What is Internal energy? — Physics

Definition 20.10 University Physics — Year 1 · Chapter 20 — Kinetic Theory and the Perfect Gas

The internal energy $U$ of a system is the sum of the kinetic energies of its molecules in the frame where the system is at rest and of their mutual potential energies; it is an extensive state function.

Examples

Example 20.12 (A room’s worth)

The $8300\,\mathrm{mol}$ of air in the classroom at $300\,\mathrm{K}$ : $U = \tfrac52 \times 8300 \times 8.314 \times 300 = 5.2 \times 10^{7}\,\mathrm{J}$ — the energy of a liter and a half of gasoline, held in molecular motion. Warming the room by $1$ K costs $\tfrac52 nR = 170\,\mathrm{kJ}$ (at constant volume).

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