Entropy and The Second Law

The Finer Points of Heat

 By 1852, Thomson had come to believe that heat could be both transformed into work, as described by Joule’s theory, and free flowing to produce no work at all, as described by Fourier’s theory. In the latter, heat was simply dissipated, but not lost in accordance with the first law. Moreover, he distinguished between high quality and low quality energy and insisted that the universal tendency for energy is to dissipate as heat, making it unavailable for work. But Thomson wasn’t the only one thinking about the finer points of heat … so was Clausius.
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The Universal Tendency of Energy

William Thomson’s Struggle with Heat and Work

In 1847, when William Thomson (later Lord Kelvin) (1824–1907) learned of Joule’s experiments (on the mechanical equivalent of heat) demonstrating that work could be converted to heat, he immediately recognized the impact of this discovery. Moreover, it was clear, although not explicitly demonstrated by Joule’s experiments (but nonetheless claimed by Joule), that this equivalence meant that one would expect the conversion of heat into work to be possible as well. This caused problems for Thomson, since at that time, he was still a proponent of caloric theory, which stood in direct opposition to Joule’s conclusion.

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The Weight of Heat

The chemist Lavoisier actually tried to weigh heat; he found it was weightless.

The Drinking Bird as a Heat Engine

The drinking bird is a nice example of a heat engine. The evaporation of water at the bird’s beak results in a cooler temperature there than at its base (around the tail feather). In turn, this temperature difference causes a pressure difference (high to low from base to beak) that causes the (very volatile) liquid to rise up, eventually toppling the bird forward causing it to “drink”.

The Drinking Bird

Heat is “Motion”

In 1798, while boring holes into cannon barrels (as part of the manufacturing process), Count Rumford concluded that heat was the result of some sort of motion within objects.

The Imponderable Fluids

By the end of the eighteenth century, heat along with its cohorts light, magnetism and electricity were regarded as an imponderable fluid capable of flowing between the spaces assumed to be present in matter.

Clapeyron’s Reformulation of Carnot’s Work

In 1834 Émile Clapeyron (1799-1864), a former classmate of Carnot’s, published a paper in the Journal de l’École Polytechnique. Here he reformulated Carnot’s work using clear concise mathematics and a new graphical presentation for Carnot’s reversible heat engine (still taught today to every chemistry major taking a good physical chemistry class) that finally brought Carnot’s work to the attention of engineers, chemists and physicists.

Heat is Energy

Heat was the biggest stumbling block to a complete understanding of energy, remaining separate from it until around 1850 when the first law of thermodynamics was inducted.