We’ve discovered several puzzles in the last two chapters:

• Why does the ideal-gas law work for every gas?

• Why is the molar specific heat the same for every monatomic gas? And for every diatomic gas? And for every elemental solid?

• Just what does temperature actually measure?

We can resolve these puzzles and understand many properties of macroscopic systems by studying the microscopic behavior of the system’s atoms and molecules. This is the micro/macro connection.

《 LOOKING BACK Sections 19.3, 19.4, and 19.5 Heat, the first law, and specific heats

Gases have pressure due to the collisions of the molecules with the walls of the container. We’ll find that we can calculate an average molecular speed, the root-mean-square speed, by relating the ideal-gas law to a microscopic calculation of the gas pressure.

At a microscopic level, temperature measures the average translational kinetic energy of moving atoms and molecules. We will use this discovery to explain why all monatomic (and diatomic) gases have exactly the same molar specific heat.

《 LOOKING BACK Section 19.7 The specific heats of gases

Two thermally interacting systems reach a common final temperature because they exchange energy via collisions. On average, more-energetic atoms transfer energy to less-energetic atoms until both systems have the same average translational kinetic energy.

Entropy measures the extent to which a macroscopic system spreads its thermal energy among all the atoms and molecules that make up the system. This is a subtle but powerful idea. The second law of thermodynamics says that a system’s entropy always increases, never decreases, until it reaches a maximum when the system is in thermal equilibrium. Entropy and the second law will help us understand why heat energy flows spontaneously from hot to cold but never from cold to hot.