Internal energy

Internal energy is a thermodynamic state property representing the microscopic kinetic and potential energies of a system’s constituents, excluding the system’s overall motion and position.

First law of thermodynamics: The energy-balance principle stating that a system’s internal-energy change equals heat added minus work done by the system. It relates internal-energy changes to heat transfer and work.
Thermodynamic system: A chosen quantity of matter or region of space studied using thermodynamic variables and laws. The system boundary determines which microscopic energies count as internal.
Kinetic energy: Energy associated with the motion of an object or collection of matter. Bulk kinetic energy from a system’s overall motion is excluded from its internal energy.
Ideal gas: A model gas whose particles have negligible volume and no intermolecular forces except during elastic collisions. An ideal gas’s internal energy depends only on temperature.
Quantum statistical mechanics: The statistical framework for predicting macroscopic properties from quantum states and energy levels. It replaces classical equipartition when microscopic energy levels are quantized.
Heat: Energy transferred between systems because of a temperature difference. Heat transfer changes internal energy without being energy stored as heat within a system.
Thermodynamic state: The condition of a system specified by its thermodynamic properties at a given time. Internal energy is fixed by the system’s state, not by the path used to reach it.
Potential energy: Energy associated with position, configuration, or interactions in a force field. Microscopic interaction energy can be internal, while external position energy is not.
Specific heat capacity: The heat required to raise the temperature of a unit mass of a substance by one degree. It quantifies the temperature change associated with an internal-energy change.
Ground state: The lowest-energy quantum state of a system. Quantum systems can retain nonzero ground-state energy even at absolute zero.

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