Thermal Expansion

Contributed by Max Prichard

NOTE: This is a more advanced experiment that will require a few more materials and setup steps, but the results are worth it! For some quicker experiments, check out some of the simple experiments in our collection.

Introduction

• You might have heard before that materials tend to expand as they heat up. This is a phenomenon called thermal expansion, and it happens to almost every object that we see around us, but how does it work? Materials are made up of atoms and molecules, and although we can’t see these individual building blocks, at any time they are vibrating and bumping into each other. As we heat things up, this vibration gets stronger and tends to move the atoms/molecules farther apart given they have more energy. However, as scientists, we can’t accept this as fact without measuring it! Usually things only expand by a tiny fraction, which makes it almost impossible to observe directly. However, in this experiment we will use a setup that will allow us to actually see thermal expansion in action!

Materials

• A long, thin piece of aluminum, like a knitting needle or a metal drinking straw. The longer, the better
• Two bottles that are the same height (should be glass)
• Candle
• Sewing needle, the thinner the better
• Two (2) erasers (both with flat surfaces on both sides)
• A piece of paper

Procedure

1. Take about half of the sheet of paper, and fold it until it is a thin slip of paper which is about 1 cm across and around 10 cm long and a few sheets thick. Stick the needle through the center of this long paper rectangle and glue it in place, such that the paper rectangle is at one end of the needle. We will use the rotation of the needle to measure the thermal expansion of the knitting needle, and the long paper rectangle will amplify this effect.
2. Secure one end of the knitting needle/rod to one of the bottles using a piece of tape, or by sticking it firmly into a piece of cork in one of the bottles. It is best for the knitting needle to naturally bend down a bit, as we will see.
3. Rest the far end of the knitting needle on the other bottle, with the sewing needle between them and perpendicular to the knitting needle so that it can roll. This step is easier if the knitting needle has a downward angle in the cork/tape, so that when we lift it to rest on the knitting needle, the sewing needle won’t move.
4. Carefully light the candle, and place it on some solid base that raises the flame to the height of the knitting needle (it’s important for the flame to be touching the knitting needle!). However, don’t move the candle under the knitting needle just yet. 
5. The effect is indeed visible with the naked human eye, but if you have a smartphone that can take an accelerated-speed capture, then start the recording, and focus on the paper strip that is attached to the needle. Now, move the candle under the knitting needle. You should see the paper strip start to turn, and this will be even more visible using an accelerated-speed capture! 

If anything in the above is unclear, a video is worth at least a thousand words, so watch the instructional video included with the experiment!

Troubleshooting

If you don’t see the needle moving at first, don’t worry! We didn’t either :), but with some checking and tweaking we can double check that all parts of the experiment are working smoothly.

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Don’t forget to check out the included instructional video, where we will also go over some key pointers to make sure the experiment is a success and some troubleshooting tips in case you are stuck.

Physics Concepts and Questions

In this experiment, there were a lot of details that we had to pay attention to in order to amplify the thermal expansion, namely using a tiny sewing needle to turn the small motion of the needle into a rotation, and further using a long paper strip to make even this noticeable. And even then, the amount that the paper strip turned was very small! Does this give us an idea as to the magnitude of the thermal expansion that we are seeing?

To further understand the magnitudes of thermal expansion, consider that we heated the knitting needle directly using a candle flame. In fact, the hottest part of a candle flame is almost 2000° F! Usually, thermal expansion is proportional to the difference in temperature that the object undergoes. That is to say, if the temperature difference gets 10 times greater, then the expansion will also get 10 times greater. Given that in this experiment we used a temperature change of almost 2000° F at one part of the needle, how much would you expect to see the needle change its length going from a cold morning (50° F) to a hot afternoon (90° F)? Are you likely to be able to see something expanding with your eyes when you walk around outside in the morning?

If you’ve ever driven across a long bridge, you might notice that there are some strange steel junctions every now and again between the concrete that have small gaps between them. Keep in mind that concrete, like the knitting needle, also has a coefficient of thermal expansion. Does our experiment give you an idea of why these steel junctions need to be placed in a long bridge? What would happen if there were not any gaps in a long concrete bridge when the temperature changes?