A piston - cylinder device initially contains 0.3 m 3 of nitrogen gas at 350 kPa
ID: 701852 • Letter: A
Question
A piston - cylinder device initially contains 0.3 m3 of nitrogen gas at 350 kPa and 35 oC. An electric heater within the device is turned on and is allowed to pass a 240W for 6 minutes. Nitrogen expands at constant pressure, and a heat loss of 3000 J occurs during the process. Determine the final temperature of the nitrogen. (Average CP for Nitrogen is 1.039 kJ/kgK and molecular weight of Nitrogen is 28.01 kg/kmol.)
Everying prior to the engery balance equation I understand if someone can explain why in the engery balance equation moles are being multiplied by Cv. Should it not be mass multiplied by Cp?
Explanation / Answer
Determine the work done by electric heater
= 240 J/s x 6 min x 60s/min
= 86400 J = 86.4 kJ
From the ideal gas equation
Mass of N2 gas m = P1 x V1 / R x T1
= 350 kPa x 0.3 m3 / (0.297 kPa m3 /kg K x (35+273))
= 1.148 kg
Energy balance
Work done by electric heater - heat loss = change in enthalpy
86.4 - 3 = m(h2 - h1) = mCp(T2 - T1)
83.4 = 1.148 x 1.039 x ( T2 - 35)
T2 = 104.92 °C
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