PROPERTIES OF GASES Equation of State For a perfect gas: pv = RT where p is pressure, N/m2, Pa, or kPa v is specific volume, m3/kg T is absolute temperature, ° K R is the gas constant, J/kgK or kJ/kgK and R = R /M where R is the Universal Gas Constant = 8.3144 kJ/kmole K M is the molecular weight, e.g. for air Mair = 28.96 kg/kmol, Rair = 0.2871 kJ/kgK. Other Properties At moderate temperatures and pressures the properties internal energy and enthaply are assumed to be independent of pressure. u = u(T ,M) or for a particular gas u = u(T) and h = h(T ,M) or h = h(T) specific heats: cv = du/dT, cp = dh/dt, and cp / cv = γ since h = u + pv = u + RT, then dh/dT = du/dT + R. Thus cp = cv + R and cp - cv = R, or R = cp(γ -1)/ γ. Second Law Tds = dh -v dp ∴ ds = dh/T -v dp/T = dh/T - R dp/p for an isentropic process ds = 0 ∴ dh/T = R dp/p. This expression may be integrated to give T2 s ∫c T1 p p dT = R * ln 2 T p1 For the special case where the specific heats remain constant this equation may be written as: R T2 s  p 2  c p  p 2  =   =   T1  p1   p1  γ −1 γ