Thermodynamics

c p c v R1kJ>kg # K2 (4–29)Chapter 4 | 183Replacing dh by c p dT and du by c v dT and dividing the resulting expressionby dT, we obtainThis is an important relationship for ideal gases since it enables us to determinec v from a knowledge of c p and the gas constant R.When the specific heats are given on a molar basis, R in the above equationshould be replaced by the universal gas constant R u (Fig. 4–29).c p c v R u 1kJ>kmol # K2(4–30)At this point, we introduce another ideal-gas property called the specificheat ratio k, defined asAIRV = constantT 1 = 20°CQ T 1 2 = 30°C∆u = c v ∆T= 7.18 kJ/kgQ 2AIRP = constantT 1 = 20°CT 2 = 30°C∆u = c v ∆T= 7.18 kJ/kgk c pc v(4–31)The specific ratio also varies with temperature, but this variation is verymild. For monatomic gases, its value is essentially constant at 1.667. Manydiatomic gases, including air, have a specific heat ratio of about 1.4 at roomtemperature.FIGURE 4–27The relation u c v T is valid forany kind of process, constant-volumeor not.EXAMPLE 4–7Evaluation of the u of an Ideal GasAir at 300 K and 200 kPa is heated at constant pressure to 600 K. Determinethe change in internal energy of air per unit mass, using (a) data from the airtable (Table A–17), (b) the functional form of the specific heat (Table A–2c),and (c) the average specific heat value (Table A–2b).Solution The internal energy change of air is to be determined in three differentways.Assumptions At specified conditions, air can be considered to be an idealgas since it is at a high temperature and low pressure relative to its criticalpointvalues.Analysis The internal energy change u of ideal gases depends on the initialand final temperatures only, and not on the type of process. Thus, thefollowing solution is valid for any kind of process.(a) One way of determining the change in internal energy of air is to read theu values at T 1 and T 2 from Table A–17 and take the difference:Thus,u 1 u @ 300 K 214.07 kJ>kgu 2 u @ 600 K 434.78 kJ>kg¢u u 2 u 1 1434.78 214.072 kJ>kg 220.71 kJ>kg∆u = u 2 – u 1 (table)2∆u =c v (T ) dT1∆u ≅ c v,avg ∆TFIGURE 4–28Three ways of calculating u.(b) The c – p(T) of air is given in Table A–2c in the form of a third-degree polynomialexpressed asc p 1T2 a bT cT 2 dT 3