热力学第一定律

热力学第一定律将先前力学中已知的能量守恒定律推广到热量。它的提出是物理学的一大进步，因为它彻底推翻了当时流行的热质说，该说认为热量是一种无重量的流体。詹姆斯·焦耳在19世纪40年代进行的实验证明了热量的功当量，这对于确立热量和功的相互转化至关重要。

The law introduces internal energy ([latex]U[/latex]) as a state function, meaning its value depends only on the current state of the system, not on how it got there. In contrast, heat ([latex]Q[/latex]) and work ([latex]W[/latex]) are path-dependent process quantities. The law’s differential form is [latex]dU = \delta Q – \delta W[/latex]. For a cyclic process, where the system returns to its initial state, the change in internal energy is zero ([latex]\Delta U = 0[/latex]), so the net heat supplied equals the net work done. This principle is the basis for all heat engines. The law also implies the impossibility of a perpetual motion machine of the first kind—a machine that produces work without any energy input.