SECOND LAW OF THERMODYNAMICS AND ENTROPY 305
dharm
/M-therm/th5-4.pm5
- In a reversible cycle 100 kJ of heat is received at 500 K ; then an adiabatic expansion occurs to 400 K, at
which temperature 50 kJ of heat is received, then a further adiabatic expansion to 300 K at which
temperature 100 kJ of heat is rejected :
(i) Find the change in entropy which occurs as the system is restored to its initial state in the remainder
of the cycle.
(ii) If during the remainder of the cycle heat is transferred only at 400 K, how much heat is transferred and
in what direction? [Ans. 0.008 kJ/K ; + 3.2 kJ] - 1 kg of air is compressed according to the law pv1.25 = constant from 1.03 bar and 15°C to 16.45 bar.
Calculate the change in entropy. [Ans. 0.255 kJ/kg K] - A quantity of gas (mean molecular weight 36.2) is compressed according to the law pvn = constant, the
initial pressure and volume being 1.03 bar and 0.98 m^3 respectively. The temperature at the start of
compression is 17°C and at the end it is 115°C. The amount of heat rejected during compression is 3.78 kJ,
cp = 0.92. Calculate :
(i) Value of n,(ii) Final pressure, (iii) Change in entropy.
[Ans. (i) 1.33 ; (ii) 1.107 bar ; (iii) 0.228 kJ/kg K]