Heat equivalent
The heat equivalent is the historical conversion factor between a mechanical or electrical amount of energy and the resulting thermal energy if a complete conversion into thermal energy takes place. The mechanical or electrical heat equivalent indicates the conversion between the units newton meter or watt second and the meanwhile outdated unit calorie . From the discovery of the heat equivalent, the first law of thermodynamics followed as a special case of the law of conservation of energy .
In modern physics, the heat equivalent has lost its meaning because since the introduction of the SI system of units, all forms of energy have the same unit joule .
history
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An old unit of heat was the calorie . It was defined by the amount of heat required to raise the temperature of a unit of weight of pure water by one unit. The mechanical and electrical energy were initially not considered in connection with thermodynamics.
In 1798, Benjamin Thompson almost discovered the mechanical heat equivalent. The size of the equivalent can be calculated from Thompson's experiments, but he himself did not recognize the full meaning of the relationships.
In 1842 Julius Robert von Mayer published in the Annalen der Chemie und Pharmacie his studies on the temperature increase in a liquid when doing mechanical work. Among other things, he dealt with the relationship between the frictional work carried out and the resulting warming.
"Let's rub z. If, for example, two metal plates are placed against each other, we will see movement disappear, but heat appear and the only question now is, is movement the cause of warmth. "
Furthermore, Mayer determined the heat equivalent quantitatively and determined that the potential energy of a body at a height of about 365 meters is equivalent to heating an equal mass of water by 1 Kelvin (correct would be 427 m height):
"[...] that the sinking of a part by weight from a height of about 365 m corresponds to the heating of an equal part by weight of water from 0 ° to 1 °."
His physical justifications for equivalence were imprecise.
In his publication, Mayer speaks of a transformation of forces . This is because, in its day, the term force denoted both the force itself and the energy. The term energy in its current form was only introduced ten years later by the Scottish physicist William Rankine .
James Prescott Joule made similar thermodynamic experiments around the same time and determined the mechanical and electrical heat equivalent. He was the first to use the term heat equivalent and published his results in the Annalen der Physik und Chemie in 1850 .
Physical justification
When mechanical or electrical work is dissipated , the particles of a substance are excited by friction, electrical or magnetic fields, for example, and their mean kinetic energy increases. The thermal energy of a system corresponds to the total kinetic energy of its particles, i.e. H. Energy of another form can be converted completely (equivalent) into thermal energy.
Web links
- Heat equivalent. In: Lueger: Lexicon of the entire technology and its auxiliary sciences. Vol. 8 Stuttgart, Leipzig 1910., pp. 809-810.
- A practical experiment to determine the heat equivalent: Conversion of mechanical work into heat. (pdf) Munich University of Applied Sciences, accessed on June 24, 2017 .
Individual evidence
- ↑ Cannons, Blood, and Conservation of Energy. On: uni-kiel.de
- ^ A b Robert Julius von Mayer: Remarks on the forces of inanimate nature . In: Annals of Chemistry and Pharmacy. Volume XLII, Issue 1, Verlag CF Winter'sche, 1842, pp. 237 and 240. Available from Google Books
- ↑ James Prescott Joule: About the mechanical heat equivalent . In: Annals of Physics and Chemistry. Volume 4, Verlag JA Barth, 1854, pp. 601ff. (German version of its publication published in 1850). Available on Google Books
- ↑ James Prescott Joule: On the Mechanical Equivalent of Heat . In: Royal Society London (Ed.): Philosophical Transactions of the Royal Society of London . tape 140 , 1850, pp. 61–82 , doi : 10.1098 / rstl.1850.0004 (English, royalsocietypublishing.org [accessed June 24, 2017]).