The negative ratio of a change in pressure applied to a block of some material to the resulting fractional change in volume of the block; symbol K, unit Pa (pascal); also called the modulus of compression. It is constant for a given material, eg for glass, K = 0·37 × 1011Pa. The higher the value of K, the more difficult is the material to compress. Compressibility is 1/K.
The bulk modulus (or incompressibility) K of a fluid or solid is the inverse of the compressibility:
where P is pressure and V is volume.
The bulk modulus thus measures the response in pressure due to a change in relative volume, essentially measuring the substance's resistance to uniform compression. Other moduli describe the material's response (strain) to other kinds of stress: the shear modulus describes the response to shear, and Young's modulus describes the response to linear strain.
Strictly speaking, the bulk modulus is a thermodynamic quantity, and it is necessary to specify how the temperature varies in order to specify a bulk modulus: constant-temperature (KT), constant-enthalpy (adiabatic KS), and other variations are possible.
The inverse of the bulk modulus gives a substance's compressibility.
For a gas, the adiabatic bulk modulus KS is approximately given by
where
κ is the adiabatic index, sometimes called γ.
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