In plain and simple terms, energy coupling can be explained as the process of initiating a reaction by using the energy produced as an output of any other previous reaction. Energy, which itself is the capacity of the possessing body to do work, exists in many varying forms in the Universe. We need energy for every biological activity, and this energy is provided by the chemical reactions taking place inside the body. Similarly, all external physical phenomena have their energy requirements.
It isn’t always possible to feed energy in any form to carry a process out as there are conditions upon the form of the initiating energy. This is where the concept of energy coupling kicks in. For instance, a fast moving body has inherent kinetic energy. If the purpose is to generate electricity, the requirement would be to set the generator in action, which further requires the rotation of a turbine. The kinetic energy of a fast jet of water is used to rotate the turbine, which eventually produces electricity, i.e., energy from the molecular motion (kinetic energy) is used as a raw material to create a different from of energy (electrical energy in this case).
On similar lines, let us consider a situation where an overhead tank contains stationary water, devoid of the above mentioned kinetic energy. However, by virtue of its height, this water pool is a source of untapped gravitational potential energy, which can be coupled from one form to the other, eventually generating useful electricity. The progression of this chain is pretty similar to the earlier discussed case. From a height, water is allowed to fall on a wheel, thus setting it in rotational motion. The potential energy of the stored water changes into mechanical energy in this way. The wheel rotation can then be used to create electricity, or electrical energy.
One of the greatest physicists of the nineteenth century, Joule was the first scholar to propose this energy coupling theory. In 1845, he undertook a very simple experiment that exhibited the concept of energy changing its form. He placed a paddle wheel in a water tank, and then cranked the wheel for some time. He observed that the temperature of the water inside the tank rose by a few degrees. Further experimentation established the fact that a fixed amount of energy, in whatever form, was required to affect an increase of a single degree. This pretty much explained the fact that energy could be coupled from one form to the other.
Another very simple and explicit example of energy coupling is exhibited by nature. The Sun reflects its heat in the form of solar energy all over the earth’s surface. This solar energy is gobbled up by plants in order to prepare their food with the aid of chlorophyll, in a process called photosynthesis. This results in a change in the form of the solar energy to an equivalent chemical form inside the plants. Animals and human beings consume these plant materials (chemical energy) and use it as mechanical energy to perform their routine tasks. And all this while, it is energy coupling in action!
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