Editing Pressure (section)

===Scalar nature===
In a static [[gas]], the gas as a whole does not appear to move. The individual molecules of the gas, however, are in constant [[Brownian motion|random motion]]. Because there are an extremely large number of molecules and because the motion of the individual molecules is random in every direction, no motion is detected. When the gas is at least partially confined (that is, not free to expand rapidly), the gas will exhibit a [[hydrostatic pressure]]. This confinement can be achieved with either a physical container, or in the gravitational well of a large mass, such as a planet, otherwise known as [[atmospheric pressure]].

In the case of planetary [[atmosphere]]s, the [[pressure-gradient force]] of the gas pushing outwards from higher pressure, lower altitudes to lower pressure, higher altitudes is balanced by the [[gravitational force]], preventing the gas from diffusing into [[outer space]] and maintaining [[hydrostatic equilibrium]].

In a physical container, the pressure of the gas originates from the molecules colliding with the walls of the container. The walls of the container can be anywhere inside the gas, and the force per unit area (the pressure) is the same. If the "container" is shrunk down to a very small point (becoming less true as the atomic scale is approached), the pressure will still have a single value at that point. Therefore, pressure is a scalar quantity, not a vector quantity. It has magnitude but no direction sense associated with it. Pressure force acts in all directions at a point inside a gas. At the surface of a gas, the pressure force acts perpendicular (at right angle) to the surface.<ref>{{Cite web |date=2023-03-28 |title=Gas Pressure Study Guide - Inspirit Learning Inc |url=https://www.inspiritvr.com/gas-pressure-study-guide/ |access-date=2024-02-11 |language=en-US}}</ref>

A closely related quantity is the [[stress (physics)|stress]] tensor ''σ'', which relates the vector force <math>\mathbf{F}</math> to the 
[[vector area]] <math>\mathbf{A}</math> via the linear relation <math>\mathbf{F} = \sigma\mathbf{A}</math>.

This [[tensor]] may be expressed as the sum of the [[viscous stress tensor]] minus the hydrostatic pressure. The negative of the stress tensor is sometimes called the pressure tensor, but in the following, the term "pressure" will refer only to the scalar pressure.<ref>{{Cite web |title=Thermal-FluidsPedia {{!}} Pressure (Thermodynamics) {{!}} Thermal-Fluids Central |url=https://www.thermalfluidscentral.org/encyclopedia/index.php/Pressure_(Thermodynamics) |access-date=2024-02-11 |website=www.thermalfluidscentral.org}}</ref>

According to the theory of [[general relativity]], pressure increases the strength of a gravitational field (see [[stress–energy tensor]]) and so adds to the mass-energy cause of [[gravity]]. This effect is unnoticeable at everyday pressures but is significant in [[neutron star]]s, although it has not been experimentally tested.<ref>{{cite journal|title=Einstein's gravity under pressure |doi=10.1007/s10509-009-0016-8 |volume=321 |issue=2 |journal=Astrophysics and Space Science |pages=151–156|arxiv=0705.0825 |bibcode=2009Ap&SS.321..151V |year=2009 |last1=Vishwakarma |first1=Ram Gopal |s2cid=218673952 }}</ref>