The Heat is on! A huge amount of the technology and comforts that we have in our world just wouldn’t exist if it wasn’t for what we’ve learned about the way heat behaves. So keep cool and use this excellent series to teach your students everything that they need to know about heat, including its effect on things and how it transfers from one thing to another.
In Episode 3, Thermal Expansion, we look at the fact that substances expand when they get hot and contract when they get cold and we explain why they do. We look at how thermal expansion can be a real nuisance, but also how we can put thermal expansion to good use for the good of the world!
A 3-minute excerpt.
The Episode 3 Question Sheet for Students:
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The Transcript (which can be used as a textbook)
Contents:
Part A: Introduction. Things expand when they get hot.
Part B: Expansion and Contraction in Solids: Steel and concrete expand when they get hot, so all large buildings and bridges have to be designed to allow for the expansion. But what causes thermal expansion?
Part C: Thermal Expansion in Liquids and Gases: Thermal expansion of liquids and gases gives us thermometers and, indirectly, electricity!
Part D: Water: An Important Exception*. *Conditions Apply: Water pipes often burst in really cold weather because water doesn’t follow the same rules as most other substances. However, if it did follow the same rules, winter would bring unimaginable destruction to aquatic life.
Part A: Introduction
In this series, we’re Shedding Light on Heat. We’re looking at all the essentials of what heat energy is, how it affects things, and how it transfers from one thing to another. In this episode, we’re going to look at the fact that things expand or contract when their temperature changes.
In Episode 1 of our series, we saw that temperature is a measure, on a scale, of the average kinetic energy of the atoms or molecules that make up a substance. In cold water for example, the water molecules on average vibrate and move relatively slowly, and in hot water, the water molecules on average vibrate and move relatively quickly.
This concept that everything is made of atoms or groups of atoms (called molecules) that are constantly moving is called the kinetic theory of matter. The word kinetic refers to movement. It’s also often called the particle theory. Rather than saying atoms or molecules we can just refer to both as particles.
The Kinetic Theory (or the Particle Theory), combined with the fact that atoms are slightly sticky thanks to being made up of positively charged protons and negatively charged electrons, neatly explains what solids, liquids and gases are and why substances exist in different states at different temperatures.
Now using a thermometer, like this one, is a very common way of measuring temperature. As the temperature increases, the red liquid rises up the tube. Why does it rise? Well, it’s because when things get hot, they expand, they literally get bigger.
And why do they get bigger? Well, in this episode of the Shedding Light on Heat series, we’re going to use the Kinetic Theory to explain why. The Kinetic Theory is a pretty brilliant theory. So let’s begin.
Part B: Expansion and Contraction in Solids
Steel and concrete, and in fact most solids expand when they’re heated, not by much, it’s not very obvious, but their expansion is hugely important in the construction of all large bridges and buildings.
Here I’ve set up a simple demonstration to show you how much this iron rod expands when heated. One end of the iron rod is fixed to the retort stand, but the other end is free to move. I’ve poked a pin through a straw and placed the pin under the iron rod so that if the iron rod moves in any way it will roll on the pin and we’ll be able to see its movement thanks to the straw.
As the iron rod heats up, you can see that the straw is rotating, which means the pin is rolling which means that the metal is expanding. I can’t even tell just by looking at the iron rod that it’s expanding because it’s barely expanding at all, but the fact that it is expanding is really important.
When I turn off the gas, the iron starts cooling down and it’s now contracting. Again I can’t tell just by looking at it, but as it contracts it’s rolling the pin to the left and the straw is also rotating as a result.
It turns out that if you heat a 20-metre-long steel beam from 10°C to 50°C, it will expand by about 1 cm. This animation is not to scale. A same-sized concrete beam will expand by about the same amount, but an aluminium beam will expand by nearly double that if you heat it by the same amount. It’s not much expansion, but it’s important, and if you don’t allow for it, a bridge for example made of steel and concrete will tear itself apart.
Concrete and steel bridges are built with these gaps in them. They’re called expansion gaps. Without them the bridge will literally collapse soon after it’s built. The expansion gaps allow for movement when the concrete and steel expand and contract in hot and cold weather.
Let’s look at a simple, single span concrete bridge that rests on two piers. To accommodate for expansion and contraction, the span is not actually attached to the piers but is free to move between the expansion gaps. When it’s hot the concrete span expands and fills the gaps and the gaps get smaller. When it’s cold, the concrete contracts, that is, it shrinks, and the expansion gaps get bigger. Here I’ve been hugely exaggerating the expansion and contraction.
