Temperature

At the molecular level, temperature is related to the random motions of the particles (atoms and molecules) in matter. Because there are different types of motion, the particles' kinetic energy (energy of motion) can take different forms, and each form contributes to the total kinetic energy of the particles.

For example, when water squirts from a hose, part of the kinetic energy of the water is due to the movement of the molecules as a collection in a single direction out the nozzle. But the individual water molecules are also moving about with random, constantly changing, speeds and directions relative to each other. This kind of kinetic energy is called molecular translational energy. This energy remains even after the squirted water becomes a quiet puddle. The temperature of the puddle, or of any object, is a measure of the average of the individual translational energies of all of its atoms or molecules.

If a swimming pool were filled from this same hose, the total molecular translational energy of the molecules in the pool would be much greater than those in the puddle because there are many more molecules in the pool. The temperatures of the puddle and the pool, however, would be the same because temperature is a measure of the average molecular translational energies.

The molecules in a kettle of boiling water have a higher average molecular translational energy—a higher temperature—than those in the swimming pool. Place the kettle on the surface of the pool and the direction of energy flow is obvious: from hotter to cooler, from higher temperature to lower, from greater average molecular translational energy to lesser. These are three ways of saying the same thing.

The reason that heat flows from an object of higher temperature to one of lower temperature is that once they are in contact, the molecular agitation is contagious. Fast-moving molecules will collide with slower-moving ones, kicking them up to higher speed and thereby raising their translational energy.