16.1 Internal energy

Internal energy U is the total microscopic kinetic and potential energy of a system. Its change depends only on initial and final states, not on the path.

Separate state variables from process quantities such as heat transfer and work, which depend on how the change occurs.

A gas can reach the same final temperature by slow compression or heating; its ∆U is the same if the initial and final states match.

Internal energy is not “heat contained” in an object, and work is not a state variable.

Raising temperature increases average microscopic kinetic energy, so internal energy generally rises; phase changes may instead change microscopic potential energy at constant temperature.

Identify whether energy enters as heating, work or both, and distinguish temperature change from state change.

Heating a solid raises its temperature until melting begins; during melting, energy continues to increase internal energy while temperature stays nearly constant.

Temperature is not a direct measure of total internal energy: mass, material and phase also matter.