# list of physics formulas class 11 chapter Surface Tension

## Class- 11 chapter Surface Tension

### Inter-Molecular force

The force between the molecules of the substances are called intermolecular forces. Types of intermolecular forces.

• Force of adhesion or adhesive force: If is the force of attraction acting between the molecules of different substances.
• Force of cohesion or cohesive force: The force of attraction amongst the molecules of the same substance.
• Surface tension: Surface tension is the molecule property of free liquid surfaces, by virtue of which it behaves like a stretched elastic membrane with a tendency to contact so as to occupy minimum surface area.
• To Measure surface tension: Surface tension of a liquid can be measured as the force per unit length on an imaginary line drawn on the liquid surface, which acts perpendicular to the line on its either side at every point and parallel to the liquid the surface. S = F/L units: Nm-1. Dimensional formula ML0T–2.
• Surface Energy: The potential energy per unit area of the surface film is called surface energy.

• Surface tension of a liquid is numerically equal to its surface energy.
• Capaillary tube: A tube of very fine bore.
• Capillarity: Rise or fall of a liquid in a tube of very find bore is called capillarity.
1. The fine pores of a blotting paper act like capillary tubes. Ink rises in them leaving the paper dray.
2. A towel soaks water on account of capillarity action.
3. Oil rises in the long narrow spaces between the threads of a wick, because they acts as fine capillaries.
• Angle of contact: The angle between the tangent to the liquid surface and tangent to the solid surface at point of contact inside the liquid in known as angle of contact.depends upon the nature of the liquid and solid in contact.
1. Depends upon the nature of the liquid and solid in contact.
2. Depends upon the medium which exists above the free surface of liquid.
3. Is independent of the inclination of the solid to liquid surface.
4. Is fixed for a given pair of solid and liquid and surrounding medium.

### Important Points:

• The angle of contact changes with the pair of solid and liquid.
• For pure water and glass angle of contact as zero.
• Angle of contact increases on increasing temperature.
• Angle of contact changes on adding impurities to liquids.
• If angle of contact is less the 90°.
1. The liquid wets the solid surface.
2. The liquid spreads on solid surface.
3. There is a capaillary rise.
4. The liquid meniscus is concave.
• If the angle of contact in greater than 90°.
1. The liquid does not wet the solid surface.
2. The liquid does not spread on solid surface.
3. There is a capillary dip.
4. The liquid meniscus is convex up.

### Important Formula

Formula to determine surface tension by capillarity method.

h: capillary height.

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