Abarometer tube, containing mercury, is lowered in a vessel containing mercury until only $50\,\, cm$ of the tube is above the level of mercury in the vessel. If the atmospheric pressure is $75 \,\,cm$ of mercury, what is the pressure at the top of the tube?
A$33.3 \,\,kPa$
B$66.7 \,\,kPa$
C$3.33 \,\,MPa$
D$6.67\,\, MPa$
Medium
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A$33.3 \,\,kPa$
a since the mercury rises only $50 \mathrm{cm}$ it implies the rest of the excess atmospheric pressure is being countered by the pressure at the top of the tube.
So pressure at top $=$ atmospheric pressure $-50 \mathrm{cm}$ of $\mathrm{Hg}=(75-50) \mathrm{cm}=25 \mathrm{cm}$ of $\mathrm{Hg}=$ $1 / 3 \times 100 k P a=33.33 k P a$
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