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Some of what was thought in the 1800's about heat has since been proven wrong. They followed the creed of observe theorize and test, and the theory was improved.
In the 1800's they thought of heat an an imponderable (think undetectable or invisible) fluid flowing through all matter, including liquids and vapors. This led to the waterwheel analogy of heat engines, in which heat was compared to water. It was thought that the same amount of heat entered and exited a heat engine, but that it has moved toward the "ground" like water does as it falls, so has less force behind it afterward.
This also led to the model of heat engines as operating between two reservoirs. A hot reservoir is possible, as heat can be replenished, but a cold reservoir is not (you can't add cold). Ironically, in the 1800's the only way to make cold temperatures was to expand a volume of vapor, which is what all heat engines do. Heat engines "make" their own cold temperature by consuming heat energy as it is converted to another form of energy
In turn, this led to today's model which at its most fundamental is still modeled as exhausting heat. |
Where did Carnot Limit come from?
In "Reflections on the Motive Power of Heat" by Sadi Carnot , and "An Account of Carnot's Theory" by Sir William Thomson (better known as Lord Kelvin) (Kelvin scale anyone?) These were published together in a single book with Carnot's title in the 1800's Thanks to www.archive.org for pdf of original scan (Cited as "Reflections" for brevity hereafter).
The combined works define "Reversibility", discuss the impossibility of creating energy from nothing, and sadly from there it gets very distorted by history.
Many physicists will tell you everything is reversible, can be run in either direction, even down to particle and anti-particle annihilation. The only difference being how likely the reverse is to occur. Annihilating into a burst of energy requires two particles to contact, but the reverse requires a simultaneous collision of many photons and sub-components, mathematically orders of magnitude less likely.
So generally when one speaks about reversing things, it is all about probability, not about gaining or losing energy.
Carnot and Kelvin observe the reversibility (~1820's) of the state of vapor (temperature, pressure) when heat is added or subtracted, or volume changed and restored. This predates the Law of conservation of energy (a generation later at about ~1850), and is in fact a major contribution to the Law, which generalizes the conservation principle beyond just applying to vapors. So when reading their work its important to keep in mind what they did and did not know at that moment in history.
Somehow, their work got twisted into concluding that the Carnot cycle is somehow special, inherently best of all cycles, which is almost the exact opposite of their conclusions.
Coupled with this warped version of their store is the notion that all possible heat engines are less efficient than the Carnot cycle. Its quite easy to show that there are an infinite number of cycle shapes which have precisely the same solution as the Carnot cycle, but the important aspect is to review what the First Law of Thermodynamics - Conservation of Energy has to say. An "irreversible" cycle must have at least one irreversible curve to construct it, a curve which follows a different path when traversed in opposite direction, which can only happen if energy (like heat) is created or destroyed. |
