### Surface Tension

• Surface tension occurs at the interface of the liquid and a gas or at the interface of two liquids.
• Surface tension (σ) is inversely proportional to temperature.
• It acts when fluid is at rest.
• It is a surface phenomenon.
• It is force per unit length (N/m).
• It is due to cohesion only.
• For water-air interface at 20 °C, its value is 0.0736 N/m.
• At the critical point, liquid-vapor states are same therefore σ = 0.

P = 4σ/d

P = 8σ/d

#### Pressure inside Jet

P = 2σ/d

Here d = diameter of a drop

P = Gauge Pressure

## Capillary Action

• Capillary Action is due to cohesion and adhesion, both.
• For capillary action diameter of the tube should be less than 3 cm.

#### Height of water in capillary Tube

h = 4σcos θ/ρgd

Where, h = rise in capillary

σ = surface tension of water and glass

d = diameter of a tube

θ = Angle of contact between the liquid and material

• For water and glass take θ = 0 degree
• For mercury and glass take θ = 128 degree

#### When liquid surface supports another liquid of density ‘ρb’ then rise is given by

h = 4σcos θ/(ρ – ρb)gd

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