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Bulk modulus
Intermediate
Resistance to uniform compression.
B = −VΔP/ΔVB = −VΔP/ΔV measures resistance to uniform compression; its inverse is compressibility. Water 2.2 GPa; gases much lower.
Pressure of 100 atm compresses water by 0.45%.
Related to speed of sound.
In more detail
The bulk modulus B = −V dP/dV describes how much a material resists uniform compression. Liquids and solids have high bulk moduli, while gases have much smaller values.
Real-world uses
- Sound speed in water depends on its bulk modulus.
- Hydraulic systems use the bulk modulus of oil to model stiffness.
- Deep-sea submersibles must withstand high pressure, and the bulk modulus of the materials tells engineers how much they compress.
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Frequently asked questions
Why is water hard to compress?
Its molecules are close together, so a large pressure change produces only a small volume change.
Is the bulk modulus of a gas constant?
No. It depends on pressure, and for an ideal gas it equals the pressure in isothermal conditions.
Why do liquids have large bulk moduli?
Liquids are almost incompressible because their molecules are already close together, so pressure changes their volume only slightly.
Related topics in Fluids & Elasticity
Strain · Hooke's law for solids · Young's modulus · Shear modulus · Stress–strain curve · Poisson's ratio