Viscosity

Intermediate

Internal friction in fluids.

Key relationF = ηA dv/dy

Viscosity η measures internal friction between fluid layers: F = ηA(dv/dy). Honey is more viscous than water; viscosity of liquids decreases as temperature rises, gases the opposite.

Worked example

Water 1 mPa·s, glycerol 1.4 Pa·s.

Common mistake

Not the same as density.

In more detail

Viscosity is a measure of a fluid's resistance to flow, arising from internal friction between layers. Honey has high viscosity, and water has low viscosity.

Real-world uses

Engine oils are chosen for their viscosity at operating temperatures.
Food industry processes depend on the viscosity of sauces and paints.
Motor engineers choose oils with the right viscosity so that engines flow easily when cold and protect parts when hot.

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Frequently asked questions

What are the units of viscosity?

Pascal seconds, though the poise is also used.

Does temperature change viscosity?

Yes. Liquids become less viscous when heated, while gases become slightly more viscous.

Is viscosity the same as density?

No. Viscosity measures a fluid's resistance to flow, while density measures mass per unit volume, so they are different properties.

Viscosity

In more detail

Real-world uses

Frequently asked questions

Related topics in Fluids & Elasticity