If the central span was bolted in to the piers, the piers would be pushed outwards and pulled inwards as the span expanded and contracted and cracks would form on the piers. The cracks would get bigger and bigger and weaken the piers to the point where they would eventually collapse.
Of course a large gap on the road surface is asking for trouble if a car drives over it, although I’ve exaggerated a little here, so engineers have to cover the gap somehow. Engineers will often fill the gap with a type of hard rubber that flexes or they use a variety of toothed patterns. When it’s hot and the concrete span expands, the gap gets smaller, and when it’s cold and the concrete span contracts, the gap gets bigger, but the tooth pattern allows tyres to roll over the gap without falling into the gap.
The span of an arch bridge, like the Sydney Harbour Bridge in Sydney, is supported by a huge steel arch. An arch can carry a greater load than just a flat steel or concrete beam. When it gets hot, the full length of the arch expands and so the road surface actually rises. In the case of the Sydney Harbor Bridge, by about 18 cm. The arch actually rests on four huge hinges that allow the arch to kind of open and close as it expands when it gets hot and as it contracts when it gets cold.
Steel train tracks expand when they get hot, so they used to be laid with gaps between them. You can sometimes hear the sound of the train going over the gaps as a clickety-clack sound. A tapered design, that is, one that goes from thin to full width, provides a flat surface for the wheel to roll on, but allows for expansion and contraction. However, a few decades ago, engineers starting bolting the train tracks down to very heavy concrete slabs. This reduces the movement caused by expansion. The steel sections of track are welded together which results in a smoother ride and less wear-and-tear on the wheels of the train.
All large structures have to be designed in a way that allows the materials that they’re made of to expand and to contract without causing damage, but expansion also affects little things.
When glassware like a test tube is heated, the glass expands by a very small fraction of a millimetre. You can’t even tell. However, if you then place the hot glass in cold water, it often cracks. This is because the water cools the outside of the glass and so the outside contracts back to its original size. However, the inside of the glass is still hot. The contracting glass on the outside pulls on the inside of the glass before the inside has a chance to cool down and to contract and this pulling causes it to crack.
When it comes to glassware, you have to be very careful. Glass baking dishes are usually made of special types of glass that hardly expand at all when they’re heated, and so they are resistant to cracking when the temperature changes, but even they can still break if they cool down too quickly. Never put a hot baking dish on a stone benchtop, because the glass or the stone or both might actually crack. Always use a wooden or cork board of some sort.
Overhead power cables are made to hang a little loosely so that when they contract in cold weather they don’t pull on the poles with too much force and break free of their connections.
Pipes used in large industrial plants have to be made to bend regularly so that when they expand and contract they don’t break. If the pipes were straight and they expanded or contracted too much they would break free from whatever they’re attached to.
If you’re ever having trouble removing the lid from a jar of jam, just hold it under hot water for a minute or so. The metal lid heats up, expands, and therefore gets looser. There’s your handy home hint for the day.
Now why do things expand when they get hot, and why things contract when they cool down?
In Episode 2, I explained how the kinetic theory neatly explains how changes of state occur. If, for example, ice is heated, the water molecules vibrate faster and faster until they’re vibrating so fast that they partly overcome the electrostatic forces that are holding them in place and they start sliding around each other. The solid melts into a liquid.
The kinetic theory also neatly explains why things expand when they get hot. The atoms in, say, a cold metal rod vibrate relatively slowly and they don’t move very far from their average position. As a result, they don’t take up much space. If the rod is heated, the atoms vibrate more and more quickly and they all kind of push each apart. The metal therefore expands. The atoms themselves don’t get bigger, they just take up more space because they’re moving more.
If the metal is cooled, the vibration slows, and so the atoms take up less space. The metal rod therefore contracts.
The expansion caused by heating something up is called “thermal expansion”. The word thermal refers to anything to do with heat. Up until now I’ve been using the word heat as in heat energy, but the expression “thermal” energy means exactly the same thing. Thermal Expansion is the expansion that occurs when something heats up. So the kinetic theory neatly explains what solids, liquids and gases are and how and why substances can change state. To that list we can now add that it also explains thermal expansion.
You know you’re on a winning theory when you can use it to explain so many things.
So solids expand when they’re heated, what about liquids and gases. Well, they do the same thing, but to a much greater extent!
