example 4-1 - Example 4-1 Nitrogen is contained in a...

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Example 4-1 Nitrogen is contained in a piston-cylinder assembly with an initial volume of 0.4 m 3 , an initial temperature of 20°C, and a pressure of 300 kPa. The nitrogen expands at constant pressure while 390 kJ of heat is added. Find the final temperature and the work done a) assuming constant specific heats at 20°C b) using variable specific heat Solution: a) For a constant pressure expansion in a closed system, QH =∆ Assuming constant specific heats () 21 p Qm cTT =− To find the mass, use 3 1 m VOLUME(N2, 300, 20) 0.29 kg vP T == = = 3 1 3 1 0.4m 1.38kg m 0.29 kg V m v = kJ CP(N2, 20) 1.04 kg K p cT = p Q TT mc =+
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() o 2 390kJ 20 C kJ 1.38kg 1.04 kg K T =+ ⎛⎞ ⎜⎟ ⎝⎠ o 2 292 C T = From the first law for a closed system UQW ∆=− WQ U =− 21 v WQm cTT kJ CV(N2, 20) 0.744 kg K v cT == = o kJ 0.744 292 20 C kg K W 111kJ W = b) In the second part, specific heat is not assumed to be constant. We begin again with
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example 4-1 - Example 4-1 Nitrogen is contained in a...

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