<Tr> <Td> <P> </P> </Td> </Tr> <P> (3) </P> <P> Internal energy is the sum of all microscopic forms of energy of a system . It is the energy needed to create the system . It is related to the potential energy, e.g., molecular structure, crystal structure, and other geometric aspects, as well as the motion of the particles, in form of kinetic energy . Thermodynamics is chiefly concerned with changes in internal energy and not its absolute value, which is impossible to determine with thermodynamics alone . </P> <P> The first law of thermodynamics asserts that energy (but not necessarily thermodynamic free energy) is always conserved and that heat flow is a form of energy transfer . For homogeneous systems, with a well - defined temperature and pressure, a commonly used corollary of the first law is that, for a system subject only to pressure forces and heat transfer (e.g., a cylinder - full of gas) without chemical changes, the differential change in the internal energy of the system (with a gain in energy signified by a positive quantity) is given as </P>

The sum of the kinetic and potential energies of all the particles in a system is called its