A frictionless 400-mm-diameter piston sits on top of water in a cylinder such that the pressure is initially at 120 kPa at a temperature of 80ºC. Heat is added until the final pressure is 800 kPa. There are stops in the cylinder wall so that the piston can rise a distance of only 200 mm. The work done by the steam is nearest:
A) 12 kJ
B) 8 kJ
C) 3 kJ
D) 2 kJ
C) 3 kJ
Work is accomplished as the piston rises at constant pressure. It’s simply P V as the piston rises. When it hits the stops and the pressure then begins to increase, no work is done since no movement results. It matters not what the final pressure is nor what the initial temperature is.
W = P V = P? r 2 h = 120 kN/m2 × ? × 0. 22 m2 × 0.2 m = 3.02 kJ
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