The first law of thermodynamics is a very general statement about energy and its conservation. A system’s thermal energy changes if energy is transferred into or out of the system as work or as heat. That is,

《 LOOKING BACK Sections 10.4 and 10.8 Conservation of energy

Work $W$ is the transfer of energy in a mechanical interaction when forces push or pull. Work is done on a gas by changing its volume.

• $W>0$ (energy added) in a compression.

• $W<0$ (energy extracted) in an expansion.

《 LOOKING BACK Sections 9.2–9.3 Work

Heat $Q$ is the transfer of energy in a thermal interaction when the system and its environment are at different temperatures.

• $Q>0$ (energy added) when the environment is hotter than the system.

• $Q<0$ (energy extracted) when the system is hotter than the environment.

Heat energy can be transferred between a system and its environment by

• Conduction

• Convection

• Radiation

• Evaporation

Heat can cause either a temperature change or a phase change. A material’s response to heat is governed by its specific heat, its heat of fusion, its heat of vaporization, and its thermal conductivity. These are thermal properties of matter. An important application is calorimetry—determining the final temperature when two or more systems interact thermally.

The first law of thermodynamics is a very general statement that energy cannot be created or destroyed—merely moved from one place to another. Much of the machinery of modern society, from automobile engines to electric power plants to spacecraft, depends on and is an application of the first law. We’ll look at some of these applications in more detail in Chapter 21.