Answer: Capillary depression occurs (mercury is pushed down).
- A Capillary depression occurs (mercury is pushed down)
- B Capillary rise occurs, exactly as it does for water in a glass tube
- C No capillary effect occurs for an obtuse angle of contact
- D Mercury overflows out of the top of the capillary tube largely
Correct answer: A. Capillary depression occurs (mercury is pushed down)
Explanation: When θ > 90°, cosθ < 0. Capillary height h = 2T cosθ/(ρgr) becomes negative. This means capillary depression - mercury is pushed down. Mercury meniscus is convex.
Since the same volume of fluid must pass every cross-section per second (continuity), the fluid speeds up where the pipe narrows; Bernoulli's equation then says this faster-moving fluid has lower pressure - the principle behind a venturi meter and an aircraft wing's lift.
Concept context
Pressure, buoyancy, Bernoulli equation, viscosity, and surface tension.