Internal energy is a state function of a system and is an extensive quantity. It is the sum of the kinetic energy due to the motion of molecules and the potential energy associated with the vibrational motion and electric energy of atoms within molecules. Internal energy also includes the energy in all the chemical bonds. The internal energy of a system is identified with the random, disordered motion of molecules; the total (internal) energy in a system includes potential and kinetic energy/Thermodynamics/Energies_and_Potentials/Internal_Energy). The internal energy of an isolated system cannot change, as expressed in the law of conservation of energy, a foundation of the first law of thermodynamics.

Key points about internal energy in thermodynamics include:

• Extensive property: Internal energy is an extensive property, meaning its magnitude depends on the amount of substance in a given state.

• State function: Internal energy is a state function of a system, meaning it depends only on the internal state of the system and not on the particular choice from many possible processes by which energy may pass into or out of the system.

• Change in internal energy: For a closed system, with matter transfer excluded, the changes in internal energy are due to heat transfer and due to work done on or by the system.

• Internal energy change equations: The first law of thermodynamics expresses the change in internal energy as ΔU = q + w, where q is heat and w is work. For an isolated system, the change in internal energy is zero, and energy is conserved/Thermodynamics/Energies_and_Potentials/Internal_Energy).

In summary, internal energy is a fundamental concept in thermodynamics that describes the total energy of a system due to the motion and interactions of its constituent particles. It is an extensive property that depends on the amount of substance in a given state and is a state function of the system. The change in internal energy is due to heat transfer and work done on or by the system, and it is expressed by the first law of thermodynamics.