Mechanical equivalent of heat

The heat equivalent is the historical conversion factor between a mechanical or electrical energy quantity and the resulting heat energy when complete conversion takes place in the thermal energy. The electrical and mechanical equivalent of heat is the conversion between the units newton meter or watt seconds and the unit calorie. From the discovery of the heat equivalent of the first law of thermodynamics was followed as a special case of conservation of energy.

In modern physics, the equivalent of heat has lost its meaning, because since the introduction of the SI unit system, all forms of energy have the same unit joules.

History

An old unit for quantities of heat was the calorie. It was defined as the amount of heat required to warm pure water at a particular temperature difference. The mechanical and electrical energy were initially not considered related to the thermodynamics.

In 1798, Benjamin Thompson discovered almost the mechanical equivalent of heat. The size of the equivalent can be calculated from the experiments Thompsons, he himself did not realize the full significance of the correlations, however.

In 1842, Julius Robert von Mayer published in the Annalen der Chemie and Pharmacie his studies on the temperature increase in a liquid while performing mechanical work. It dealt, inter alia, the relationship between verrichteter friction work and the warming occurring.

" We rubbing example, two metal plates at each other, so we are moving away, see heat occur, however, and the question is now only the movement is the cause of heat. "

Furthermore, Mayer determined the heat equivalent quantitatively and found that the potential energy of a body in 365 meters heats the same water mass to 1 Kelvin.

" [ ... ] That the sinking of a weight Theiles from a height about 365 m corresponds to the heating of an equal weight Theiles water from 0 ° to 1 °. "

Its physical justification of equivalence were false or inaccurate from today's perspective. He did not use the energy term, he went from a transformation of the forces.

James Prescott Joule introduced around the same time at similar thermodynamic experiments and determined the mechanical and electrical equivalent of heat. He used the term first equivalent of heat and published in 1850 in the Annals of Physics and Chemistry its results.

Physical justification

If mechanical or electrical work is dissipated, then the particles of a substance are excited, for example by friction, electric or magnetic fields and its kinetic energy increases. The heat energy of a system corresponds to the total kinetic energy of its particles, i.e., energy of a different form can ( equivalently ), to be completely converted into heat energy.