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Two moles of helium gas are taken over the cycle ABCDA shown in the P-T diagram. The work done on the gas in taking it from D to A is

Asked in JEE Main 2009 · Isothermal Work and Heat

Figure: Isothermal Work and Heat
Answer: (2) +414R

Step-by-step solution

On the P-T diagram DA runs at constant temperature, so it is isothermal:

W_by=nRT ln (V_A)/(V_D)=-nRT ln (P_A)/(P_D).

Reading the figure, D and A both sit at 300 K, with the pressure rising from

1×10⁵ to 2×10⁵ Pa, so the gas is compressed.

|W|=2R×300×ln 2=600R×0.69=414R.

Compression, so the work done on the gas is +414R.

Why the other options are wrong

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