Question

what is thermal expansion?explain

Answer 1

Thermal expansion is the ability of solids to expand in size due to the application of temperature (heat). That is the molecules of solids when heated start to move fast due to the increase of its kinetic energy. Example, railway tracks gets expanded a bit during summer because it gets heated up.
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Answer 2

Thermal expansion is the tendency of matter to change in shape, area, and volume in response to a change in temperature, through heat transfer. Temperature is a monotonic function of the average molecular kinetic energy of a substance. When a substance is heated, the kinetic energy of its molecules increases.