Venturi tube

Meaning: A Venturi tube is a pipe that narrows to a throat and then widens again. An incompressible fluid has a constant flow rate, so the fluid must speed up in the narrow part (continuity). Bernoulli's principle then says that, at the same height, the faster section has the lower pressure. The pressure difference is measured with a manometer or with vertical tubes, and it gives the speed and flow rate.

Equation: P₁ − P₂ = ½ρ(v₂² − v₁²), with A₁v₁ = A₂v₂. P is pressure (Pa), ρ is fluid density (kg/m³), v is speed (m/s), A is cross-sectional area (m²). Subscript 1 is the wide section and 2 is the throat. If the pipe changes height, add the term ρg(h₂ − h₁) to the right side, where g is 9.8 m/s² and h is height (m).

On the MCAT: A passage shows a horizontal pipe with a constriction and vertical tubes above it. You predict which fluid column is lower (the one over the throat), or use the height difference (ΔP = ρgΔh) to calculate speed. A common quick check is that halving the radius cuts the area to one quarter and makes the speed four times larger.

Common confusion: Students think squeezing the fluid raises the pressure in the narrow part. It does not. The fluid gains kinetic energy at the expense of pressure, so the throat has lower pressure. The equation assumes steady, nonviscous flow; in a real viscous fluid pressure also falls along the pipe.