Physics · Mechanical Properties Of Solids · NEET
For almost all materials, heating lowers elasticity and cooling raises it. When you heat a metal, the atoms vibrate more and the inter-atomic bonds become weaker, so the material regains its shape less strongly. That is why a hot wire stretches more easily under the same load. The rare exception is INVAR steel, whose elasticity barely changes with temperature (which is why it is used in precision instruments). For NEET, the safe rule is: temperature up means elasticity down.
It depends on the impurity. A suitable impurity increases elasticity, an unsuitable one decreases it. The classic example is steel: adding a small amount of carbon to pure iron (potassium is not used here) greatly raises its elasticity, so steel is far more elastic than pure iron. But if the impurity is not compatible with the base metal's structure, it disturbs the bonds and lowers elasticity. NEET usually tests the steel example, so remember carbon in iron increases elasticity.
Metals are made of many small crystal blocks called grains. Hammering and rolling apply repeated force that breaks large grains into many small grains. More grain boundaries make it harder for the layers of atoms to slip past each other, so the material resists permanent deformation better and springs back more strongly. This is why cold-worked (hammered/rolled) metal is more elastic. Remember: hammering makes grains small and raises elasticity.
Annealing means heating a metal to a high temperature and then cooling it very slowly. During slow cooling, the small grains merge and grow into large crystal grains. Fewer, larger grains let atomic layers slip more easily, so the metal becomes softer and less elastic. Annealing is the opposite of hammering. For NEET: annealing grows big grains and lowers elasticity.
This is the most common trap. A more elastic material stretches LESS for the same force and springs back more strongly. Elasticity is the tendency to regain original shape, not the ability to stretch a lot. Steel has higher elasticity than rubber even though rubber stretches far more, because steel needs a much larger force to produce the same strain and returns to shape almost perfectly. So a factor that 'increases elasticity' makes the material stiffer, not stretchier.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Temperature, impurities, hammering and rolling, and annealing. A quick memory aid is TIHA. Temperature up lowers elasticity, a suitable impurity raises it, hammering/rolling raises it, and annealing lowers it.
The impurity factor. Adding a controlled small amount of carbon to iron produces steel, which has much higher elasticity than pure iron. This is the standard example NEET uses for the impurity factor.
For nearly all materials, elasticity decreases as temperature rises. The special exception is INVAR steel, whose elasticity stays almost constant with temperature, so it is used in clocks and precision measuring instruments.
No, they are opposites. Hammering (and rolling) breaks grains into small ones and increases elasticity. Annealing heats the metal and cools it slowly so grains grow large, which decreases elasticity.
Questions on mechanical properties of solids often ask which treatment increases or decreases elasticity, or compare materials like steel and rubber. Knowing the direction of each factor lets you answer these one-liners quickly without a formula.
