![]() In the analysis to follow the "ave" notation is dropped. The reference state for these tables is defined as Since the parameters P r, v r, and s o apply to air in a particular process, call isentropic, you should ignore these parameters until we study Chapter 6. When you review Table A-17, you will find h and u as functions of T in K. These last two relations form the basis of the air tables (Table A-17 on a mass basis) and the other ideal gas tables (Tables A-18 through A-25 on a mole basis). Similarly, the internal energy change relative to the reference state is ![]() Let's perform the integral relative to a reference state whereĪt any temperature, we can calculate the enthalpy relative to the reference state as Let's take a second look at the definition of D u and D h for ideal gases. Sometimes adequate (and most often used) values are the ones evaluated at 300 K:.See Table A-2c for variable specific data. The best average value (the one that gives the exact results):.Some ways to determined these values are as follows: To find D u and D h we often use average, or constant, values of the specific heats. These ideal gas processes have the same change in internal energy and enthalpy because the processes occur between the same temperature limits. Process 1-2b: P = a + bV, a linear relationship In the above figure an ideal gas undergoes three different process between the same two temperatures. Where C V,ave and C P,ave are average or constant values of the specific heats over the temperature range. The change in internal energy and enthalpy of ideal gases can be expressed as The differential changes in internal energy and enthalpy for ideal gases become The ideal gas specific heats are written in terms of ordinary differentials asįigure 3-76, page 143 in the text shows how the specific heats vary with temperature for selected ideal gases. For ideal gases, we use thermodynamic function relation theory of Chapter 11 and the equation of state (Pv= RT) to show that u, h, C V, and C P are functions of temperature alone.
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