Stuck on physics A2 applied topic question Watch

lambda98
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http://filestore.aqa.org.uk/subjects...W-QP-JUN10.PDF
http://filestore.aqa.org.uk/subjects...W-MS-JUN10.PDF

I was doing this paper from June 2010, and it asks you to calculate the work done by an expanding gas from a p-V diagram, but in the mark scheme it only calculates the area under the graph up to atmospheric pressure, rather than zero. Does anyone know why this is? The examiner's report said many 'failed to read the question carefully' and calculated to 0 pressure but I've read it a good few times now and can't see it!
It's question 4.a. in the link attached-thanks
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Teenie2
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The clue is in the question - "Use the graph to estimate the work done by the air as it expands from a pressure of3.6 × 10^5Pa to ATMOSPHERIC PRESSURE" not to zero as the gas never reaches zero pressure.
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lambda98
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(Original post by Teenie2)
The clue is in the question - "Use the graph to estimate the work done by the air as it expands from a pressure of3.6 × 10^5Pa to ATMOSPHERIC PRESSURE" not to zero as the gas never reaches zero pressure.
But does that matter? Isn't the equation for work done by the gas (not the whole system) just pdV, using absolute not relative pressure?
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Teenie2
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The work done by the gas is dW = delta p * dV where delta p = p (internal) - p (outside) = pressure difference between the internal and the external air, and dV is the volume change.

So the total work done is the area under the graph down to atmospheric pressure not 0 as this would imply that the external pressure was a vacuum.
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