The First Law is a statement of the 能量守恒. It posits that the change in a closed system’s internal energy ([latex]\Delta U[/latex]) is equal to the heat supplied to the system ([latex]Q[/latex]) minus the work done by the system on its surroundings ([latex]W[/latex]). The governing equation is [latex]\Delta U = Q – W[/latex]. This law links heat, work, and internal energy, establishing heat as a form of energy transfer.
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.
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