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Pressure (p) 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. The SI units for pressure are Pascals (Pa). In thermodynamics the pressure is given by

p = - \left.\frac{\partial A}{\partial V} \right\vert_{T,N} = k_BT \left.\frac{\partial \ln Q}{\partial V} \right\vert_{T,N}

where A is the Helmholtz energy function, V is the volume, k_B is the Boltzmann constant, T is the temperature and Q (N,V,T) is the canonical ensemble partition function.

See also