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Pressure = force per unit area (Definition!)

Force

Pressure = Area

Gas samples exert a force pushing against any surface they contact, due to the impact of molecules striking the surface. Per unit area, it’s the same for all surfaces in or walls containing the same gas sample, unless the gas is flowing somewhere collectively.

That pressure increases with molecular velocity and mass, and with the number of molecules per unit volume (number density).

A sheet suspended within a gas sample gets same force on both sides, so it does not move.

Average molecular velocity increases with temperature but decreases with molecular mass, in such a way that the mass effect cancels.

This leaves pressure nearly proportional to temperature and to number density, but independent of the molecular mass:

P = R(T)(n/V), where R is the gas constant, n = # moles = N/NA

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