Heat energy and temperature

Over the past few centuries scientists have put forward some very strange theories concerning the nature of heat. One of these was that heat was some sort of fluid that you added to a body to make it hot and removed from a body to cool it down! Whatever heat was, the result of its addition or removal was clear - the temperature of the body rose or fell.

We must therefore consider the change in temperature of a body to be related to the change in the heat content of that body.

During the last century two men, Rumford and Joule, proposed that heat was related to energy, indeed that heat was itself a form of energy. Davy showed that even cold objects like blocks of ice could be melted if they were rubbed together. In 1843 Joule performed his classic paddle wheel experiment, in which water was heated by friction from a rotating paddle wheel driven by the loss of potential energy from a falling mass. We can summarise their results as:

To heat up a body requires energy. This energy increases the internal energy of the body by increasing the kinetic energy of its molecules and so the temperature of the body rises.

Fixed points

Standard reference temperatures (fixed points) are used when calibrating thermometers.

Primary reference temperatures

Secondary reference temperatures

The following temperatures are used as practical fixed points in different temperature ranges: