What Is Thermal Expansion?

If it wasn't for our understanding of thermal expansion, bridges would collapse, jars would remain unopened for years and we would have no way to measure the temperature. But on the bright side, a few less people would run out of gas next summer. Confused? All will make sense after today's lesson.

Thermal expansion is where materials expand while being heated, causing them to take up more space. Some materials expand more than others - metals, for example, tend to expand a lot. But this all happens due to the motion of tiny little molecules.

Temperature is the average kinetic energy of the molecules in a substance. So that means, if we heat something up, we are making the molecules move faster. Molecules that move faster tend to take up more space, and that's why materials expand when you heat them and contract when you cool them.

Examples of Thermal Expansion

Okay, so let's go through a few examples of thermal expansion. I said at the start of the lesson that bridges would collapse if we didn't understand thermal expansion, and that's true. Bridges have a feature called an expansion joint, containing little jagged teeth with a cap in-between. If the bridge material expands and the bridge gets longer, the teeth move closer together, and if it contracts, they move further apart. This allows the bridge to change in length without it, you know . . . collapsing in a chaotic mess of broken metal and falling cars.

Another example? Opening a tight jar; when you can't open a jar, what do you do? Well, there are a lot of tricks. Maybe you tap the lid of the jar against the counter to break the seal. Maybe you use one of those grippy rubbery things - that's a technical term. But one way you can open a tight jar is by running the lid under a hot tap. This causes the lid to expand. But wouldn't an expanding lid make it tighter?

To explain this, imagine heating up a doughnut-shaped piece of metal. Mm . . . doughnuts. Does the doughnut hole get larger or smaller? People sometimes think smaller, because they imagine the expanding doughnut filling the hole, but if the complete shape expands, the hole also gets larger. So if you heat up a jar lid, the gaps between the lid and the glass threads get larger, making it easier to open.

A thermometer also works by expansion. The liquid in a thermometer - which used to be mercury, but is now usually an alcohol of some kind - expands as it gets hotter, and the marks on the glass are calibrated to tell you the temperature based on how much the liquid's expanded.

Okay, one more example: the gas gauge on a car. People are much more likely to run out of gas in the summer than the winter. Why is that? Well, the gas in your car expands, like anything else, and changes the reading on your fuel gauge. In the summer, the gas takes up more space in the tank, and so when it reads empty, there's significantly less fuel left in the tank than in winter. Maybe you get an extra 35 miles after it hits empty in the winter, but that won't be the case in the heat of summer. So, be careful out there! Don't be the fool who has to walk three miles to the gas station because they don't know about thermal expansion.

Thermal Expansion Equation

The equation for thermal expansion is pretty simple. This equation represents the amount something expands in one direction. Imagine a metal bar expanding, for example... this will tell you how much the length of that bar changes. Delta-L is the change in length of the material measured in meters, L is the original length of the material also measured in meters, delta-T is the change in temperature the material experiences measured in either Celsius or Kelvin and alpha is the coefficient of linear expansion of the material, which is just a number that represents how much different materials expand - you can look it up in a data booklet, and you should be given it in an exam question.