Internal Energy is the microscopic energy contained within a system formed by the disordered movement of molecules (kinetic energy), Potential energy, and the nuclear energy present within the atoms of these molecules. It should be noted that the kinetic energy of molecules present in the system and not the kinetic energy of the system is calculated in the Internal Energy.
The Internal Energy is denoted by ‘U’ and is measured in Joules (J). This Internal Energy can increase with the increase in temperature and change of state or phase (from solid to liquid to gas). Heat Reservoirs store this Internal Energy.
Different Substances will have different Internal Energies depending on the atom, temperature, bonds, pressure, etc.
There are two forms of Internal Energy namely Kinetic Energy and Potential Energy