Internal energy is a state function of a system and is an extensive quantity, meaning it depends on the size of the system or the amount of substance it contains
. It is defined as the energy associated with the random, disordered motion of molecules, and is separated in scale from the macroscopic ordered energy associated with moving objects
. Internal energy is a thermodynamic potential and an extensive property
. Some key aspects of internal energy include:
- Microscopic origin : Internal energy arises from the molecular state of motion of matter, involving energy on the atomic and molecular scale
- Energy units : The symbol for internal energy is U, and its unit of measurement is the joule (J)
- Specific internal energy : This is an intensive thermodynamic property, represented by the lowercase letter u, which is the internal energy per mass of the substance in question. The SI unit of specific internal energy is J/g
- Kinetic and potential energy : In an ideal gas, the internal energy is the sum of the kinetic energies of the particles in the gas, as well as their potential energy due to intermolecular attractive forces
- Phase changes and chemical reactions : Internal energy is important for understanding phase changes, chemical reactions, nuclear reactions, and many other microscopic phenomena, as the possible energies between molecules and atoms are important
- Factors affecting internal energy : Internal energy can be altered by modifying the object's temperature or volume without altering its composition
. As a system's temperature increases, the internal energy also increases
