A level Physics · Edexcel 9PH0 · Topic 4B
Fluid flow and viscosity
Key points
- In laminar flow the fluid moves in smooth layers with no mixing; velocity at a point stays constant. In turbulent flow there are eddies and the velocity changes randomly.
- Flow tends to become turbulent at higher speeds and around sharp edges; streamlining keeps flow laminar and reduces drag.
- Viscosity (, unit Pa s) is a measure of a fluid's resistance to flow. For liquids it decreases as temperature rises.
- Stokes' law: viscous drag on a small sphere moving slowly with laminar flow is .
- At terminal velocity: weight = upthrust + viscous drag, so the sphere stops accelerating.
- Core practical: time a ball bearing falling between marks in a tall tube of liquid (e.g. glycerol), check it has reached terminal velocity, then calculate . Keep the temperature constant.
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Fluids resist objects moving through them and resist flowing themselves. This sub-topic describes the two kinds of flow, defines viscosity and uses Stokes' law in core practical 4 to measure the viscosity of a liquid.
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