For a closed system the internal energy is essentially defined by
ΔU = q + W
Where
- U is the change in internal energy of a system during a process
- q is the heat
- W is the mechanical work.
If an energy exchange occurs because of temperature difference between a system and its surroundings, this energy appears as heat otherwise it appears as work. When a force acts on a system through a distance the energy is transferred as work. The above equation shows that energy is conserved.
The different components of internal energy of a system is given below.
| Thermal energy | Sensible heat | Energy change of a system associated with:Molecular translation, rotation, vibration.Electron translation and spin.Nuclear spin of molecules. |
| Latent heat | Energy required or released for phase change, change from liquid to vapour phase requires heat of vaporization. | |
| Chemical energy | Energy associated with the chemical bonds in a molecule. | |
| Nuclear energy | The large amount of energy associated with the bonds within the nucleus of the atom. |
The physical and chemical processes that can change the internal energy of a system is given below.
| Transferring energy across the system boundary by | Heat transfer | Energy transfer from a high temperature to low temperature state. |
| Work transfer | Energy transfer driven by changes in macroscopic physical properties of a system such as compression or expansion work. | |
| Mass transfer | Energy transfer by mass flowing across a system boundary. | |
| Change through internal processes | Mixing | Heat releases upon components mixing that may lead to lower internal energy. |
| Chemical reaction | Heat required or released during a chemical reaction that changes chemical energy. | |
| Nuclear reaction | Heat released during a nuclear reaction that changes nuclear energy. |