2. 1st law

2. 1st law - Tan 2. THE FIRST LAW OF THERMODYNAMICS The...

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Tan 1 2. THE FIRST LAW OF THERMODYNAMICS The First Law of Thermodynamics is an extension of the energy conservation principle of mechanics to include also mainly the effect of heat changes. This is based on the observation that work can be converted into heat and vice versa, and transferring heat and doing work to a system have equivalent effects. The expenditure of a fixed amount of work leads always to the produc- tion of "heat" (more exactly, energy change) of the same amount. To give the law an analytical formulation, it was only necessary to define a new energy function that includes also the effect of heat. 2.1 The Relation Between Heat and Work: Historical Development The acceptance of the fact that heat and work can turn into each other and both can produce the same effect on a system has been a long process. Consider the quantitative experiments car- ried out by Benjamin Thompson. Upon becoming Count of Rumford of The Holy Roman Em- pire, he was commissioned by the King of Bavaria to supervise the boring of cannon at the Mu- nich Arsenal. He was impressed by the tremendous temperature rise associated with the boring operations and noticed that the heat produced was roughly proportional to the work performed. This led him to suggest in 1798 that heat is generated by the mechanical energy expended. How- ever, at the time heat had been regarded as a fluid, called the caloric gas or fluid, residing inside a matter. In this theory the temperature of a matter was determined by the amount of caloric gas it contained; and, when coming into contact, two bodies of different temperatures will reach a common intermediate temperature by the flow of caloric gas between them. In the caloric theory this intermediate temperature or equilibrium state is one with the caloric gas pressure being the same in all parts of the (combined) matter. Thus, the caloric theory accounted for Count Rum- ford's observation of heat production in canon boring not by the performance of work but by the hypothesis that the amount of caloric that can be contained in a body is proportional to its mass raised to a power larger than unity. Thus, smaller pieces of metal produced by the boring con- tained less caloric per unit mass, and consequently in reducing the original mass to a number of small pieces caloric gas was evolved into measurable heat. When it was then demonstrated that when a blunt borer was used, which produces very few turns, the same heat production accom- panied the same amount of work done, the caloric theory "explained", in this case, the heat pro- duction as being due to the action of air on the metal surfaces during the performance of work.
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Tan 2 The caloric theory, however, was discarded in 1799 when Humphrey Davy melted two ice blocks by rubbing them together in vacuum, since in this case the heat needed to melt the ice can only be due to the mechanical work done by the rubbing. The time was, however, not scientifi- cally ready for a mechanical theory of heat. This was true until the work of Dalton and others
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This note was uploaded on 07/13/2011 for the course ME 218 taught by Professor Dr.tan during the Fall '11 term at Duke.

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2. 1st law - Tan 2. THE FIRST LAW OF THERMODYNAMICS The...

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