TITLE.PM5

FIRST LAW OF THERMODYNAMICS 135

dharm

M-therm/th4-2.pm5

l

Constant

volume

Constant

pressure

n m

V

p

Fig. 4.20

∴ Un = 195 – 77 = 118 kJ

Qn–l = 0 being adiabatic process

∴ zδQ = 90 – 95 = – 5 kJ

and zδW = – 18 + Wn–l = – 5

∴ Wn–l = – 5 + 18 = 13 kJ
Hence, Wn–l = 13 kJ ; Ul = 105 kJ ; Um = 195 kJ ; Un = 118 kJ. (Ans.)
Example 4.22. A movable frictionless piston closes a fully insulated cylinder on one side
and offers a constant resistance during its motion. A paddle work is drawn into the cylinder and
does work on the system.
Prove that the paddle work is equal to change in enthalpy.
Solution. Refer Fig. 4.21.

Fig. 4.21