Please explain elasticity and plasticity as properties of a material?

Elasticity and Plasticity

The property of a material body by virute of which it regains its original configuration (i.e. shape and size) when the external deforming force is removed is called elasticity. The property of the material body by virtue of which it does not regain its original configuration when the external force is removed is called plasticity.

Deforming Force

An external force is applied to a body which changes its size or shape or both is called deforming force.

Perfectly Elastic Body

A body is said to be perfectly elastic if it completely regains its original form when the deforming force is removed. Since no material can regain completely its original form so the concept of perfectly elastic body is only an ideal concept. A quartz fiber is the nearest approach to the perfectly elastic body.

Perfectly Plastic Body

A body is said to be perfectly elastic if it does not regain its original form even slightly when deforming force is removed. Since every material partially regain its original form on the removal of deformig force, so the concept of perfectly plastic body is an ideal concept. Paraffin wax, wet clay are the nearest approach to a perfectly plastic bodies.

Cause of Elasticity

In a solid, atoms and molecules are arranged in such a way that each molecule is acted upon by the forces due to the neighbouring molecules. These forces are known as intermolecular forces. When no deforming force is applied on the body, each molecule of the solid(i.e. body) is in its equilibrium position and the inter molecular forces between the molecules of the solid are maximum.

On applying the deforming force on the body, the molecules are displaced from their equilibrium position. In other words, intermolecular forces gets changed and restoring forces are developed on the molecules. When the deforming force is removed, these restoring forces bring the molecules of the solid to their respective equilibrium positions and hance the solid (or the body) regains its original form.