Thermal image of a space shuttle coming through the upper atmosphere back to Earth. The yellow areas of the nose, wings and rear flap experience the greatest heating. NASA imageEverything is made of atoms and molecules, and they don’t sit still. The amount of their movement is what we call temperature, which is a measure of how hot or cold something is.

 

Atoms and molecules within a gas move rapidly, colliding with each other and the walls of their container. A rubber birthday balloon full of helium atoms – billions of them – stays inflated because of the collisions of the atoms against the rubber. Moving the balloon into sunlight adds heat to it, causing the atoms to move faster, and the balloon expands. Putting the balloon in a freezer will remove heat from it, and the helium atoms will slow down and press less against the rubber and the balloon will shrink slightly. The amount of the gas doesn’t change, just how fast the atoms and molecules move.

 

 

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The same thing happens in solids, but the movement of the atoms is more restricted. Atoms within molecules also move, but they are connected by chemical bonds – like springs – that limit their motions. The thermal energy of a material is its total energy, combining kinetic energy from movement and potential energy from the molecular springs. Temperature is a measure of an object’s average thermal energy.