The system does not have to pass through only equilibrium states. It describes transitions between equilibrium states but is not concerned with the intermediate states. This is because the first law is not subject to such a restriction. Notice that in Example 3.2, we did not assume that the transitions were quasi-static. Example 3.3 takes into consideration what happens if friction plays a role. If friction were to play a role in this example, less work would result from this heat added. When a closed cycle is considered for the first law of thermodynamics, the change in internal energy around the whole path is equal to zero. The negative sign indicates that the system loses heat in this transition. The first law of thermodynamics is stated as follows: This sign convention is summarized in Table 3.1. Thus, W is positive when work is done by the system and negative when work is done on the system. When a gas expands, it does work and its internal energy decreases. Since added heat increases the internal energy of a system, Q is positive when it is added to the system and negative when it is removed from the system. The first law states that the change in internal energy of that system is given by Q − W Q − W. Suppose Q represents the heat exchanged between a system and the environment, and W is the work done by or on the system. We will see in this chapter how internal energy, heat, and work all play a role in the first law of thermodynamics.

His humorous statement of the first law of thermodynamics is stated “you can’t win,” or in other words, you cannot get more energy out of a system than you put into it.

Snow (1905–1980) is credited with a joke about the four laws of thermodynamics. This interaction is given by the first law of thermodynamics. Now that we have seen how to calculate internal energy, heat, and work done for a thermodynamic system undergoing change during some process, we can see how these quantities interact to affect the amount of change that can occur. Explain how heat transfer, work done, and internal energy change are related in any thermodynamic process.State the first law of thermodynamics and explain how it is applied.

By the end of this section, you will be able to:

Interactive thermodynamics 3.2