It is also my understanding that pressure is at least partially a manifestation of atomic movement. You can increase both heat and pressure of a gas, for example, by reducing volume. You can then remove the heat, but the pressure will remain. Why
does pressure remain? Is it from atoms hitting the walls of the container?

Edited April 18, 2013 by Gen1GT

[Enthalpy]

If you remove heat from a gas, the pressure will decrease too.

 

In a monoatomic gas like argon, under reasonable conditions, the internal heat is just the kinetic energy of the molecules. Heat given to or taken from this gas at constant volume changes just this internal energy; but at constant pressure for instance, or under different conditions, the internal energy would share the change with other forms of energy.

 

Polyatomic gases also store internal energy in other movements ot state changes. Take nitrogen: the atoms can rotate around their common center or mass, which is kinetic energy, but without a global movement of the molecule's center of mass. Or bromine: the bond is elastic enough to elongate or contract even at room temperature due to shocks against among molecules, so the bromine atoms not only move with the molecule's center of mass, they also rotate around it like nitrogen does, and they also vibrate around it, which adds more kinetic energy and deformation energy. Somewhat stiffer molecules like carbon dioxide vibrate a bit at room temperature and more at heat, while light ones like hydrogen or stiff ones like nitrogen vibrate little even at heat.

 

Pressure is the shocks of molecules against the walls, yes. Temperature is the amount of energy in each movement (or state) capable of storing some energy, among others in each translation of the molecule's center of mass. So a warmer temperature means that the molecules translate faster and hit the walls with more energy and moment, pushing them stronger: more pressure.

 

But at the same pressure, more molecules hitting the surface per time unit also push the surface stronger, so temperature isn't the sole cause of a gas' pressure; density is one other. Or written differently, the volume of a unit amount of gas, for instance a "mole" of gas - but that's nothing more than the reciprocal of the density, moles per volume unit versus volume per mole.

 

Gasses fortunately happen to give a very simple relation between temperature, volume and pressure, under reasonable conditions: just P=R*T/V where R is measured as 8.3145J/mol/K and V is the volume of a mole, a mole being 6.022e23 molecules - chosen as the number of carbon atoms that weigh 12g.