The lengths of air column are 10 cm and 8 cm in a faulty barometer when it reads 72 and 71 cm of Hg. The value of atmospheric pressure is:

68 cm of Hg

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71.5 cm of Hg

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76 cm of Hg

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74 cm of Hg

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Solution

The correct option is C 76 cm of Hg
Let the height of air column = $l$ cm and $P$ be the atmospheric pressure
In a barometer, the enclosed air column obeys Boyle's law
For the enclosed air column,
$(P - r e a d i n g) \times l$ = constant
Case 1: $(P - 72) \times 10$
Case 2: $(P - 71) \times 8$
Since both are equal,
$(P - 72) \times 10$ = $(P - 71) \times 8$
2P = 720 - 568 = 152 cm of Hg
P = 76 cm of Hg

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