SAT Physics Subject Test

Chapter 15 Thermal Physics


Unlike the condensed phases of matter—solid and liquid—the atoms or molecules that make up a gas do not move around relatively fixed positions. Rather, the molecules of a gas move freely and rapidly, in a chaotic swarm.

A confined gas exerts a force on the walls of its container because the molecules are zipping around inside the container, striking the walls and rebounding. The magnitude of the force per unit area is called pressure, and is denoted by P.

P = 

The SI unit for pressure is the N/m2, the pascal (abbreviated Pa). As we’ll see, the faster the gas molecules are moving, the more pressure they exert. Be careful not to confuse pressure and force. Pressure is force divided by the area over which the force acts. For example, if you placed a box weighing 1,000 N on a tabletop whose area was 1 m2, then the pressure would be 1,000 pascals. But if that same box were placed on a thin vertical column whose top had an area of only 1 cm2 (or 10–4 m2), then the pressure would be 10,000,000 pascals. In each case, the force was the same,but the areas over which the force acted were different, which is why the pressures were different.

We also need a way to talk about the typically vast numbers of molecules in a given sample of gas. One mole of atoms or molecules contains

NA = 6.022 × 1023

of these elementary quantities. The number NA is known as Avogadro’s constant, and the mass of one mole of any substance is its atomic or molecular mass (these values are given in the Periodic Table of Elements). For example, the mass of a carbon-12 atom (the most abundant isotope of carbon) has a mass of exactly 12 atomic mass units, and a mole of these atoms has a mass of 12 grams. Oxygen has an atomic mass of 16 g, so a mole of carbon dioxide (CO2), which is made up of one carbon atom and two oxygen atoms, has a mass of 12 g + 2(16 g) = 44 g.