Hot body

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A depiction of the Papin engine (Papin, 1690), Carnot engine (Carnot, 1824), and Clausius engine (Clausius, 1865), wherein the "hot body" (e.g. fire or hot boiler), i.e. a highertemperature body, "cold body" (e.g. spray of cold water or air), i.e. a lower ↓ temperature body, and the "working body" (Clausius, 1865), aka "working substance" (Carnot, 1824), which is made to expand or contract, in so-called work cycles, defined originally via the "reversible" Carnot cycle (1824), then modified, by changing "caloric" to "entropy", into the "irreversible" Clausius cycle (1865).

In thermodynamics, hot body (TR:80) (LH:5) (TL:86), as compared to the "cold body" or "working body", refers to any body in the universe at a higher temperature than another body; albeit, generally defined as the "boiler", i.e. tank of boiling water, heated by fire, which is put in contact with piston and cylinder, to make the contents inside of the volume of the piston and cylinder, expand, in the so-called standard heat engine or steam engine model.


In 1824, Sadi Carnot, developed a abstract “heat engine” model, wherein he conceptualizing all steam engines, as being Papin engines in basic principle, and where the boiler was the “hot body”, the spraying cold water was the “cold body”, and the matter inside of the piston and cylinder was the “working substance” or “working body” (Clausius, 1865).

In 1834, Emile Clapeyron extended Carnot’s “hot body” and “cold body” terminology and logic, by presenting it in graphical form, using the “indicator diagram” of Watt.

In 1848, William Thomson began working on the heat engine ideas of Carnot.

In 1850, Rudolf Clausius, via Thomson, began to expand on these models to form the science of thermodynamics.


The following are quotes:

“The production of motive power [work] in the steam engine, is due the transportation of caloric [heat] from a warm body [hot body] to a cold body, i.e. to its re-establishment of equilibrium — an equilibrium considered as destroyed by any cause whatever, by chemical action, such as combustion, or by any other.”
Sadi Carnot (1824), On the Motive Power of Fire (pg. 7)[1]

End matter


  1. Carnot, Sadi. (1824). Reflections on the Motive Power of Fire: and on Machines Fitted to Develop that Power (editor: Eric Mendoza) (pg. 7). Dover, 1960.

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