# Difference between revisions of "Pressure"

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− | '''Pressure''' (<math>p</math>) is the force per unit area applied on a surface, in a direction perpendicular to that surface, i.e. the scalar part of the [[stress]] tensor | + | '''Pressure''' (<math>p</math>) is the force per unit area applied on a surface, in a direction perpendicular to that surface, i.e. the scalar part of the [[stress]] tensor. |

In thermodynamics the pressure is given by | In thermodynamics the pressure is given by | ||

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[[Boltzmann constant]], <math>T</math> is the [[temperature]] and <math>Q (N,V,T)</math> | [[Boltzmann constant]], <math>T</math> is the [[temperature]] and <math>Q (N,V,T)</math> | ||

is the [[Canonical ensemble | canonical ensemble partition function]]. | is the [[Canonical ensemble | canonical ensemble partition function]]. | ||

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+ | ==Units== | ||

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+ | The SI units for pressure are Pascals (Pa), 1 Pa being 1N/m<math>^2</math>, or 1J/m<math>^3</math>. Other usual units are bars and millibars (mbar); 1mbar=100Pa=1hPa, 1 hectopascal. 1 bar is <math>10^5</math>Pa by definition. This is very close to the standard atmosphere (atm), approximately equal to typical air pressure at earth mean sea level: | ||

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+ | atm, standard atmosphere = 101325 Pa = 101.325 kPa = 1013.25 hPa = 1.01325 bar | ||

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==See also== | ==See also== | ||

*[[Pressure equation]] | *[[Pressure equation]] |

## Revision as of 10:24, 4 June 2008

**Pressure** () is the force per unit area applied on a surface, in a direction perpendicular to that surface, i.e. the scalar part of the stress tensor.
In thermodynamics the pressure is given by

where is the Helmholtz energy function, is the volume, is the Boltzmann constant, is the temperature and is the canonical ensemble partition function.

## Units

The SI units for pressure are Pascals (Pa), 1 Pa being 1N/m, or 1J/m. Other usual units are bars and millibars (mbar); 1mbar=100Pa=1hPa, 1 hectopascal. 1 bar is Pa by definition. This is very close to the standard atmosphere (atm), approximately equal to typical air pressure at earth mean sea level:

atm, standard atmosphere = 101325 Pa = 101.325 kPa = 1013.25 hPa = 1.01325 bar