Q.A bimetallic strip is made of aluminium and steel ($\alpha_{Al} > \alpha_{steel}$). On heating, the strip will

Thermal Expansion: Why Things Grow When Heated

You already know that matter is made of atoms or molecules. In a solid, these particles are locked into a lattice, but they are not still — they vibrate around fixed positions. When you heat a substance, you give its particles more energy. They vibrate faster and with larger amplitude. That larger vibration pushes the particles slightly farther apart from each other. The result? The whole object gets bigger. That is thermal expansion.

The key intuition: heat increases atomic vibration, which increases the average distance between atoms, which makes the object expand in every direction.

The Precise Physics

For small temperature changes, the expansion is directly proportional to the temperature change. This is an excellent approximation for most engineering and exam problems.

Linear expansion (change in one dimension — length):

$\Delta L = \alpha L_0 \Delta T$

where $\alpha$ is the coefficient of linear expansion, $L_0$ is the original length, and $\Delta T$ is the temperature change.

Area expansion (change in two dimensions):

$\Delta A = \beta A_0 \Delta T$

where $\beta$ is the coefficient of area expansion.

Volume expansion (change in three dimensions):

$\Delta V = \gamma V_0 \Delta T$

where $\gamma$ is the coefficient of volume expansion.

This ratio is not a coincidence — it follows directly from geometry. Imagine a cube of side $L$. Its area is $L^2$ and its volume is $L^3$. If each side expands by a factor $(1 + \alpha \Delta T)$, then:

• New area = $L^2 (1 + \alpha \Delta T)^2 \approx L^2 (1 + 2\alpha \Delta T)$ → so $\beta = 2\alpha$
• New volume = $L^3 (1 + \alpha \Delta T)^3 \approx L^3 (1 + 3\alpha \Delta T)$ → so $\gamma = 3\alpha$

The approximations hold because $\alpha \Delta T$ is tiny (typically $10^{-5}$ per °C), so squares and cubes of it are negligible.

Why Liquids Are Different

Liquids have no fixed shape, so they have no "linear" or "area" expansion — only volume expansion matters. A liquid expands to fill its container. The coefficient of volume expansion for a liquid is typically much larger than for a solid (about 10 times larger for water compared to steel).

A classic exam point: when a liquid is heated in a container, both the liquid and the container expand. The apparent expansion you see (liquid rising in a graduated cylinder) is the difference between the liquid's true expansion and the container's expansion.

Real-World Examples You Already Know

• Railway tracks have small gaps between them. In summer, the tracks expand; without gaps, they would buckle.
• Thermometers work because mercury or alcohol expands more than the glass tube, so the liquid column rises visibly.
• Bimetal strips (two metals riveted together) bend on heating because one metal expands more than the other. This is used in thermostats and old-fashioned fire alarms.
• Tight glass lids on jars: run hot water over the metal lid. The metal expands more than glass, loosening the seal.

The One Formula You Must Memorize

For any solid, for a small temperature change $\Delta T$:

$\text{Final length} = L_0 (1 + \alpha \Delta T)$

$\text{Final area} = A_0 (1 + \beta \Delta T)$

$\text{Final volume} = V_0 (1 + \gamma \Delta T)$

And always: $\beta = 2\alpha$, $\gamma = 3\alpha$.

That is the entire concept. Everything else — bimetallic strips, expansion gaps, thermometer design — is just this idea applied to real situations.