The concerted and unrelentingefforts and the rational attitudes of scientific people during the centuries following Renaissance to understand the rules of Physical Universe are most wonderful and quite awe inspiring.As an example we can study the efforts made by scientific minded people to understand the thing called “heat”.
The heat we get from flames and fires, the endless heat coming from sun, the nature of heat flow in metals, the phenomenon of melting and boiling, heat radiation coming from hot hill sides and hot boulders in summer– all such things made the scientific peopleto study and investigatethe subject of heat in a scientific way. But such a study 300 or 400 years back was like a long journey to a pathless land..
Luckily necessary environment was already existing after the very accurate definitions ofprimary physical units for Length, Mass and Timeand secondary units like Density,Pressure etc,invention of physical and chemical balances,start of study of pure chemical substances, codifying their physical and chemical reactions etc.
At that time the studies about nature of “Heat” were also being conducted by some other “natural philosophers” (the physicists as they were called in those times). The scientists who want every thing to be measured and to be proved by experiment wanted units for “heat” too.For this purpose they carried out a systematic study of the ordinaryboiling and melting. They felt that “melting point of ice” and “boiling point of water” can be taken as the two “standard heat levels” because every person on earth daily comes across these two phenomena.This initial decision (to treat these two heat levels as very important to science) helped a lot to further the systematic research in the subject.
The scientific people soon proposed use of a “mercury thermometer” (a device to measure levels of heat of different bodies). The steady levels of the mercury at the melting point of ice and boiling point of water were “declared” as two standard heat levels and were marked on the “thermometer”. The distance in between these two levels was divided into one hundred equal parts called “degrees of temperature”. The investigators might first have been wonderstruck when they saw that when ice melted or water boiled the mercury level did not change until the change of state ended. They found laterto their surprise thatthe stoppage of temperature during the melting and boiling processes to be true for all pure solids, crystals and liquids. This finding now a very ordinary thing for usmust have been treated in those times as a great discovery !
Defining unit of temperature was a great step forward. But from here we should go by big leaps to reach the modern age in Physics. Temperature was only heat level. For example by using the mercury thermometer we can most accurately differentiate between heat levels of different substances(manly liquids in those distant times)by immersing the thermometer bulb in those substances. Early scientists could not fully realize the fact that heat was a form of energy. In those early times the term “energy” meant only mechanical energy, muscular energy etc.
They treated “heat” as some thing of a fluid like the water hiding inside wet cloth. It was called “caloric fluid”.They felt that when one heated a rather big sized iron ball the “caloric fluid” i.e. heat goes on filling the ball just as water poured into a drum slowly fills it! To measure this “caloric fluid” they defined a unit –the quantityof the mysterious“caloric fluid” that would newly enter inside a gram weight of water when it is heated by one degree. This unit quantity of the invisible caloric fluid was called “Calorie”. The most important idea (which we now know is wrong as per quantum theory of radiation but is still very useful in getting a preliminary understanding of heat) was that each homogenoussubstance likemetal, wood, rubber,stone ,water, alcoholshould have very specific “heat capacity” orindividualheat absorbing capacity peculiar to that substance. This very specific “heat capacity” of a substance was defined as the number of calories of heat fluid that would be required by just one gram of that substance for a rise of one degree temperature.
A French scientist –engineer Count Rumford proved in 1798 that this idea is wrong.. He accidentally observed with amazementthat asmall but thick solid piece ofiron fixed in a frame when it is subjected to very strong friction (as when being bored into thick-walledtubes for making guns) release large unlimited quantities of heat for unlimited timenot in keeping with thesmall size of the iron piece. Drums of water could be heated by the heat coming out of the friction from this small iron piece!
So scientists had a feeling that heat is a form of energy just like friction, mechanical energy etc and that the caloric theory is quite wrong. They came to conclusion that heat is just like mechanical energy. we knowkinetic energy ( mechanical energy of motion) which is possessed by objects in motion like fly wheels, heavy objects falling down and we knowpotential energywhich is possessed by bodies like tied springs, heavy objects lying at a height trying to roll down but preventedby an obstacle from rolling. Scientists boldly concluded that heat is similar to those forms of energy. Soon it was proved that mechanical energy can be converted into heat and vice versa and that there is an equivalence factor (the mechanical equivalent of heat). .
