Suppose the pressure at the surface of mercury in a barometer tube is P₁ and the pressure at the surface of mercury in the cup is P₂.

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$P_{1} = 0, P_{2} atmospheric pressure$

$P_{1} = P_{2} = 0$

$P_{1} = P_{2} = atmospheric pressure$

$P_{1} = atmospheric pressure, P_{2} = 0$

answer is A.

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Detailed Solution

Since there is nothing above the mercury's surface in a barometer, $P_{1}$ is equal to zero.
However, in cup, atmosphere is above the surface of the mercury. As a result, atmospheric pressure is $P_{2}$ .

$P_{A} = 0; P_{B} = a t m o s p h e r i c p r e s s u r e$

As there is no air above, $P_{A} = 0$

And $P_{B} = 1 a t m$ ,since it is exposed to the atmosphere and feels the weight of the air above it.

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