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Question

A tube of length 1 m is completely filled with mercury and is inverted in a mercury cup for constructing a barometer which reads 76 cm. Suppose a 1 m tube is filled with mercury upto 76 cm and closed by the cork.
It is inverted into a tray containing mercury and the cork is opened. The height of mercury column now in the tube over the surface of mercury in the tray will be:

A
76 cm
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B
Zero
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C
>76 cm
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D
<76 cm
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Solution

The correct option is D <76 cm
When a tube of length 1 m is inverted over mercury cup and the cork is opened, the inverted tube is not completely filled with mercury as some air is filled in the top of the tube.
When the tube is completely filled, the barometer reads 76 cm.
Since, some air is present, it applies pressure against the atmospheric pressure.
Therefore, the height of the column is less than 76 cm

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