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3-1: What is heat?

When an object is heated, its temperature rises. Everyone knows that, when an object of higher temperature is in contact with another of lower temperature, heat is transmitted from the higher to the lower temperature object. Here, what is heat?
It is known well at present that the heat transfer (heat conduction) is a transfer of energy; namely, heat is a form of energy. How can we explain this?
This problem was just the clue to make a relation between the macroscopic and the microscopic world.

[The Phlogiston Theory, the Caloric Theory]
Up to the beginning of the 19th century, combustion had been understood by supposing an invisible negative-mass substance named phlogiston. According to this theory, every combustible object contains phlogiston and combustion is explained as a process to lose the phlogiston. When such an object as mercury burns, the mass would increase. Looking at this fact, people thought that the mass of the phlogiston is negative.
The heat conduction had been explained by introducing an imponderable fluid called caloric. It was considered that a higher temperature object contains more caloric than a lower one, and the transfer of the caloric implies the heat conduction.
Lavoisier confirmed by carrying out quantitative chemical experiments precisely that combustion of an object is a process in which some substances combine with others. Thereby the theory of phlogiston was denied. Moreover, looking at an ultimate supply of heat by friction, people suspected the caloric theory and abandoned it after Joule's experiment that will be described below.

[Nature of Heat]
Using the apparatus shown in the following figure, J. P. Joule (UK, 1818 - 89) confirmed that heat is equivalent to energy, and measured the mechanical equivalent of heat that represents the quantitative relation between heat and mechanical energy.

The apparatus used by Joule
This is the apparatus used by Joule who confirmed the equivalence between heat and mechanical energy. When the weights go down, the impellers in a water tank rotate to stir the water. The potential energy of the weights is converted into the kinematical energy of the rotation of the impeller, and the temperature of the water rises.

[The Mechanical Equivalent of Heat]
The unit to measure heat is cal (calorie). The heat required to raise the temperature of 1 g of water by from to at 1 atmospheric pressure is 1 cal. The currently accepted value is

[Statistical Mechanics]
As learned in Part 1: (Prologue: Atomic Nature of Matter and Electricity, it became clear that a matter consists of a huge number of molecules and atoms. At first, the motions of these molecules and atoms are considered to be described according to the laws of the classical theory, i.e. Newtonian mechanics and Maxwellian electromagnetism.
Although we cannot watch the motions of molecules and atoms directly, we can connect them with the quantities in the macroscopic world, e.g. the pressure or the temperature, calculating an appropriate statistical average over a great number of degrees of freedom of those molecules or atoms. For example, the pressure of a gas is the average value of the forces acting on the wall of the container when the gas molecules colliding with it. And the temperature of the gas denotes the average value of the energies of individual molecules.
Thus the attempt in which the various laws in the macroscopic world are derived from the microscopic degrees of freedom of individual molecules and atoms is just statistical mechanics developed by J. C. Maxwell (UK, 1831 - 79) and L. Boltzmann (Germany, 1844 - 1906).
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