मराठी

Surface Tension and Surface Energy

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Topics

Estimated time: 7 minutes
  • Surface Tension
  • Surface Energy
  • Relation between the surface energy and surface tension
Maharashtra State Board: Class 11

Definition: Surface Tension

The property of a liquid due to which its free surface tries to have minimum surface area and behaves as if it were under tension somewhat like a stretched elastic membrane is called surface tension.

Maharashtra State Board: Class 11

Definition: Sphere of Influence

An imaginary sphere drawn round a molecule (taken as centre) with a radius equal to the range of molecular attraction is called the sphere of influence of that molecule.

Maharashtra State Board: Class 11

Definition: Surface Film

A thin film of liquid near its surface having thickness equal to the molecular range of attraction is called surface film.

Maharashtra State Board: Class 11

Definition: Surface Energy

The work per unit area done by the force that creates a new surface is called surface energy.

Maharashtra State Board: Class 11

Definition: Excess Pressure

The difference of pressure between the two sides of a liquid surface, which arises in equilibrium because the pressure inside a bubble or drop is greater than outside, is called excess pressure.

Maharashtra State Board: Class 11

Formulas: Excess Pressure

Surface Excess Pressure
Plane surface Δp = 0
Convex surface Δp = \[\frac {2T}{R}\]
Drop Δp = \[\frac {2T}{R}\]
Bubble in liquid Δp = \[\frac {2T}{R}\]
Bubble in air Δp = \[\frac {4T}{R}\]​
Bubble at depth h (density d) Δp = \[\frac {2T}{R}\] + hdg
Cylindrical liquid surface Δp = \[\frac {T}{R}\]
Maharashtra State Board: Class 11

Key Points: Surface Tension

  • Surface tension depends only on the nature of liquid and is independent of area of surface or length of line considered.
  • Surface tension of a liquid decreases with rise of temperature; it is zero at boiling point and vanishes at critical temperature.
  • Due to surface tension, a drop or bubble tends to contract, which increases internal pressure — this difference between inside and outside pressure is called excess pressure.
  • For a drop and bubble in liquid: Δp = \[\frac {2T}{R}\]​; for a bubble in air: Δp = \[\frac {4T}{R}\]​(two free surfaces).
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