Internal Energy

When temperature rises, what happens inside a body?

This question has puzzled scientists for a few hundred years. Some thought that heat was a form of motion in the body. Others thought that heat was a substance. Today, we know that the only substance inside a body are atoms or molecules, and the only change when heated is their motion.

When temperature goes up, the particles move faster. They have more kinetic energy. In a solid, this means faster vibrations of atoms about their rest positions. In a liquid or a gas, it means faster movement from one place to another. We think of the total kinetic and potential energies as the internal energy of the body. So when temperature increases, internal energy increases.

In everyday life, there is evidence that heat is due to motion of particles. When water freezes to ice, its volume increases. This can be quite obvious if you put a cup of a water in a freezer, or when a frozen water pipe bursts in winter. If you measure the mass of the larger volume of ice, it is the same as the original volume of water. This suggests that the temperature change does not involve an additional substance, and that the increase in volume is due to water particles moving further apart.

When we rub our hands together, when we pump up a bicycle tyre, when we drill a hole, it always gets hotter. All these motions suggest that the temperature change happens because the particles in a substance are somehow disturbed and move faster.


Copyright 2010 by Kai Hock. All rights reserved.
Last updated: 19 October 2010.