http://www.ck12.org Chapter 13. Heat
13.3 Heat
Objectives
The student will:
- Explain the relationship between heat and energy transfer.
- Describe how the calorie is a measure of energy.
Vocabulary
- calorie: The amount of energy needed to increase the temperature of 1 gram of water by 1 °C at standard
atmospheric pressure (101.325 kPa). This is approximately 4.2 joules. - heat: The sum of all the kinetic energies of all the molecules in an object or substance.
Introduction
Heatis yet another common word that has a very specific meaning in physics. We place our hands in front of a
fire and speak of heat. We step with bare feet onto a tiled floor in an unheated room during winter and our feet feel
cold. We speak of “losing heat.” Human beings have expressed comfort and discomfort concerning the pleasant and
unpleasant effects of heat since the advent of language. But an understanding of the nature of heat was not a simple
affair. Even as late as the 18th century it was still not properly understood. For example, it was still commonly
accepted that if two objects had the same temperature then they “contained” the same amount of heat. We know
today that this is not true. During the latter half of the 18th century, the idea that heat was a form of matter called
caloric became generally accepted. This, too, was proven incorrect. It was not until the 19th century that physicists
were finally able to explain the nature of heat.
Understanding Heat
By the mid-19th century, R.J. Mayer constructed the principle of the conservation of energy. He based his premise
on an experiment comparing mechanical energy and heat. Mayer’s ideas were taken up by James Prescott Joule,
whose work was discussed earlier. One of Joule’s famous experiments involved heating a fluid mechanically, using
friction. In Joule’s experiment, a paddle wheel inside a container of fluid was rotated, resulting in an increase of the
fluid’s temperature. The rise in the temperature of the fluid indicated that the work done in rotating the paddle wheel
was equivalent to heating up the fluid (with a flame, for example). Heat could therefore be viewed as a transfer of
energy, involving a temperature change. Thus the definition of heat is given as:
Energy that is transferred from one object to another object due to a temperature difference between the objects.
To restate: When we speak of heat, we do not mean a material substance, nor do we mean a form of energy. Again,
by heat we mean a transfer of energy.