Physics Bootcamp

Prior to the nineteenth century, scientists thought that the sense of hot and cold was related with the amount of heat a body contained. The theory was called the caloric theory. In the caloric theory, heat is considered to be a “weightless fluid” that flowed from a hot body to a cold body with the total quantity being conserved. This viewpoint of heat is no longer held since experiments have suggested better ways of looking at the phenomena involving heat as we will see below.

In 1798 Count Rumford, born as Benjamin Thompson in Massachusetts, USA, presented a paper to the Royal Society of Great Britain in which he described the results of a series of experiments conducted in Munich, Germany concerning heat associated with the boring of cannons. He had observed that in the drilling of canons, the cannons became blazingly hot and water poured to cool them also became very hot. After a series of careful measurements he found that the amount of heat released into water would be enough to completely melt the canon if it could be returned to the metal. Therefore, he concluded that more heat was being released in the drilling process than could have been originally contained in the metal. This observation contradicted the caloric theory. Rumford concluded that it was the mechanical process of boring that was responsible for heat production and not the supposedly heat present in the metal beforehand. In his own words,

“It appears to me extremely difficult, if not impossible, to form any distinct idea of anything capable of being excited and communicated in the manner that heat was excited and communicated in these experiments, except it be motion.”

As a result, physicists think of heat as a form of energy in transit as opposed to an energy, such as kinetic energy, that an object contains; i.e., no object contains heat but flow of heat can change the energy of an object. When a body at higher temperature comes in a direct contact with another body at a lower temperature, then, the energy flowing from the body at higher temperature to the body at lower temperature is said to be heat. As a result the energy of the hotter body goes down and the energy of the colder body goes up.

Rumford also calculated the amount of heat produced in the boring of a cannon and related it to the mechanical energy put in for boring thereby finding the first reported value of the mechanical equivalent of heat. His value expressed in the unit of heat (calories) and the British unit of mechanical energy (Foot-pound) would be \(3.15 \) foot-pound per calorie, which is approximately \(1.5\%\) higher than the presently accepted value. Count Rumford also invented many other useful devices, such as the kitchen range, the double boiler, the pressure-cooker and the drip coffee maker.

A clearer demonstration of the mechanical equivalence of heat was provided by James Joule in England in 1840’s. To demonstrate the equivalence of mechanical energy and heat, Joule constructed a device where he could measure mechanical energy and thermal energy at the same time. In his device, a sketch of which is shown in Figure 19.1, paddle wheels attached to a shaft were placed in a set amount of water in a copper cylinder. The shaft rotated when the strings wrapped over it unwound when the two masses fell. Joule expected that the rotating paddle wheels would force the water molecules to rub against one another and produce heat as a result of the friction between the paddle wheel and the liquid.

The loss in potential energy of the falling masses provided a measure of the mechanical energy introduced in the system and the change in temperature was measured to figure out the amount of thermal energy. In Joule’s set-up, the change in temperature was expected to be very small, approximately 0.2 degrees Celsius. To measure such small differences in temperature Joule constructed his own thermometer, and conducted experiments in a wine cellar, which provided a stable temperature environment. He found that the loss in the mechanical energy was directly proportional to the rise in the temperature. He was also able to find a numerical value of the proportionality constant between work and thermal energy, which we now call mechanical equivalence of heat.