Thermodynamics:
Heat and Enthalpy

As described in the Energy module, any moving thing has kinetic energy. Since all matter is made up of atoms, ions, and molecules which are constantly moving, all objects have a form of energy within them. This energy is called heat .

Heat is often described as if it were a fluid (like water) that can be transferred from one object to another. Even scientists talk this way when they say things like "50 J of heat were added to the metal bar." It is important to realize that this is just a simplification of what is really going on.

You will often hear descriptions of processes that sound like heat is being scooped out of or being poured into objects. Remember what is really going on: atoms are speeding up or slowing down so that their kinetic energy increases or decreases.

For example, take the metal bar that was described above. When someone says "50 J of heat were added to the metal bar," what he or she is really saying is that the bar was heated so that it had 50 J more heat. It was probably placed in a hot water bath or heated in a bunsen burner flame, for example. The atoms in the bar were bombarded by the water molecules or ions in the burner flame. On average, the atoms in the bar started to move faster than they were, while the water molecules or flame ions started to move more slowly. Thus the kinetic energy of the metal atoms increased while the water molecules or ions in the flame lost kinetic energy. Thus heat energy was transferred from the water or flame to the metal.

Imagine that you have two metal bars with the same composition and the same mass. The only difference between them is that one bar has more heat (a higher average kinetic energy) than the other. What do you think you would feel if you were to touch the two bars?

Click on the metal bars below to see the macroscopic effect of an object's higher or lower heat. Click on the mouse icon at left to reset the image.

The macroscopic feature of objects with a higher heat is a higher temperature . Hotter objects have a higher average kinetic energy and a higher temperature; cooler objects have a lower average kinetic energy and a lower temperature. Temperature is measured by comparing the object you want to measure to a calibrated standard, like a tube of mercury or alcohol in a thermometer. If your object and the alcohol in the thermometer have the same average kinetic energy, then they must have the same temperature.

One other important note about heat and temperature: the kinetic energies we are talking about are just averages. The atoms, molecules, and ions in objects have a range of speeds and kinetic energies: some are moving faster and others are moving slower. But it is their average speed that determines the amount of heat an object has and what its temperature is.

Heat and Temperature