Surface Tension and Capillarity
Surface tension is the force per unit length in a liquid surface; capillary rise is h = 2 S cos t/(r ρ g).
What is Surface Tension and Capillarity?
Surface tension is the force per unit length in a liquid surface; capillary rise is h = 2 S cos t/(r ρ g).
Key formula / rule: Excess pressure: drop 2S/r, soap bubble 4S/r.
Key points
- Compute capillary rise and excess pressure.
- Explain surface tension in terms of intermolecular forces.
Common exam trap
Using the drop formula for a soap bubble with its two surfaces.
Definitions
- Term
Surface Tension and Capillarity
- Meaning
Surface tension is the force per unit length in a liquid surface; capillary rise is h = 2 S cos t/(r ρ g).
- Term
Surface Tension and Capillarity — explanation
- Meaning
Molecules at the surface have unbalanced cohesive forces, so the surface behaves like a stretched membrane and minimises area. Excess pressure inside a bubble is 4S/r and inside a drop is 2S/r.
Learning objectives
Compute capillary rise and excess pressure.
Explain surface tension in terms of intermolecular forces.
Formulae
- Key point
Excess pressure: drop 2S/r, soap bubble 4S/r.
- Key point
Capillary rise h = 2S cos t/(r ρ g); narrow tubes give greater rise.
- Key point
Surface tension falls with rising temperature.
Prerequisites
PHY-U3-MPM-T2-S1-C1
Common mistakes
Using the drop formula for a soap bubble with its two surfaces.
Ignoring the contact angle for mercury, which depresses instead of rising.
Keywords
Surface
Tension
Capillarity
Practice preview
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