Part C: Thermal Expansion in Liquids and Gases.
This conical flask has some water in it which I’ve coloured with a drop of food dye and this rubber stopper has a thin hollow glass tube in it. When I push the rubber stopper into the conical flask, the water rises up the tube a little. So, the amount of water I started with fills the whole conical flask and a small part of the glass tube.
Now what happens if the water is heated? Well, it’s probably no surprise that liquids also expand when they’re heated, just like solids do. The water is expanding and therefore rising up the tube. Water in fact expands much more as a percentage than most solids do, and you can even see the water here rising up the tube. It took about 6 minutes for the water level to reach the top. As you can see the water has expanded quite a lot.
If I now let the water cool down again, the water starts to contract, and so the water level drops. It actually took about 15 minutes to contract back to near the volume that it had started with.
In a very real sense, this conical flask and tube make a thermometer. When it’s cold, the water level is low, and when it’s hot, the water level is high.
Actual thermometers work in exactly the same way. There’s a small bulb at the bottom and a thin tube coming out of it. When the liquid gets hot, it expands and rises up the tube. (The liquid by the way in this case is a type of alcohol that has been coloured with a red dye.) When the temperature falls, the liquid contracts and so the alcohol level drops.
Now I’m guessing that you’ve heard the terms “global warming” or “Climate Change”. Over the past 150 years or so, we’ve seen that average temperatures around the world have increased by just under 1°C.
Since the world’s oceans are a little warmer now than they were a century and a half ago, the water has actually expanded and it’s believed that average sea levels have risen by somewhere between 10 and 20 cm. The exact number is uncertain. In this simple animation where the thermal expansion is hugely exaggerated, you can see that the ocean rises and covers some land.
If the average water level, not including tides and waves, was about here on the day we filmed this, but it rose by 10 cm due to thermal expansion, the new water level would be about here.
Now gases also expand when they get hotter and in fact by a much greater amount than solids and liquids. This conical flask has air in it. If I trap the air with a balloon and then heat the air, it expands, taking up more space, and the balloon gets bigger. Of course, just like solids and liquids, when the air cools down, it contracts and the balloon therefore deflates.
Of course the gas only expands if it’s free to expand. If you seal the gas in a rigid container, then instead of expanding when it gets heated, the pressure inside the container increases. The atoms get faster and faster as they get hotter and hotter and they smash into the walls of the container with more and more force. If the container isn’t strong enough it will eventually explode. Pretty impressive simulation, now watch the live action explosion. POP. Cool! Okay, I’ll do a bigger one in a minute.
If I do a similar demonstration as before, but this time start with a little bit of water in the conical flask, the water starts to boil and the expansion is much greater than before. This is because of the fact that in a liquid state, the water molecules are kind of close together but when the liquid boils and turns into a gas, the water molecules spread out and take up much more space.
If you pour a small amount of water into a balloon, tie the balloon up and then heat it up in a microwave oven, the liquid water that turns into steam inflates the whole balloon in about 40 seconds. It’s actually an easy way of blowing up balloons for a party, but the downside is that if your party atmosphere is less than 100°C your balloon’s gonna deflate! Oh well, nice try.
It turns out that if you boil just 1 litre of water and turn it into a gas, that is steam, at 100°C, at normal atmospheric pressure, the steam will take up 1700 litres of space. It’s a huge amount of expansion. (0.1 m x 0.1 m x 0.1 m à 1.2 m x 1.2 m x 1.2 m) If the water is in a rigid container, the pressure will build up instead.
Never put unopened containers like cans of baked beans into an oven, or a sealed plastic container with food in it into a microwave oven. If you do, the water and air might expand so much that the container will burst. If you want to heat up some leftovers, place the lid loosely onto the container to allow the air and steam to escape but to stop any splattering. There’s another handy home hint.
People have actually been severely injured when sealed cans filled with ordinary water have exploded because the hot steam has burst out of them. Imagine these sharp pieces of metal hitting you in the eye. I don’t want to think about it.
We can actually put the huge expansion of water into steam to good use because we can use it to make things move.
In this conical flask, the water is turning into steam. Because of the fact that its volume expands and the pressure builds up in the flask, the steam rushes up the glass tube and pushes on the little pin wheel, making it turn. The pin wheel is just a round piece of paper I had cut out, which I then cut little slits in. I attached it with a pin to a straw and folded up four kind of triangles to make fan blades.
