Thermal expansion and its applications

Uuuughhh! Linaa! Can you help me open this jar? It’s stuck! How do we open it?

Try this trick! Take a bowl and pour some hot water into it. Place the closed jar in the bowl, upside down, so that the lid is immersed. Leave it there for a couple of minutes. Take the jar out, try to open it now.

Oh, wow, that was easy! How did this happen?! The jar and the lid, like all substances, are made up of particles that move. This movement is affected by temperature. When a substance or an object is heated, particles inside start moving faster and moving away from each other.

This causes the material to expand. This tendency of substances to expand when heated is called thermal expansion. On the other hand, when a substance is cooled, the opposite happens. The moving particles in the substance slow down and are pulled closer together. The volume of the substance decreases; the substance contracts.

This is thermal contraction. How much a material expands or contracts, depends on the temperature change, the amount of material, and the material’s properties. Different materials expand at different rates. With our jar, when the hot water heats up the metal lid, the lid’s volume changes and its diameter increases. The glass jar does not expand as much, so the difference in diameter becomes just enough to twist the lid off.

The different rates at which materials expand — the expansion coefficients — are very important when constructing buildings, bridges, or other big structures. These are often built using concrete, reinforced with steel rods. Steel rods need to expand and contract at approximately the same rate as concrete. Otherwise, concrete can crack and crumble over time. Train tracks are often exposed to significant temperature changes.

If the rails are welded together with no space between them, there is no room for them to expand when temperature increases. The rails might buckle and deform, which can lead to a train derailing! One way to avoid buckling, is to use shorter rail segments and leave a small gap between them. This is because expansion is proportional to the amount of material - shorter segments of rails expand less than longer ones. Gaps between rails allow for rails to expand, without causing them to deform.

Thermal expansion is often used to indicate temperature. Most commonly, it’s used in traditional liquid-in-glass thermometers. These have a capillary tube that contains liquid, such as alcohol. The scale on the thermometer corresponds to how much the liquid inside the capillary tube expands, depending on temperature change. Another type of thermometer involves a special strip made of two different types of metal fused together along their length - bimetallic strip.

The two metals expand and contract at different rates when heated or cooled. This causes the bimetallic strip to bend. Such strips are often used in devices that control temperature — thermostats. Gases are affected by thermal expansion too! Take, for instance, car and bike tyres.

Tyres often have recommendations from the manufacturer that state how much air and pressure they can hold. Why? Well, when a car or bike is moving quickly, the air inside warms up and expands. If a tyre is overinflated, it might burst! Most materials in our everyday life are exposed to changing temperatures, so they expand and contract all the time.

Knowledge about thermal expansion allows designers, engineers and manufacturers to create better solutions and products. And it can come in handy next time you’re struggling to open a jar!