Question

A piston cylinder device initially contains air at 125 kPa and 300 K. At this state,resting on a pair of stops and enclosed volume is 100 L. The mass of the pistor 250 kPa pressure is required to move it. The air is now heated till its volume be times. Use the following table.

T(K)	300	600	900	1200	1500
u (kJ/kg)	212	389	594	818	1114

What is the net heat transferred to the air?

• A. 37.5 kJ
• B. 130.9 kJ
• C. 168.4 kJ
• D. 189.4 kJ

Answer 1

The correct option is C 168.4 kJ

(C)
The process is desired on P- V as

$m=\frac{PV}{RT}$

$=\frac{125\times 0.1}{(0.287\times 300)}=0.145kg$

From ideal gas equation
$\frac{P_1{V}_1}{T_1}=\frac{P_s{V}_s}{T_3}$

$\Rightarrow \frac{125\times 0.1}{300}=\frac{250\times 0.25}{T_3}$

$\Rightarrow T_3=1500K$

Work done in the process$=P_2(V_3-V_2)$

=250 (0.25-0.10) = 37.5kJ
from first law of thermodynamics
$Q-W=\Delta U=m(u_3-u_1)$

$Q=W+m(u_3-u_1)$

=37.5 + 0.145(1114 - 212 ) =168.4 kJ