Thermal Expansion and Contraction

Instructor: Dina El Chammas Gass

Dina has taught college Environmental Studies classes and has a master's degree in Environmental and Water Resource Engineering.

In this lesson, you'll learn how materials change when temperature changes. You'll also explore how this change is measured and how the coefficient of thermal expansion helps measure this change.

What Is Thermal Expansion?

Imagine yourself in an open space, chairs laid out in a certain area. All the chairs fill up,and everyone sits side by side. Everyone coexists in a very harmonious way, occupying that set space. Now imagine a skunk popping up from underneath the chair right squat in the middle. What's going to happen? You guessed it, everyone is going to jump out of the chair and scatter everywhere! The crowd will expand.

This is very much what thermal expansion is all about. At a certain temperature, atoms within a material are going to occupy a set space. This set space defines the boundaries of that material. Atoms are in constant motion, bouncing off of each other, but at that specific temperature they are moving and bouncing against each other in that set space. When you heat that material, the atoms get agitated. They move faster. The hotter the temperature, the faster they move. They start bouncing off of each other more frequently and with more force. They start to need more space to coexist in, and they get it. They expand the space they exist in; expand the boundaries of the material they make up. What ends up happening is the material itself expands.

Solids, liquids and gasses all expand when they are heated. Different materials expand at different rates although a general rule of thumb is that liquids and gasses expand at higher rates than solids.

What Is Thermal Contraction?

Thermal contraction is the opposite of thermal expansion. When the temperature drops, atoms calm down and shrink. They aren't bouncing so aggressively off of each other and don't need that much space to coexist. When they shrink, the boundaries of the material shrink. The material contracts.

Most solids, liquids and gasses contract when they are cooled. One notable exception is water. Water is a magical element that expands at freezing point. Liquid water contracts when cooled, and you would expect that it continues to contract as it gets colder. When it hits freezing and turns into a solid, it expands. That's why water pipes may burst when the water inside freezes over. Ice takes up more space than liquid water.

Ways to Measure Thermal Expansion

There are three basic types of thermal expansion:

  • Linear thermal expansion
  • Area thermal expansion
  • Volumetric thermal expansion

Linear thermal expansion is only used for solids and indicates an expansion in one dimension. Imagine a measuring tape pulled out to measure half a meter. Grab the measuring tape and pull it out to measure one meter. This is what linear expansion looks like.

Area thermal expansion indicates expansion in two dimensions. Let's say you draw a square on a piece of paper. Then, around your original square, you draw another square with sides twice as long. If your smaller square expanded in two dimensions, it would end up looking like your second square.

Volumetric thermal expansion indicates expansion in three dimensions - like a balloon that you blow more air into.

So measuring linear, area, or volumetric thermal expansion gives you an idea of how your materiel is reacting to a change in temperature. If your measurement for thermal expansion is a negative value, that means your material contracted.

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