Different Forms of Energy

Energy can manifest itself in a number of forms. We have already seen that it can be in the form of mechanical energy, which is subdivided into kinetic energy and potential energy. Potential energy can arise due to different causes. For instance, as we have seen, there is potential energy due to gravitation and in the case of a spring, it is due to the elastic forces acting within the spring. We shall describe other forms of energy very briefly.

Sub Topics

Internal energy
Heat or thermal energy
Chemical energy
Electrical energy
Mass-energy equivalence
Nuclear energy

Internal energy

Kinetic and potential energies that we have so far considered, refer to the object under consideration, as a whole. On the other hand, any given body contains energy because of the motion of the molecules in it. As molecules vibrate with respect to one another, there will be both kinetic energy of motion and the potential energy, due to mutual attractions and repulsions. The totality of this energy is the internal energy. The temperature of the body goes up as thermal energy or the internal energy increases. This is the case when work is done against friction. We rub our hands together, to keep them warm. Kinetic energy of our hands is converted into internal energy by friction.

Heat or thermal energy

A hot body can do work as it cools. For example, in a steam engine, steam expands and cools, moving a piston which does mechanical work, say, on the wheels of the locomotive. Thus, some energy is associated with heat. In terms of the molecules of the hot body, heat is related to their disorderly motion and to their mutual potential energy, i.e., to internal energy. The science of relation between heat, internal energy and temperature is thermodynamics.

Chemical energy

A stable chemical compound has less energy than its separated parts, the difference being in the specific arrangement and motion of electrons and nuclei in the compound. This difference is called chemical energy or energy of chemical binding. In a chemical reaction, energy can be absorbed or released, depending on whether the total energy of the reactants is less or more than that of the products (endothermic or exothermic reaction - examples are hydrolysis and burning of coal, respectively).

Electrical energy

Electric charges and current attract or repel each other, i.e., they exert forces on each other. Thus, work needs to be done in order to move them with respect to one other. The energy associated with this work is called electrical or electromagnetic energy.

Mass-energy equivalence

Albert Einstein, in the beginning of this century, showed that matter (whatever the form) was equivalent to energy. The energy associated with a mass 'm' (just because of its existence) is given by the famous formula of Einstein, E = mc² ------------ (1).

where 'c' is the speed of light in vacuum, which is approximately equal to 3 x 10⁵ km/s. Thus, even a minute amount of matter gives rise to an enormous amount of energy, if this conversion can be made. A collision between an electron and a positron (oppositely charged version of the electron), can lead to their total annihilation and the production of pure energy (electromagnetic radiation).

Nuclear energy

Neutrons and protons attract each other at distances of order of 10^-¹⁵m, and bind (come together) to form nuclei. The associated energy is called nuclear energy. For example, because of this attraction, a helium nucleus (consisting of two protons and two neutrons) has less energy than the separated constituents. Infact, a Helium nucleus weighs less than two protons and two neutrons. The fusion of the latter produces the former, and therefore, releases energy equal to mass difference (excess) times c². This is the basic process of energy production in stars. Nuclear fission, in which a less stable heavy nucleus such as ²³⁵U, breaks up into two more stable nuclei and some neutrons, is also a well known example of nuclear energy production (nuclear reactors and atom bomb).