Now a steam-powered pin wheel may seem like just a toy, but in fact it’s a type of turbine, not unlike this giant steam-powered turbine in this power station. Large power stations, like this one, generate electricity by harnessing the power of moving steam.
Coal or natural gas is burned inside a huge boiler to boil water. The water turns to steam which, as we’ve seen, takes up much more space than the equivalent mass of water. The high-pressure steam is directed through a series of turbines which spin as the steam passes through them. They’re basically giant steel pin wheels! The turbines are attached via a steel shaft to a generator and as the generator spins it generates electricity. The generator is similar to this hand-cranked generator that I’m turning, but it’s absolutely huge. Generators generate electricity by either moving magnets near coils of wire or moving coils of wire near magnets.
Now the steam that passes through the turbines doesn’t drift off into the air like it’s doing here. The turbines are fully enclosed in huge steel shells, but the upper part of the shells had been removed when we filmed this because the turbines were undergoing maintenance. Once the steam passes through the turbines, it condenses in the condenser, which is cooled by a separate cooling system that I’m not showing here, and then the water that has condensed is pumped back into the boiler to be boiled again. This cycle repeats continuously and generates electricity continuously.
Steam trains typically use burning coal or wood to boil water which then turns to steam. The expanding steam rushes out of the boiler into the cylinder and pushes on the piston, first one way and then the other. The back-and-forth movement of the piston forces the wheels to turn.
Part D: Water: An Important Exception*. *Conditions Apply
So, we’ve now seen lots of examples of things expanding when they get hot and contracting (or shrinking) when they get cold. However, water is a very important exception to this rule at certain temperatures.
As water gets colder and colder it contracts. The water molecules vibrate with less and less energy and they get closer and closer together. However, the contraction stops at 4°C. As the temperature drops from 4°C to 0°C, the water starts to expand again until it freezes.
This can of sugary drink contains mostly water. I’ll place it into the freezer in this pot so that it cools down and freezes. The reason I put it in the pot first will become pretty obvious. The can has been in the freezer for twenty four hours now. Let’s take a look at it.
When the temperature dropped below 4°C, the water expanded and at some point probably when it was half frozen the can split open spilling the contents into the container. A can won’t always split open, but it often will.
The reason for this is that below 4°C the vibration of the water molecules slows down to the point where they start lining up in what’s called a crystal structure. The hydrogen atoms, coloured white here are more attracted to the 
oxygen atoms, coloured orange, and a series of rings is produced. The crystal forms a 3D structure but I’m only showing it in 2D. This lining up leaves large spaces between the molecules (which I’ve exaggerated a little on the right) and this causes the water’s overall volume at 0°C to increase from when it was at 4°C.
Here you can see two containers each holding exactly one litre of water (and some food dye). Now if I place one container into the freezer, wait a day because that’s how long it takes to freeze, and then place the frozen water next to the liquid water, you can see that the ice now has a volume of more than 1 litre.
In fact, if you start with 1 litre of water at 4°C its volume will expand to 1.087 litres when it freezes, a rise of about 9%.
Now a metal can splitting open is bad enough, but glass containers are an especially big no no for freezers. That’s your third handy home hint in just five minutes.
Sometimes in really cold climates, the water in water pipes freezes and it bursts the pipes open, so this expansion of water can be a real nuisance.
However, there’s a really big upside. Because say 1 kg of ice has a greater volume than 1 kg of liquid water, ice is less dense than liquid water which means that ice floats on water.
Therefore, when ice forms on lakes, rivers, or oceans (in places where in winter it gets really really cold), the ice floats on the surface, which allows the fish and plants and other organisms to survive underneath. Even though the plants don’t really grow much, or at all, during the colder months, the fish that eat plants can still continue to feed on the plants, and life goes on.
So ice floats on liquid water although today this ice is melting pretty quickly. Now, living in Australia, I use the Celsius scale, so I would say that ice melts at a temperature of 0°C.
I said in Episode 1 of our series that of the Celsius scale and the Fahrenheit scale one isn’t any better than the other it’s just what you get used to. However, there is a temperature scale that scientists and engineers use that in many ways is better than both the Celsius scale and the Fahrenheit scale. It’s called the Kelvin scale, and it’s what we’ll be looking at in our next episode. See you then.
Credits:
Written, directed, and presented by Spiro Liacos.
The simulations of the solids, liquids, and gases were created by
PhET Interactive Simulations
University of Colorado Boulder
https://phet.colorado.edu