HP Flow r⁴ — FRCA Primary MCQ

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EasyPhysics & Clinical MeasurementHP Flow r⁴FRCA Primary

Regarding the physics of flow laminar flow through a tube is described by the Hagen-Poiseuille equation. Flow is proportional to:

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Correct answer: E — The fourth power of the radius

Hagen-Poiseuille: Q = πΔPr⁴/8ηl. Laminar flow is proportional to the fourth power of the radius and inversely proportional to viscosity and length. Halving the radius reduces flow by 2⁴ = 16-fold. For turbulent flow resistance depends on density (not viscosity) and flow is proportional to r⁵.

Reference: Al-Shaikh Stacey 2019 Essentials of Anaesthetic Equipment