There are three directions for energy conversion:
1, internal energy, the candle melted, which is the performance of internal energy improvement.
2, light energy, candles will glow when burning.
3, heat energy, candle burning gives off heat.
From a microscopic point of view, internal energy is the statistical average of the sum of the energies of irregular molecular movements. The energy of random motion of molecules includes kinetic energy of molecules, the potential energy of intermolecular interaction and the energy of internal motion of molecules. The internal energy of an object does not include the kinetic energy when the object moves as a whole and the potential energy in the gravity field.
In principle, the internal energy of an object should include the sum of nuclear kinetic energy, potential energy, chemical energy, ionization energy and nuclear energy of all microscopic particles, but in the process of general thermodynamic state changes, the molecular structure, atomic structure and nuclear structure of a substance do not change, so these energy changes can be ignored. However, when chemical reactions are involved in thermodynamic research, chemical energy needs to be included in internal energy.
The essence of internal energy:
Internal energy is the inherent property of objects and systems, that is, all objects or systems have internal energy, regardless of whether the outside world exists or not, whether the outside world has an influence on the system. Internal energy is a kind of extensive quantity (or capacity property), that is, the size of internal energy is directly proportional to the quantity (quantity or quality) of matter when other factors remain unchanged.
When a system changes from the original equilibrium state to another new equilibrium state, the change of internal energy only depends on the system state before and after the change, and has nothing to do with how the change occurs (such as the speed of change) and how the change goes through a tortuous process (such as isothermal process, isobaric process or arbitrary process).
This property of internal energy is essentially different from work and heat.