# Compressibility

The **bulk modulus** *B* gives the change in volume of a solid substance as the pressure on it is changed,

The **compressibility** *K* or , is given by

## Isothermal compressibility[edit]

The **isothermal compressibility**, is given by

(Note: in Hansen and McDonald the isothermal compressibility is written as ).
where is the temperature, is the *particle number density* given by

where is the total number of particles in the system, i.e.

## Adiabatic compressibility[edit]

The **adiabatic compressibility**, is given by

where is the entropy.