venturi effect

Meaning: The Venturi effect is the drop in fluid pressure where a pipe narrows and the fluid speeds up. An incompressible fluid must carry the same volume per second through every cross-section, so it moves faster in the narrow part (continuity). Bernoulli's principle, which is energy conservation per unit volume, then says the gain in kinetic energy comes at the expense of pressure, so static pressure is lower in the constriction (horizontal flow, ideal fluid).

Equation: P₁ + ½ρv₁² = P₂ + ½ρv₂² and A₁v₁ = A₂v₂, where P is pressure (Pa), ρ is density (kg/m³), v is speed (m/s), and A is cross-sectional area (m²). Points 1 and 2 are at the same height.

On the MCAT: A passage shows a tube with a narrow section and vertical pressure-gauge tubes, or a device such as an aspirator or atomizer. You predict faster flow and lower pressure at the constriction, or find v₂ from continuity and then ΔP from Bernoulli. Arterial narrowing may be used to illustrate it.

Common confusion: Narrower does not mean higher pressure. Speed rises in the narrow part and static pressure falls there. Real viscous fluids also lose pressure steadily along a tube, which Bernoulli's ideal model ignores.