Ideal Gas Law
p = ZRT
Ideal Gas Processes
Polytropic processes: exponent
Energy to raise temperature of unit
mass one degree
C = Q/ (mT)
SI (kJ/kg K)
Gas has two values
Cv – constant volume process
Cp – constant pressure process
Internal energy as f(T)
Enthalpy as f(T)
System contains 2lbm water vapor at
300oF & 20 ft3. The system is
compressed isothermally to 9.05 ft3;
the work of compression is – 90.8
Btu. In the next process, the system
is heated isometrically to 120psi.
Kinetic and potential energy effects
are negligible. Find the heat
transfers in the two processes.
A gas undergoes three processes in
series returning to its initial state,
called a cycle.
1 2 compression from 10 psi to 50psi.
Initial volume is 4 ft3. The process is described
by p = C.
2 3 Isometric process; P1 = P3.
3 1 Isobaric process.
Find net work of the cycle.
Piston/cylinder assembly contains
1.4kg saturated water at 200oC.
Heat is transferred to the system until
only saturated vapor present, and the
volume has quadrupled.
Find the final volume, temperature,
and pressure; find U.
First law of thermodynamics
Q – W = (m/2)(V22 – V12) + mg(z2 – z1 ) + m(u2 - u1)