Mechanical Properties of Solids: Difference between Elasticity and Plasticity (For CBSE, ICSE, IAS, NET, NRA 2022)

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What is Elastic Behaviour of Solids?

Elastic Behaviour of Solids
  • 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 inter molecular forces.
  • Elastic Behavior Of Solids – What happens to a rubber band when you stretch it and let go? It deforms but regains its original nature when you stop applying a force.
  • But if you take an aluminium rod and try to bend it using your arm strength. You somehow do manage to bend it a little and then stop applying force.

Elasticity

The Elasticity
  • This difference in the behavior of the material is based on their elastic and plastic nature.
  • The rubber band has high elasticity. Elasticity is the ability of a body to resist any permanent changes to it when stress is applied.
  • The body regains its original shape and size when stress application ceases.
  • The change in the shape or size of a body when external forces act on it is determined by the forces between its atoms or molecules.
  • These short-range atomic forces are called elastic forces.

Plasticity

The inability of a body to return to its original size and shape even on removal of the deforming force is called plasticity and such a body is called a plastic body.

Difference between Elasticity and Plasticity

  • All materials have an elastic limit beyond which, if continuous stress is applied, they will start losing their ability to exhibit perfect elastic behavior and start deforming.
  • Contrastly, plasticity is the non-reversible deformation of solid materials on the application of forces.
  • Looking at the elasticity in the atomic level, solids are made of atoms (or molecules) .
  • They are surrounded by other such atoms which are held in a state of equilibrium by inter atomic forces.
  • When an external force is applied these particles are displaced, resulting in the deformation of the solid. When the application of the deforming force is stopped, inter atomic forces drive the atoms to regain their state of equilibrium.
  • The concept of elasticity is an idealization as no material is perfectly elastic.
  • If you use a hair tie to groom yourself, you may have noticed that its size tends to deform after prolonged use. After a point, it may snap as well. This is because the hair tie eventually loses its elastic nature.

Stress

Stress is defined as the ratio of the internal force F, produced when the substance is deformed, to the area A over which this force acts. In equilibrium, this force is equal in magnitude to the externally applied force. In other words,

Important Points on Elastic Behavior of Solids

  • An elastic body is one that regains its original shape and size when deforming forces are removed.
  • A plastic body is one that succumbs to deforming forces (however small) and cannot return to its original shape and size.
  • Elasticity is the property of a body to regain its original shape and size when deforming forces are removed. It exhibits an opposition to change.