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How did Joule “disprove” caloric theory? According to the Blundells in their Concepts in Thermal Physics, they mention:

Joule let a mass tied to a string slowly descend a certain height, while the other end of the string turns a paddle wheel immersed in a certain mass of water. The turning of the paddle frictionally heats the water. After a number of descents, Joule measured the temperature rise of the water. In this way he was able to deduce the ‘mechanical equivalent of heat’. He also measured the heat output of a resistor (which, in modern units, is equal to $I^2 R$, where $I$ is the current and $R$ the resistance). He was able to show that the same heat was produced for the same energy used, independent of the method of delivery. This implied that heat is a form of energy. Joule’s experiments therefore consigned the caloric theory of heat to a footnote in history.

I can’t understand how the authors justify the disproval by Joule.

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    $\begingroup$ From en.wikipedia.org/wiki/Caloric_theory "<i>Count Rumford had found that boring a cannon repeatedly does not result in a loss of its ability to produce heat, and therefore no loss of caloric. This suggested that caloric could not be a conserved "substance"</i> I would guess the paddle wheels replace the boring machine. $\endgroup$ – StudyStudy Aug 6 '19 at 16:10
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As I understand it the caloric theory was that heat is a invisible unique substance that flowed into and out of things as well as be stored in things causing a temperature change. The cannon experiment showed it wasn’t stored in things.

Joule, in his experiment, showed the equivalence of heat and mechanical work with respect to increasing the temperature of a liquid. No flow of an invisible substance was needed to increase the temperature of the liquid.

Hope this helps

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  • $\begingroup$ I understand your argument but not of the Blundells’ when they say “Thisimplied that heat is a form of energy.” How did resistor experiment implied that? $\endgroup$ – Atom Aug 6 '19 at 18:07
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    $\begingroup$ @Atom If caloric exists then it probably is a conserved quantity. If caloric is a conserved quantity then it cannot be created out of nothing. How come things become hot when you drill a hole in it? Supposedly because the destruction releases trapped caloric. Joule showed a consistent pattern of transforming mechanical energy to heat, without any destruction. The consistency suggested the existence of some conserved quantity of unknown nature, and that heat is a subset of that more general conserved quantity. $\endgroup$ – Cleonis Aug 6 '19 at 18:29
  • $\begingroup$ @Cleonis That cleared much! $\endgroup$ – Atom Aug 6 '19 at 18:41

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