1.a sample of gas has a volume of 5 L at a pressure of 120 kPa.at this fixed pressure what is the work done by the gas if the volume is (A)doubled;(B)halved?
2.two moles of an ideal gas are initially at a pressure of 100 kPa.the gas is compressed isothermally at 0 ℃ to a pressure of 250 kPa.find the work done by the gas.
work in thermodynamics?
2007-03-11 5:57 pm
回答 (3)
2007-03-11 7:26 pm
✔ 最佳答案
1a) In this case, the gas expands with △V = 5 dm3 since the volume is doubled.Hence work done = P(△V) = 120 × 103 × 5 × 10-3 = 600J
The work done is positive since the gas expands that it needs to do work against the atmospheric pressure.
1b) In this case, the gas expands with △V = -2.5 dm3 since the volume is doubled.
Hence work done = P(△V) = 120 × 103 × (-2.5) × 10-3 = -300J
The work done is negative since the gas contracts that work is done by the atmospheric pressure on it.
2) Applying the equation PV = nRT, we have
Initial volume:
V = (2 × 8.314 × 273)/(100 × 103) = 0.0454 m3
Final volume:
V' = (2 × 8.314 × 273)/(250 × 103) = 0.0182 m3
So the work done by the gas is given by:
W = RT ln (V'/V)
= 8.314 × 273 × ln (0.0182/0.0454)
= -2080 J
Which means that work is done on the gas by the external pressure.
參考: My physics knowledge
2008-09-19 5:45 am
2007-03-11 6:37 pm
1. W = F s
F = P A
W = P A s
V = A s
W = P V
i.e.(a) W = 120000 X (5 x 2), (b) W = 120000 x (5/2)
2. PV = nRT
100000V = 2R x 273
V = 0.00546R
250000v = 2R x 273
v = 0.002184R
volume change = V - v
work done = extra energy needed to compressed the gas
= (250000-10000) x (V-v)
F = P A
W = P A s
V = A s
W = P V
i.e.(a) W = 120000 X (5 x 2), (b) W = 120000 x (5/2)
2. PV = nRT
100000V = 2R x 273
V = 0.00546R
250000v = 2R x 273
v = 0.002184R
volume change = V - v
work done = extra energy needed to compressed the gas
= (250000-10000) x (V-v)
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