统从A到B，然后通过另一条路线将其返回到A状态，从而产生或破坏能量。相反，在循环过程中(系统返回到其原始状态)，B在一个方向上的增加与B在相反方向上的减少完全相等，因此在循环中没有净增益或能量损失。
The change in internal energy of a system may be determined by measurement of the heat absorbed by the system from its surroundings, W, and the work done by the system on its surroundings, W. In these terms,
系统内能的变化可以通过测量系统从其周围的W吸收的热量和系统在其周围所做的功W来决定。
△E=Q-W
It is important to keep in mind the conventions regarding the signs of these quantities: Q, positive=heat absorbed by the system; negative=heat evolved by the system
W, positive=work done by the system; negative-work done on the system
The values of Q and W involved in changing a system from state A to state B depend upon the way in which the change is carried out. However, the value of (Q-W) is a constant, equal to for the change no matter how it is brought about. If a system undergoes a change in which the internal energy of the system remains constant, the work done by the system equals the heat absorbed by the system. If a system undergoes a change that reduces its internal energy, the energy released will be the sum of the heat evolved and the work done.
重要的是要记住关于这些量的符号的惯例：
Q，正=系统吸收的热量；负=热由系统演化而来， W, 正=系统所做的功；负功-在系统上所做的负功-
从状态A到状态B所涉及的Q和W值取决于进行改变的方式。然而，无论发生什么变化，(Q-W)的值都是常数，等于 。如果一个系统经历了系统内能保持不变的变化，系统所做的功等于系统吸收的热量。如果一个系统经历了减少其内部能量的变化，释放的能量将是热量的总和完成的工作。