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why do solids expand when they are heated

Thermal expansion of solids, liquids and gases
All three states of matter (solid, liquid and gas) expand when heated. The atoms themselves do not expand, but the they take up does. is heated, its atoms vibrate faster about their fixed points. The relative increase in the size of solids when heated is therefore small. Metal railway tracks have small gaps so that when the sun heats them, the tracks expand into these gaps and dont buckle. expand for the same reason, but because the bonds between separate molecules are usually less tight they expand more than solids. This is the principle behind liquid-in-glass thermometers. An increase in temperature results in the expansion of the liquid which means it rises up the glass. are further apart and weakly attracted to each other.

Heat causes the molecules to move faster, ( ) which means that the volume of a gas increases more than the volume of a solid or liquid. However, gases that are contained in a fixed volume cannot expand - and so increases in temperature result in increases in pressure. First - the expansion question. What Prof Fink said is true as far as it goes. But it's worth saying too that the interatomic forces that hem the vibrating atom in as it oscillates on either side of its equilibrium position are not symmetrical. That is, the repulsive force felt as the atoms approach one another by one unit distance from the absolute zero separation is greater than the attractive force as they move one unit distance apart from it.

Thus, as the atoms oscillate due to their kinetic energy (heat) they spend slightly more time beyond their absolute zero equilibrium separation than they do within it. This means that the time-average separation of atoms that vibrate is slightly more than those of atoms with no vibrational energy, i. e. the absolute zero separation. The length/breadth/height of an iron bar is a multiple of the time-average separation between the adjacent atoms. Hence the expansion. As the kinetic energy increases this imbalance between attractive and repulsive forces becomes more marked and the material continues to expand. Of course the degree of expansion depends on the particular values of the force and potential functions for that material.

So, one can have say an alloy of Fe and Ni (~36% Ni) called Invar whose rate of thermal expansion at normal temperature ranges is so small that it hard to measure. It used be the material of choice for clock's pendulums so their frequency did not increase unduly in hot weather. Water in the liquid state expands on heating. Be clear on that. When it freezes it does not contract like most (but not all) materials - it expands. Grey cast iron does the same thing. (This has broken many a badly designed mould! ) Other materials do the same. The change of phase going from the liquid to the solid on cooling, from one configuration of atoms to another, each with their own potential energy need not always involve a contraction in volume. Tak.

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