Forms of energy can be classified into several different methods based on different aspects. First of all, if classification is made by its essence, energy can be classified as external energy, internal energy, thermal energy, mechanical energy, chemical energy, and nuclear energy. These forms of energy have their own nature and moreover supply thermal, electrical, and mechanical energy that are needed in everyday life by converting into each other.
External energy is a type of energy related to the motion and position of an object. It consists of Kinetic Energy and Potential Energy. An object moving at a certain velocity has kinetic energy (E=1/2 mv2, m: mass, v: velocity) that works by exerting force on another object. An object in a high place has potential energy (E=mph, m: mass, g: acceleration of gravity, h: height) equivalent to its height. When the object falls, its potential energy decreases as the height decreases, whereas its kinetic energy increases as velocity of the falling object increases. The process of a satellite, launched from the ground, going into orbit of a certain attitude is an example of kinetic energy being converted into potential energy.
Internal Energy is a type of molecular energy that forms from objects or certain systems. When air in a sealed container (a system) is heated up by an external force, the kinetic energy of the air molecules increases and the system temperature also increases. In this case, thermal energy exerted is converted into internal energy of the air in the system and increases its temperature.
For example, in the case of hot-water heating, internal energy of hot water is converted into thermal energy and transferred to the air in a room. The transferred heat is used increasing the internal energy in the air resulting in an increase in room temperature.
Thermal energy is a type of energy that transfers between two objects with a temperature difference. Therefore, it exists only when it transfers from a hotter object to a colder object. When using the expansion properties of gas or steam, thermal energy can be converted into mechanical energy. A gas turbine or steam turbine is a device that converts thermal energy into more efficient mechanical energy.
Mechanical energy is classified into work of compression and expansion of gas and work of the rotation of axes. An example of the previous can be found in the reciprocating motion of a piston, and an example of the latter can be found in the rotation of axes of a steam turbine. A heat engine is a type of engine that converts thermal energy into mechanical energy, such as automobiles, steam engines, or gas turbines.
Chemical Energy is bond energy and potential energy between the atoms of molecules that form chemical species. All substances including coal, oil, and natural gas have their own chemical energy depending on types and bond structures of atoms that form their molecules. By converting a chemical species with a high energy level into ones with a low energy level through combustion or other chemical reactions, chemical energy releases energy that is relevant to the difference made in conversion in the form of thermal energy. Automobiles, aircrafts, and rockets have engines that convert the chemical energy of their fuels into thermal energy and again into mechanical energy—an efficient type of energy in our daily life.
Nuclear energy is a type of energy stored in the form of fusion power of particles, such as protons and neutrons, that compose the nucleus of an atom. This is converted into a form of thermal energy through a nuclear fission or fusion process. A heavy and unstable atomic nucleus releasing enormous thermal energy (E=mc2, m: mass defect, c: speed of light) equivalent to mass defect accompanied by splitting in two similar atomic nuclei by neutrons is called nuclear fission.
On the other hand, nuclear fusion is two light nuclei changing into one heavier nucleus through fusion. This also generates enormous energy as mass defect occurs in the process.
In addition, there is elastic energy inherent in a compressed spring, radiant energy contained in sunrays, and electric energy generated by difference in voltages.