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Currently I'm taking an introductory course on thermodynamics. I've got a problem with understanding what is the meaning of pressure of a solid body. The question arose when I looked at phase diagram in P, T coordinates.

When we're talking about pressure of gas or liquid, it can be defined thus: it is the force per unit area with which the molecules of the substance hit the surface. This force is due to the molecular motion.

But in the case of solids, molecules don't "move", they just oscillate about their fixed positions. Therefore, they don't hit (in usual meaning) a testing surface and we have problems with pressure's definition. How then the pressure can be defined?

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The operational macrosopic definition of pressure in a solid is the same as that for a gas : when the solid's wall is in mechanical equilibrium with its surroundings, that means the force exerted on the wall by the surroundings is exactly compensated by the pressure within the solid times the area of the wall.

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