Now that you know that insulators don’t conduct thermal energy as well as conductors, it’s time to look at what happens to those conductors when they heat up. Adding thermal energy causes the molecules to move faster and spread out so that materials actually expand. This can have some surprising effects on lots of things, from pickle jars to power lines.
Watch this video to find out how heat energy causes thermal expansion. As you watch the segment, pay special attention to what happens to the molecules during thermal expansion. You might want to check the definitions of specific terms as you go.
Stop the animation at any time, watch it again, or read the transcript.
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As a material heats up, its molecules move around more. And as its molecules move around more, the space between molecules increases. This is called thermal expansion. The volume, shape or area of a material changes as it heats up. And the opposite happens as a material cools down. The molecules settle down close to one another, and the material shrinks or contracts. (footnote: *exception is solid water (ice))
Thermal contraction is why that pickle jar is a bear to open. And thermal expansion is why running it under warm water is the solution. That metal lid expands as it warms up, which loosens its grip on the jar.
Different materials expand and contract in different ways and at different temperatures. And knowing at what temperatures and how much materials expand and contract is key in engineering lots of things. Bridges are built with metal strips so that they can expand and contract in hot or cold weather without breaking. Tires lose pressure on cold days. Power lines droop in hot weather. And old school thermometers tell your temperature. All of these are examples of thermal expansion and contraction.
In the space below explain why some materials expand when they get hot. Give two examples.