Here we consider changes in volume caused by a force exerted
uniformly over the entire exposed surface of the solid.
Definitions:
Volume Stress ( P ) is the ratio of the
magnitude of the
normal force F to the area A .
Volume Strain is the fractional change in volume,
V/V .
Here, V is the initial volume and V is the change in the
volume.
Bulk Modulus ( B ) is the ratio of the volume stress
to the
volume strain:
B = = - = - .
(4)
The negative sign ensures that the Bulk Modulus is always a
positive number: forces
pressing on a solid from the outside always cause the volume to
decrease (compression), so that the fractional change in volume is
negative. B has S.I. units of N/m ^{2}.