bar

Bar (bar mark) is a unit of pressure. The bar is not an SI unit, but can be used as a decimal multiple of the SI unit (the pascal) (similar to, for example, the ton or litre). The bar is still used for its clarity, as 1 bar is approximately the pressure of the atmosphere.

Pressure is a physical quantity expressing the ratio of the force {\displaystyle F} (the so-called pressure force) acting perpendicularly, uniformly and continuously on a surface and the contents of this surface {\displaystyle S}.{\displaystyle p={\frac {F}{S}}}

The unit of pressure is pascal (Pa), that is, newtons per square meter - N.m−2. Pressure does not only act at the point of application of the force, but is transmitted through the volume of the body. It belongs to the basic thermodynamic quantities.

If the force vector acts only perpendicularly, we speak of "simple", "pure" or "uniform" pressure, if the force is not evenly distributed over the surface, then we speak of "medium" or "average" pressure, if the force acts at a point we speak of "local" pressure, i.e. j. by the pressure acting at the point of the considered surface we understand the differential proportion

{\displaystyle p={\frac {\mathrm {d} F}{\mathrm {d} S}}}.

By generalizing the definition of pressure (for forces acting in any direction on a general surface), we can write the equation:

{\displaystyle \mathrm {d} \mathbf {F} _{s}=p\mathrm {d} \mathbf {S} },where {\displaystyle \mathrm {d} \mathbf {F} _{s}} is the component of the force vector perpendicular to the surface element {\displaystyle \mathrm {d} \mathbf {S} } on which it acts, while the direction of the element vector {\displaystyle \mathrm {d} \mathbf {S} } has the direction of the normal to this face.

The physical dimension of a quantity is

{\displaystyle (p)=L^{-1}MT^{-2}}.

If the material is flexible, or compressible, pressure causes deformation. When a force is applied to a solid body, a distinction is made between tension and pressure. The tensile force will cause the body to expand, the compressive force will cause the body to be compressed. Tensile and compressive forces differ only in the direction of force action (opposite force vector).

Pressure is a physical quantity expressing the ratio of the force {\displaystyle F} (the so-called pressure force) acting perpendicularly, uniformly and continuously on a surface and the contents of this surface {\displaystyle S}.{\displaystyle p={\frac {F}{S}}}

The unit of pressure is pascal (Pa), that is, newtons per square meter - N.m−2. Pressure does not only act at the point of application of the force, but is transmitted through the volume of the body. It belongs to the basic thermodynamic quantities.

If the force vector acts only perpendicularly, we speak of "simple", "pure" or "uniform" pressure, if the force is not evenly distributed over the surface, then we speak of "medium" or "average" pressure, if the force acts at a point we speak of "local" pressure, i.e. j. by the pressure acting at the point of the considered surface we understand the differential proportion

{\displaystyle p={\frac {\mathrm {d} F}{\mathrm {d} S}}}.

By generalizing the definition of pressure (for forces acting in any direction on a general surface), we can write the equation:

{\displaystyle \mathrm {d} \mathbf {F} _{s}=p\mathrm {d} \mathbf {S} },

where {\displaystyle \mathrm {d} \mathbf {F} _{s}} is the component of the force vector perpendicular to the surface element {\displaystyle \mathrm {d} \mathbf {S} } on which it acts, while the direction of the element vector {\displaystyle \mathrm {d} \mathbf {S} } has the direction of the normal to this face.

The physical dimension of a quantity is

{\displaystyle (p)=L^{-1}MT^{-2}}.

If the material is flexible, or compressible, pressure causes deformation. When a force is applied to a solid body, a distinction is made between tension and pressure. The tensile force will cause the body to expand, the compressive force will cause the body to be compressed. Tensile and compressive forces differ only in the direction of force action (opposite force vector).

 

Pressure is a physical quantity expressing the ratio of the force {\displaystyle F} (the so-called pressure force) acting perpendicularly, uniformly and continuously on a surface and the contents of this surface {\displaystyle S}.

{\displaystyle p={\frac {F}{S}}}

The unit of pressure is pascal (Pa), that is, newtons per square meter - N.m−2. Pressure does not only act at the point of application of the force, but is transmitted through the volume of the body. It belongs to the basic thermodynamic quantities.

If the force vector acts only perpendicularly, we speak of "simple", "pure" or "uniform" pressure, if the force is not evenly distributed over the surface, then we speak of "medium" or "average" pressure, if the force acts at a point we speak of "local" pressure, i.e. j. by the pressure acting at the point of the considered surface we understand the differential proportion

{\displaystyle p={\frac {\mathrm {d} F}{\mathrm {d} S}}}.

By generalizing the definition of pressure (for forces acting in any direction on a general surface), we can write the equation:

{\displaystyle \mathrm {d} \mathbf {F} _{s}=p\mathrm {d} \mathbf {S} },

where {\displaystyle \mathrm {d} \mathbf {F} _{s}} is the component of the force vector perpendicular to the surface element {\displaystyle \mathrm {d} \mathbf {S} } on which it acts, while the direction of the element vector {\displaystyle \mathrm {d} \mathbf {S} } has the direction of the normal to this face.

The physical dimension of a quantity is

{\displaystyle (p)=L^{-1}MT^{-2}}.If the material is flexible, or compressible, pressure causes deformation. When a force is applied to a solid body, a distinction is made between tension and pressure. The tensile force will cause the body to expand, the compressive force will cause the body to be compressed. Tensile and compressive forces differ only in the direction of force action (opposite force vector). 

Kinetic pressure
The pressure in gases (for example, the pressure stretching a bicycle tire) is of a different nature than the pressure exerted by, for example, a brick lying on the ground. It is caused by the collisions of individual gas molecules, which move through space and at the same time collide with the walls of the container (and sometimes also with other molecules). That's why we sometimes talk about the so-called kinetic pressure - this highlights the fact that the main reason for the presence of pressure is actually the motion of the molecules. The movement of gas molecules is dealt with by the so-called kinetic theory of gases, one of its important results is the so-called equation of state for an ideal gas.

Hydrostatic pressure
see main article: Hydrostatic pressure
Hydrostatic pressure in gases and liquids is determined by the mass of the upper layers, which exert a force on the layers below them in the gravitational field. This force acting on the surface is called hydrostatic pressure. It is calculated using the relation
​

{\displaystyle p=\varrho .g.h}

where

• {\displaystyle \varrho } je density of liquid or gas
• {\displaystyle g\,} is the gravitational acceleration (on Earth 9.81 m.s−2)
• {\displaystyle h\,} is the depth (height of the liquid or gas column)

Atmospheric pressure

Atmospheric pressure is a special case of hydrostatic pressure exerted by the weight of the air making up the atmosphere. Under the influence of weather, atmospheric pressure fluctuates slightly, so the so-called standard atmospheric pressure or normal pressure has been established with a value of 101,325 Pa and is approximately equal to the typical value of pressure at sea level. Atmospheric pressure decreases with altitude, as the column of air above the measurement location shrinks.

Overpressure and underpressure
It is a relative expression of pressure, usually as the pressure in some closed space against the pressure in the surrounding space (often atmospheric). Overpressure means higher than the reference pressure, underpressure lower than the reference pressure.