Heat and Thermodynamics
Prior to the 19th century, it was believed that the sense of how
hot or cold an object felt was determined by how much "heat" it contained. Heat
was envisioned as a liquid that flowed from a hotter to a colder object; this
weightless fluid was called "caloric", and until the writings of Joseph Black
(1728-1799), no distinction was made between heat and temperature. Black
distinguished between the quantity (caloric) and the intensity (temperature) of
heat.
Benjamin Thomson, Count Rumford, published a paper in 1798
entitled "an Inquiry Concerning the Source of Heat which is Excited by
Friction". Rumford had noticed the large amount of heat generated when a cannon
was drilled. He doubted that a material substance was flowing into the cannon
and concluded "it appears to me to be extremely difficult if not impossible to
form any distinct idea of anything capable of being excited and communicated in
the manner the heat was excited and communicated in these experiments except
motion."
But it was not until J. P. Joule published a definitive paper
in 1847 that the the caloric idea was abandoned. Joule conclusively showed that
heat was a form of energy. As a result of the experiments of Rumford, Joule, and
others, it was demonstrated (explicitly stated by Helmholtz in 1847), that the
various forms of energy can be transformed one into another.
When heat is transformed into any other form of energy, or
when other forms of energy are transformed into heat, the total amount of
energy (heat plus other forms) in the system is constant.
This is the first law of thermodynamics, the
conservation of energy. To express it another way: it is in no way possible
either by mechanical, thermal, chemical, or other means, to obtain a perpetual
motion machine; i.e., one that creates its own energy (except in the fantasy
world of Maurits Escher's
"Waterfall"!)
A second statement may also be made about how machines operate. A steam
engine uses a source of heat to produce work. Is it possible to completely
convert the heat energy into work, making it a 100% efficient machine? The
answer is to be found in the second law of thermodynamics:
No cyclic machine can convert heat energy wholly into other forms of energy.
It is not possible to construct a cyclic machine that does nothing but
withdraw heat energy and convert it into mechanical energy.
The second law of thermodynamics implies the irreversibility of certain
processes - that of converting all heat into mechanical energy, although it is
possible to have a cyclic machine that does nothing but convert mechanical
energy into heat!
Sadi Carnot (1796-1832) conducted theoretical studies of the efficiencies of
heat engines (a machine which converts some of its heat into useful work). He
was trying to model the most efficient heat engine possible. His theoretical
work provided the basis for practical improvements in the steam engine and also
laid the foundations of thermodynamics. He described an ideal engine, called the
Carnot engine, that is the most efficient way an engine can be constructed. He
showed that the efficiency of such an engine is given by
efficiency = 1 - T"/T',
where the temperatures, T' and T" , are the hot and cold "reservoirs" ,
respectively, between which the machine operates. On this temperature scale, a
heat engine whose coldest reservoir is zero degrees would operate with 100%
efficiency. This is one definition of absolute zero, and it can be shown
to be identical to the absolute zero we discussed previously. The temperature
scale is called the absolute, the thermodynamic , or the kelvin
scale.
The way that the gas temperature scale and the thermodynamic temperature
scale are shown to be identical is based on the microscopic
interpretation of temperature, which postulates that the macroscopic
measurable quantity called temperature is a result of the random motions of the
microscopic particles that make up a system.
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