. What is Internal energy (U or E)
It is the energy associated with a system by virtue of its molecular constitution and the motion of its molecules. The contribution of energy due to molecular constitution is known as internal potential energy and the contribution of energy due to the motion of molecules is called internal kinetic energy. Internal energy of a system is given by the sum of these two types of energy.
Every substance possesses a definite amount of energy which depends upon factors such as composition of the substance, temperature and pressure. This is called intrinsic or internal energy (E).
E = ET + ER + EV + EB + EE ………