Page 1of 6Lecture 8 – September 18, 2008 Work & an Introduction to the First Law ()pdVAdzA/FFdzdW−=−=−=dVpWop∫−=dVpWfiVVop∫−=()ifopVVpW−−=Expansion: Vf>Vi, W<0 Compression: Vf<Vi, W>0 Reversible vs. Irreversible Processes
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Page 2of 6During compression in three steps, a higher pressure than the starting state is required for compression. Hence, the three blue triangular regions above the isotherm represent excess work above the minimum required for gas compression. During expansion in three steps, a lower pressure is required at each stage for expansion. The three unshaded triangular regions under the curve represent work lost during expansion relative to the maximum possible work that could be obtained. The difference between the two steps is termed hysteresis. Isothermal Reversible Compression = Minimum amount of work to compress a gas. Isothermal Reversible Expansion = Maximum amount of work done by gas