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The energy transfer Q into or out of a system by heat also depends on the process. Consider the situations depicted in Figure 20.6. In each case, the gas has the same ini- f , just enough energy is transferred by heat from the reservoir to the gas to maintain a constant temperature T i . Now consider the completely thermally insulated system shown in Figure 20.6b. When the membrane is broken, the gas expands rapidly into the vacuum until it occu- f and is at a pressure P f . In this case, the gas does no work because it does not apply a force—no force is required to expand into a vacuum. Furthermore, The initial and final states of the ideal gas in Figure 20.6a are identical to the initial and final states in Figure 20.6b, but the paths are different. In the first case, the gas energy transfer by heat, like work done, depends on the initial, final, and intermediate states of the system. In other words, because S E C T I O N 2 0 . 4 • Work and Heat in Thermodynamic Processes 617 At the Active Figures link at http://www.pse6.com, you can choose one of the three paths and see the movement of the piston in Figure 20.3 and of a point on the PV diagram in this figure. f P f P i V V i V f P i (a) f P f P i V V i V f P i (b) f P f P i V V i V f P i (c) Active Figure 20.5 The work done on a gas as it is taken from an initial state to a final state depends on the path between these states. Energy reservoir at T i Gas at T i (a) Insulating wall Final position Initial position Insulating wall Gas at T i (b) Membrane Vacuum Figure 20.6 (a) A gas at temperature T i expands slowly while absorbing energy from a reservoir in order to maintain a constant temperature. (b) A gas expands rapidly into an evacuated region after a membrane is broken. |