What is Thermal Expansion?
Thermal expansion. You would probably not be the only person in the world to say that those two words might not sound that interesting or important. But, it’s actually really important in structural engineering. For example, problems with thermal expansion were involved in a major airport terminal collapse, killing 4 and injuring 3 others, as described in this video:
So, basically, thermal expansion is really important. And, based on what you’ve already learned, the concept really isn’t that complicated.
You’ve learned already that gases and liquids (with the exception of liquid water) both expand when they are heated. To recap, this is because temperature is kinetic energy: an increase in temperature (like on a hot day, or when you turn on the stove) is an increase in kinetic energy of the molecules. So, when heated, molecules start bouncing around more, end up pushing out on their container more, and end up taking up more space. This is demonstrated nicely in this video:
Solids do the exact same thing. The molecules are a lot closer together than they are in a liquid or gas, which is what makes it a solid, but they will still move faster and expand with an increase in temperature. Likewise, they will contract, or come closer together, with a decrease in temperature. This simple demonstration shows the expansion of solids at work.
Calculating Thermal Expansion
There is also an equation that lets you calculate just how much a solid will expand with a given increase in temperature, shown below.
This says that the change in length is equal to a coefficient of expansion (α), which is basically just a fancy way of saying that the amount of expansion depends on the type of solid (aluminum versus glass versus concrete, etc.), times the original length of the metal times the change in temperature. These should all seem like fairly intuitive relationships: different solids behave differently, more metal to begin with means more expansion, and a bigger temperature change means more expansion.
It’s important to note that the relationship between temperature change and expansion is linear, which means that the same change in temperature will always cause the same change in length, no matter what that temperature is. So, if you go from 1000 ºC to 1001 ºC, the change will be just as much as if you had gone from -20 ºC to -19 ºC. It also means that the contraction from 20 ºC to 19 ºC will be the same amount as the expansion from 20 ºC to 21 ºC. Don’t worry about calculating anything with this equation, but the relationships are important to keep in mind.
How Do We Engineer Around Thermal Expansion?
Understanding thermal expansion is useful for more than just nerdy lab demonstrations. For example, you could also take advantage of the expansion of solids to loosen a tight lid on a jar by running it under hot water. It is also very important in real life: remember that terminal collapse at the airport? That was partly because the metal in the structure kept expanding and contracting with changes in temperature, which stressed the system. It’s also important to keep thermal expansion in mind for things like building bridges, especially when they’re made out of metal (concrete will also expand, but not as much). Here is a video of a bridge collapsing for entertainment purposes. What happens? It gets super hot, the metal expands, and everything collapses. I haven’t seen the movie, but I assume it involves aliens.
Think about it: when you build a bridge, you want it to be a very particular length. If it contracts too much, the pieces that go into the bridge will pull apart, and it will collapse. If it expands too much, the pieces will push into each other, making cracks, and the bridge will collapse. Basically, a lot of potential death. The solution? Engineers came up with these things called expansion gaps and expansion joints.
Expansion gaps are basically just little spaces we leave when laying down blocks of metal (or concrete, or hardwood floor) to give them space to expand under natural conditions and prevent cracking. Expansion joints are similar, but also connect the two things when they pull apart so you don’t get too big of a gap (when you’re building a bridge, you don’t want huge gaps in it). The video below shows an expansion joint at work.
Fascinating, right? Well, admittedly, this bridge moving ever so slightly back and forth to some catchy royalty-free music does not have the same entertainment value as an alien heat wave taking down the Golden Gate. But, that’s exactly the point. The goal of construction isn’t to cause a bridge to collapse with all the fiery entertainment of a blockbuster movie, it’s to keep things boring, functional, and reliable. Thanks, boring joint!
You should understand:
- That solids, like liquids and gases, expand when they are heated and contract when they are cooled.
- That the thermal expansion of solids can be calculated and depends on the type of solid we’re talking about, the initial length of that solid, and the change in temperature.
Content contributors: Emma Moulton and Emily Zhang