History Weblecture for Unit 47
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In the four decades after the "invention" of Watt's steam engine, devices to convert heat energy made possible a new kind of livelihood in England, France, and the northeastern United States. In Manchester, England, Josiah Wedgewood revolutionized pottery-making, and his kilns fired both delicate and practical porcelain patterns. He invented methods of mass production, direct mail sales, and money-back guarantees. In Wales, furnaces using anthracite coal smelted iron at higher temperatures than had been possible before, producing steel that could support the weight of carts on bridges, or the pressure of steam in tanks. The new steam engines built from this iron provided the energy to drive spinning jennys to make thread, and looms to make cloth. With cheap clothing, and cheap pottery, made possible by cheaper transportation, more people could afford to keep higher standards of hygiene and health. Populations in England and New England on the American continent boomed, and where before children had inherited and stayed on their family farms, now there was not enough land to support the available farm labor, and younger heirs migrated to the cities to find work in factories. Efficiency became a driving force in both social and scientific arenas.
The rising prosperity had many consequences, among them leisure for the middle classes to interest themselves in scientific and other intellectual pursuits. It is worth noting that Josiah Wedgewood was one of the founding directors of the Manchester Literary and Philosophical Society, which numbered among its later members secretary John Dalton, whom we have already discussed, and James Prescott Joule, who is part of our energy story in this unit. Wedgewood is also remembered as a staunch abolitionist whose porcelain medallion Am I not a man and a brother?, which depicted a black man in chains pleading for freedom, fueled popular support for the anti-slavery movement in England, ultimately leading the the abolition of slave trade in the British Empire in 1807, and slavery itself in 1833. Wedgewood was Charles Darwin's grandfather. Darwin married another of Wedgewood's grandchildren (and therefore his second cousin); the double-barreled inheritance of Wedgewood's pottery wealth is what gave Darwin the money and leisure to travel and write up his theories of evolution.
The mathematical study of heat energy begins with the work of Joseph Fourier, a French mathematician who developed a method of adding trigonometric series together. He treated heat forces as completely different from mechanical forces, following the understanding of caloric that Lavoisier had popularized. The use of heat energy in steam engines to drive turning shafts forced scientists to re-evaluate the relationship between heat, energy, work, and power.
Sadi Carnot laid the foundations not only of heat science but of thermodynamics with his work on "the perfect engine".
Carnot proposed that heat (in its fluid form as caloric) was conserved as it flowed from a hotter region or heat source to a colder region or heat sink in the engine system. Carnot's principle stated that the amount of work done by the piston of a perfect steam engine depends only on the temperature difference between the boiler and the condenser, and the amount of heat flow between them. Carnot's perfect engine is "reversible": that it, it should run either way, allowing heat transfer from the sink to the source (and in fact, this is how refrigerators work).
Another consequence of this rule is that perpetual motion is impossible: two engines operating between the same two temperature levels have the same maximum efficiencies. You can't get one engine to produce more energy than another, and use it drive the second engine in reverse to keep both going indefinitely on the initial amount of fuel.
Carnot assumed at first that no heat was converted to work, but he realized over time that this was not true, and he was forced abandon the theory of caloric as an explanation of heat flow. He wasn't able to work out a new explanation before his death from cholera in 1832.
One of John Dalton's students, James Prescott Joule became interested in the equivalence between heat and other forms of energy. Although Carnot had called the theory of caloric into question because it couldn't completely account for all heat phenomena, Joule broke down the idea that heat energy was somehow a substance. Instead, he was able to show that it was another form of energy, and that one could convert heat energy to mechanical energy (energy of motion) and that this conversion was reversible.
The German physicist Rudolf Clausius was able to show that there was a serious problem with Carnot's principle. He began by showing that the maximum work that could be obtained by heat flow depended on the temperature difference between the two substances exchanging heat, and not on the nature of the substances themselves. This idea ran counter to the concept of caloric, which stated that the heat flow or motion of caloric between two substances depended on the type of substance. Clausius stated in his most important paper (Über die bewegende Kraft der Wärme-- On the moving force of heat).
Clausius came up with two rules for the movement of heat, which became the first and second laws of thermodynamics:
Clausius is the first person to name and explain the concept of entropy, the amount of "disorder" in a system.
The following readings are optional.
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