Physics · Thermal Properties Of Matter · NEET
In a solid, atoms are held in fixed positions by bonds that act like tiny springs. At any temperature the atoms vibrate about their mean positions. When you heat the solid, the atoms get more energy and vibrate with a larger amplitude. Because the bond force is not perfectly symmetric (the atoms are pushed apart more easily than they are pulled together), the average spacing between atoms increases. More spacing between every pair of atoms adds up to a larger overall size. This is thermal expansion.
The atom itself does not grow. Its size stays essentially the same. What increases is the average distance between neighbouring atoms. The object gets bigger because there are more gaps and each gap is slightly larger, not because the atoms swell. This is a very common NEET misconception.
They describe the same effect measured in different dimensions. Linear expansion is the increase in length (one dimension), area expansion is the increase in surface area (two dimensions), and volume expansion is the increase in volume (three dimensions). A heated rod mainly shows linear expansion; a heated metal sheet shows area expansion; a heated block or sphere shows volume expansion. Their coefficients are linked: alpha (linear), beta (area) = 2 alpha, and gamma (volume) = 3 alpha, for an isotropic solid.
Yes. Liquids and gases expand too, and usually by much more than solids because their particles are held together more weakly. For liquids and gases we mostly talk about volume expansion since they have no fixed shape. Solids have all three types because they keep a definite shape. Water is a special exception between 0 C and 4 C, where it contracts on heating (anomalous expansion).
The coefficient of linear expansion for common metals is only about 10^-5 per degree Celsius. So a 1 metre steel rod heated by 100 C grows by only about 1 millimetre. The change is small but it matters a lot in engineering: railway tracks, bridges, and metal rings all leave gaps or are pre-heated to allow for it. In NEET numericals you compute this small change using delta L = alpha times L times delta T.
The temperature of a metallic sphere of radius R is increased by a small amount delta T. If the linear coefficient of thermal expansion of the metal is alpha, the approximate increase in the volume of the sphere is:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
It is the increase in the length, area, or volume of a body when its temperature goes up. Almost all materials expand a little when heated and shrink when cooled.
Heat gives the atoms more vibrational energy. They vibrate more widely and the average gap between atoms increases, so the whole solid becomes slightly larger.
Linear expansion (change in length), superficial or area expansion (change in surface area), and cubical or volume expansion (change in volume). For a solid their coefficients relate as beta = 2 alpha and gamma = 3 alpha.
Gases expand the most, then liquids, then solids. This is because the particles in gases are held together very weakly, so the same rise in temperature produces a much larger increase in size.
Yes. Between 0 C and 4 C water contracts as it warms and expands as it cools. This anomalous behaviour is why ponds freeze from the top and aquatic life survives underneath.
