Editing Bulk modulus (section)

==Definition==
The bulk modulus <math>K</math> (which is usually positive) can be formally defined by the equation
:<math>K=-V\frac{dP}{dV} ,</math>
where <math>P</math> is pressure, <math>V</math> is the initial volume of the substance, and <math>dP/dV</math> denotes the [[derivative]] of pressure with respect to volume.  Since the volume is inversely proportional to the density, it follows that 
:<math>K=\rho \frac{dP}{d\rho} ,</math>
where <math>\rho</math> is the initial [[density]] and <math>dP/d\rho</math> denotes the derivative of pressure with respect to density. The inverse of the bulk modulus gives a substance's [[compressibility]]. Generally the bulk modulus is defined at constant [[temperature]] as the isothermal bulk modulus, but can also be defined at constant [[entropy]] as the [[adiabatic]] bulk modulus.