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Two moles of helium gas are taken over the cycle ABCDA shown in the P-T diagram. Assuming the gas to be ideal, the work done on the gas in taking it from A to B is

Asked in JEE Main 2009 · Isobaric Heat and Work

Figure: Isobaric Heat and Work
Answer: (3) 400R

Step-by-step solution

On the P-T diagram AB runs at constant pressure, so it is isobaric and

W_by=nRΔ T.

Reading the figure, A is at 300 K and B at 500 K on the 2×10⁵ Pa

line, so Δ T=+200 K for two moles.

W_by=2R×200=+400R, and the work done on the gas is therefore -400R.

The printed key, 400R, is the magnitude. The three questions of this 2009

block do not use one sign convention between them. Listed in DEFECTS.md.

Why the other options are wrong

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