A piston-cylinder device contains 3 kg of air at 300 kPa and 25°C. During a quasi-equilibrium isothermal expansion process, 20 kJ of boundary work is done by the system, and 8 kJ of paddle-wheel work is done on the system. The heat transfer during this process is
A)28 kJ
B)20 kJ
C)36 kJ
D)12 kJ
E)4 kJ

Question

Quizplus · Accepted Answer

Solved by EES Software. Solutions can be verified by copying-and-pasting the following lines on a blank EES screen. 
R=0.287 "kJ/kg.K"
Cv=0.718 "kJ/kg.K"
m=3 "kg"
P1=300 "kPa"
T=25 "C"
Wout_b=20 "kJ"
Win_pw=8 "kJ"
"Noting that T=constant and thus dE_system=0, applying energy balance E_in-E_out=dE_system gives"
Q_in+Win_pw-Wout_b=0
"Some Wrong Solutions with Common Mistakes:"
W1_Qin=m*Q_in "Multiplying by mass"
W2_Qin=Win_pw+Wout_b "Adding both quantities"
W3_Qin=Q_in/m "Dividing by mass"
W4_Qin=Wout_b "Setting it equal to boundary work"

A Piston-Cylinder Device Contains 3 Kg of Air at 300