Bulk Modulus vs Young Modulus
All the substances / materials are made up of atoms. The type of atoms, number and their connection vary from material to material, and which defines each of their unique characteristics. No matter how much atoms get together to form a certain substance, atoms do not tend to arrange in a compact way where there is no space between them. The attraction and repulsion forces between the atoms, always keep a certain space between them. Therefore, in any substance, not matter how compact they are, there is enough and more space between the atoms. We divide substances mainly into three classes as solid, liquid and gas. Their atomic arrangements are different. Solids have a highly compact atomic arrangement whereas, in gas, atoms are dispersed in a larger volume with very low interactions. In liquids, an intermediate stage between the solids and gas can be seen.
Most substances reduce its volume when exposed to a uniform, externally applied pressure. However, this decrease is not a linear curve, rather, as the pressure increases, the volume decreases exponentially. Bulk modulus refers to the reciprocal of compressibility or, in other words, it is a measure of resistance to compressibility. Moreover, it describes the elastic properties of a substance.
Bulk modulus can be defined as the pressure increase needed to decrease the volume by a factor of 1/e. When a substance is compressed, it will be somewhat resistant to the compression depending on the atomic arrangement it has. Bulk modulus indicates this resistance of a substance upon uniform compression. It is measured in Pascal/ bar or any other pressure unit. Bulk modulus gives an idea of change in volume of a solid substance as the pressure on it is changed. As for the solid, bulk modulus is a property of fluids too, it indicates the compressibility of a fluid. Fairly compressible fluids have a low bulk modulus and slightly compressible fluids have a high bulk modulus. Following is the equation to calculate the bulk modulus K.
K = -V(∂P/∂V)
V is the volume of the substance and P is the pressure applied.
The bulk modulus of steel is 1.6 × 1011 P, and this is three times the value for glass. Therefore, glass is three times compressible than the steel.
Young modulus describes the elastic properties of a substance undergoing compression or stretch in only one direction. For example, when a metal rod is stretched or compressed from one side, it has a capability to return to its original length (or closer to that). This shows how far the metal can withstand a tension or compression. Young modulus is the measure of this elastic property of a substance. Young modulus was named after the physicist Thomas Young. This is also known as the modulus of elasticity. Young modulus also has the units of pressure as bulk modulus. Young modulus, E is calculated as shown below.
E = tensile stress/tensile strain
What is the difference between Bulk Modulus and Young Modulus?
• Bulk modulus is defined for a uniform compression where the pressure is applied from all directions uniformly. Young modulus is defined only for one axis of the substance.
• Bulk modulus measure the change in volume when a pressure is applied, and Young modulus measures the change is length.
• In bulk modulus amount of pressure applied is measured. In Young modulus applied tensile stress (compression or stretch) is measured.