# First main principle

In thermodynamics, first main principle, in contrast to the second main principle, of the mechanical theory of heat is defined by the following expression:

which is the general form, showing how an amount of heat dQ can change the energy dU of a body and force the body to do work dW. [1] In the case of the work being of the pressure-volume type PdV of work (pressure-volume work):

such as in the body of water in a steam engine, where dQ is an amount of heat imparted to the body, dU is the change in the energy (or internal energy) of the body, P is the pressure of the body, and dV is the change in volume of the body. [2]

This terminology and expression was used by German physicist Rudolf Clausius, later coming to be known as the first law of thermodynamics. Clausius also called this the “principle of equivalence of heat and work”, on the logic that heat can be transformed into work and work into heat according to the mechanical equivalent of heat.

References
1. Clausius, Rudolf. (1879). The Mechanical Theory of Heat (§:First Main Principle of the Mechanical Theory of Heat, or Principle of the Equivalence of Heat and Work, pgs. 21-38; pg. 110). Macmillan & Co.