Q.

A rectangular narrow U-tube has equal arm lengths and base length, each equal to l. the vertical arms are filled with mercury up to l/2 and then one end is sealed. By heating the enclosed gas all the mercury is expelled. If atmospheric pressure is  P0, the density of mercury is ρ and cross-sectional area is S, then[Neglect thermal expansion of glass and mercury]

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a

Work done by the gas against the atmospheric pressure is 5l2P0S

b

Work done by the gas against the gravity is  74Sρgl2

c

Work done by the gas against the atmospheric pressure is  P0Sl

d

Work done by the gas against the gravity is  Sρgl2

answer is A, B.

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Detailed Solution

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Total displacement of mercury against atmospheric process =[l+l2+l]
PEinitial=[(ρSl2)l4]×2+0 {Assuming zero at ground level}; 

PEmax[(ρSl2)×2+ρSl]l

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