help A piston-cylinder device contains air initially at 150kPa, 20 degree C and

Question

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A piston-cylinder device contains air initially at 150kPa, 20 degree C and 0.5m3. The air is now compressed in a polytropic process to 400kPa and 140 degree C. Determine the amount of heat transfer and identify if it is the heat loss or gain during this process. (Hint: you need to solve the value of n the polytropic exponent based on the pressures and volumes at the initial and final states). A closed piston-cylinder device contains 5L of saturated liquid water at a pressure of 150 kPa. Water is stirred and heated by a paddle wheel and a resistor placed in the water (see the figure). After the resistor and paddle-wheel are turned on 45 minutes, only one quarter mass of the water remains in liquid phase. During this process, the pressure is kept constant. Given that the paddle wheel work is 400kJ. the heat loss is 50kJ, and the current flows through the resistor is 8A. please determine the voltage of the resistor source.

Explanation / Answer

(5) We have

     P1=150kPa,T1=273+20=293K,V1=0.5m3

      P2=400kPa,T2=273+140=413K

for polytropic process we know

T1/T2=(P1/P2)^(n-1 /n)

293/413=(150/400)^(n-1 /n)

taking log both side

ln(293/413)=( n-1)/n ln(150/400)

on solving we get n=1.54

and T1/T2=(V2/V1)^(n-1)

293/413=(V2/0.5)^(1.54-1)

on solving we get V2=0.265 m3

heat transfer=(P1V1-P2V2)/(n-1)= -54.4kJ

since sign is negative so heat is lost by the system

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