# Difference between revisions of "Pressure"

## Description

Pressure [Pa = J·m-3] is the concentration of the force at the point of action. More generally, pressure is the force times concentration at the interphase of interaction.

In addition to mechanical pressure, hydrostatic pressure, barometric pressure, gas pressure (oxygen pressure), isomorphic pressures are distinguished as osmotic pressure, diffusion pressure, reaction pressure, and even electric pressure. Gas pressure is defined in the fundamental Gas law as p = C·kT (the product of particle concentration [x·m-3] times kT [J·x-1]) or p = c·RT (amount of substance concentration [mol·m-3] times RT [J·mol-1]), where k is the Boltzmann constant, R is the gas constant, T is the absolute temperature, and the unit of pressure is [Pa] = [J·m-3] = [N·m-2].

In ergodynamics, the pressure in a transformation, ΔtrΠ, is the product of free activity times force, ΔtrΠ = αtr·ΔtrF [mol·m-3 · J·mol-1 = J·m-3 = Pa].

In the classical physicochemical literature, there is confusion between the terms force and pressure: "This force is called the pressure of the gas" by Maxwell (1867); "This pressure is osmotic pressure. .. Osmotic forces are in fact .." by van't Hoff 1901; "Pressure-forces" by Einstein (1905); presentation of Fick's law of diffusion (which represents a flux-pressure relationship) as a flux-force relationship by Prigogine (1967).

Abbreviation: P, p, Π [Pa]

```Comunicated by Erich Gnaiger 2018-09-16 (last update 2020-02-13)
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• The prevailing hypothesis of light at the time was that of Descartes. He believed that light was a 'pressure' transmitted through the transparent medium of the ether. Sight, he claimed, was due to this pressure impinging upon the optic nerve. - (White 1997: p 58-59)
• In both the "Hypothesis" of 1675 and the student notebook of 1661-65, Newton tended to attribute gravity to the pressure of a descending aetherial shower. - (Dobbs 1975: p 210)