Elastic Behaviour:
All bodies can be deformed by suitably applied forces. In most cases, the bodies regain their original shape and size, at least partially. The property of regaining the original shape and size is known as elasticity. Thus
Elasticity is the property of a material by virtue of which it tries to regain its original shape and size when deforming forces are removed.
The elastic property of a material is directly related to the internal forces within the material. These internal forces are, infact, the forces between the molecules of the material. We know that when two molecules are not very close, the resultant force between them is one of attraction. These attractive forces are the binding forces that are responsible for holding the molecules within the material. However, if two molecules are pushed very close then the electronic orbits of the atoms of the molecules begin to overlap and repulsive forces come into play. The intermolecular force F, therefore varies with the separation r in a typical manner as shown below. We observe that at r = r0, the resultant force on each molecule is zero. This is the equilibrium separation corresponding to the normal state of the body.
When the material is in the solid form, it has a definite arrangement of molecules in three-dimensional space such that the resultant force on each molecule is zero. Now suppose, the solid body is deformed. As a consequence of this molecules are either pulled apart or pushed close to each other. The equilibrium is thus disturbed. If the body is extended, the molecules are pulled apart. The intermolecular separation increases and attractive forces appear within the body which try to bring the molecules back to their respective equilibrium positions. Likewise, when the body is compressed, intermolecular forces of repulsion come into play, which try to restore the original shape and size of the body.
Although there is no definite arrangement of molecules inside a liquid, but the forces resisting any change in equilibrium separation are still sufficiently large so that a liquid has a definite volume. Similarly, at a given pressure and temperature, a gas has a definite volume. Since liquids and gases do not resist changes in their shapes, the only elastic deformation that one should consider for them is the deformation due to changes in the volume. However, for solids, three kinds of elastic deformations, namely deformation due to changes in length in a particular direction, deformation due to changes in volume, and deformation due to changes in shape must be considered.
Comments (No)