In a mercury barometer, let P1 be the pressure at the mercury surface inside the (closed, evacuated) tube and P2 the pressure at the mercury surface in the open cup. Which relation is correct?

(A) P1 = 0, P2 = atmospheric pressure
(B) P1 = atmospheric pressure, P2 = 0
(C) P1 = P2 = atmospheric pressure
(D) P1 = P2 = 0

Correct answer: (A) P1 = 0, P2 = atmospheric pressure

Solution

The space above the mercury in the closed barometer tube is essentially a vacuum, so the pressure at that top mercury surface is P1 = 0. The mercury in the cup is open to the air, so its surface pressure equals the atmospheric pressure, P2 = atmospheric. The atmospheric pressure supports the column height.

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