What the heat is???

Heat has always been perceived to be something that produces in us a sensation of warmth, and one would think that the nature of the heat is one of the first things understood by mankind. Actually the mankind is looking in wrong direction. The nature of heat is very different from what we are thinking about, from millions of years. It is quite amazing to know that the thing which we are considering the simple one, and we are claiming ourselves to be the true interpreters of it, is still unknown to us until the beginning of middle of nineteenth century.

It is the middle of nineteenth century, when we had a true physical understanding of the nature of heat due to the development of kinetic theory. Actually the kinetic theory treats the molecules of a substance as tiny balls that are in motion and thus possess kinetic energy. These tiny balls (molecules) are capable of rotating about their own axis, vibrating about their mean position, and translation from one place to another. These movements arise due to intermolecular interactions, external disturbances and any other driving force. These movements, especially the translational one, are what we called heat. And the warmth we sense is actually the effect of these movements and nothing else. We are considering this warmth as a substance, and that is totally wrong. But there is another surprise here. In the mid of nineteenth century although we are able to relate that warmth with the movement of molecules but still we are not looking in true direction. At that time a French chemist Antoine Lavoisier proposed caloric theory. He says that although heat is generated due to molecular movements but it flows like a fluid like substance that is massless, colorless, odorless and tasteless substance that can be poured from one substance to another. Note that he is thinking the heat as generated by the movement of molecules rather than saying that heat itself is the movement of molecules. He says that when heat (which he called caloric) is added to a body its temperature increased; and when caloric was removed from the body, its temperature decreased. When a body could not contain any more caloric, much the same way as when a glass of water could not dissolve any more salt or sugar, the body was said to be saturated with caloric. And you will be surprised to know that the words like saturated liquid and saturated vapors which we are using today are based on this interpretation of caloric theory. So that is how the words saturated liquid and saturated vapor emerge. But there is another surprise here. Recent experiments shows that the thermal conductivities of the same material at same operating conditions are not same on NANO scale. But our recent concepts about heat suggests that it must be same. But a great deviation is experienced when two specimens of same material, one on large scale and other on NANO scale are tested for thermal conductivities. These results astonished the scientists, leaving room to again think about that what heat is??

Source: Heat and Mass transfer by Cengel ….

Research By: Engr. Ammar Aziz