To construct a barometer, a tube of length 1 m is filled completely with mercury and is inverted in a mercury cup. The barometer reading on a particular day is 76 cm. Suppose a 1 m tube is filled with mercury up to 76 cm and then closed by a cork. It is inverted in a mercury cup and the cork is removed. The height of mercury column in the tube over the surface in the cup will be

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76 cm

<76 cm

>76 cm

Zero

answer is D.

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Detailed Solution

The tube has air in it (because it is not fully filled).

By the relation

$P_{a t m} = P_{0} + ρ g h A s, P_{0} > 0 a n d P_{a t m} > ρ g h ∴ 76 c m o f H g > ρ g h h < 76 c m$

Due to the air pressure's opposition to atmospheric pressure, the column's height is less than76 cm.

Hence the correct answer is <76 cm.

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