Thermal physics is essentially the study of heat, temperature, and heat transfer. As we shall see—particularly when we look at the Second Law of Thermodynamics—these concepts have a far broader range of application than you may at first imagine. All of these concepts are closely related to thermal energy, which is one of the most important forms of energy. In almost every energy transformation, some thermal energy is produced in the form of heat. To take an example that by now should be familiar, friction produces heat. Rub your hands briskly together and you’ll feel heat produced by friction.

When you slide a book along a table, the book will not remain in motion, as Newton’s First Law would lead us to expect, because friction between the book and the table causes the book to slow down and stop. As the velocity of the book decreases, so does its kinetic energy, but this decrease is not a startling violation of the law of conservation of energy. Rather, the kinetic energy of the book is slowly transformed into thermal energy. Because friction acts over a relatively large distance, neither the table nor the book will be noticeably warmer. However, if you were somehow able to measure the heat produced through friction, you would find that the total heat produced in bringing the book to a stop is equal to the book’s initial kinetic energy.

Technically speaking, thermal energy is the energy associated with the random vibration and movement of molecules. All matter consists of trillions of trillions of tiny molecules, none of which are entirely still. The degree to which they move determines the amount of thermal energy in an object.

While thermal energy comes into play in a wide range of phenomena, SAT II Physics will focus primarily on the sorts of things you might associate with words like heat and temperature. We’ll learn how heat is transferred from one body to another, how temperature and heat are related, and how these concepts affect solids, liquids, gases, and the phase changes between the three.