Scientists then turned to study of “radiant heat energy” i.e. heatcoming directly from hot bodies like Sun, fire, burning candle, burning electric bulb etc without direct .Heat is receivedin two ways by direct contact with a hot body or through a medium (either by conduction or convection) or by“radiation” without need of a medium. In a medium heat flows from one molecule to another by conduction and convection.In cases like heat energy coming from sun there is no need of a medium. It can radiate through vacuum for even millions of miles. Scientists realized that all objects above “-273 degrees centigrade” radiate heat energy. Not only hot objects but even cold objects radiate energy if they are above “-273 degrees centigrade”. It is a two way process.
(If a hot body and a cold body are put side by side both radiate heat outwardly and towards each other. We observe only the net effect and feel as if only the hot body is radiating heat towards cold body. Also there is nothing like “cold” or a “cold energy” radiating outward like heat.We should clearly understand that the entire space in universe lying far away from a star (which is a “sun” and a hot burning sphere exactly like our sun) is at an extremely cold subzero temperature. When the sun sets in the eveningthe air all around us on the earth retains the heat and keeps us all night “air-conditioned”. In early hours before sunrise wefeel a little pinch of the “great cold” of the universe because the wind grows a little cold. But soon the hot burning sphere of sun is up and heats all the air. In very cold winter times it is the extremely cold temperature of the universe which makes our hot bodies shiver day and night.You are face to face with the all enveloping cold of the universe. If there is winter in your part of the world it also means that the small area of the earth in your and neighboring districts are located is relatively very far from the sun and so the sun is not so hot as it was a few weeks back. In addition the sun light is falling at a very slanting angle. We are surrounded by terrible cold of universe and keep warm only because of the hot sun.)
The spectroscopic study of “light” (the ordinary light with which we see) completely changed views of scientists about radiant energy.Heat can also be treated as a radiation just like light. Some radiations affect the skin while light radiation is recognized only by our eyes. By studying the visible light spectra of from hot self luminous bodies like flame of candle, burning wood etc with the help of spectroscope scientists learnt many facts about nature of both light and heat.In addition to visible light, there are many other radiations.Radiation does not mean justradiant light but also other invisible radiations like infrared, heat and ultra violet radiations known in first part of nineteenth century and also the most harmfulx-radiation and gamma radiation which were discovered towards end of nineteenth century.
It was realized that heat is just a part of “electromagnetic” spectrum i.e. electromagnetic wave system.At that time velocity of light was calculated very accurately and ingeniously by using optical apparatus (using high speed rotating mirrors). Advanced studies in fields of electricity and magnetism and the realization of electrical nature of chemical reactions, production of electric currents by voltaic cells, prediction of existence of radio waves –all lead to realization that all radiations are part of a single electromagnetic spectrum. Prof.Clerk Maxwell one of the greatest mathematical physicists of all timefound that the theoretical calculation of velocity of just predicted radio waves ( the “real electromagnetic waves” produced by electrical and magnetic apparatus) exactly equaled “the velocity of light” calculated in purely optical experiments by others. ProfMaxwell declared in 1865 thatall radiations like light ,heat, ultraviolet radiationare only electromagnetic waves and are all same but only of different sizes (just as waves in a tub, waves in a pond and waves in a sea are different in size but identical in nature)
It was a great luck for scientific community that Dr.Max Planck of Germanypropounded in 1900 that heat waves travel as “quanta”( i.e as little fixed size pieces of the continuous wavejust like fixed size pieces of thread cut from an endless reel of sewing thread). He had to do this to satisfactorily explain some anomalies in experiments on heat radiation which were beyond experimental error. This new idea of propagation of heat in fixed size “quanta” was forgotten entirely for a few decades and was even laughed at for a long time.
.Dr. Rutherford declared in 1911 that electrons should orbit a positive heavy nucleus but could not explain some anamolies in respect of Hydrogen spectrum . Dr Niels Bohr with his extraordinary intellect examined minutely ( from the view point of quantum theory of dr max planck)the already existingand most accurate experimental data for Hydrogen spectral linesthe famous Lyman,Balmer and Paschen series . Dr.Bohr applied in 1922 quantum theory of Dr.Max Planck very successfully to explain the hydrogen spectra to a large extent..soon scientists applied the quantum theoryto all electromagnetic radiations including “heat radiation”.
So finally we understand nature of Heat energy. Heat radiation like any other electromagnetic radiation is given out when electrons jump out ward from high-energy inner orbits to lower energy outer orbits in atoms comprising all matter.
Heat is just a part of the electromagnetic spectrum. It is the ever present energy radiation which is seen in almost all chemical reactions (including the chemical reactions we encounter everyday or in ordinary heating of objects). Some times heat is released in the reactions and sometimes heat is absorbed.