1. Rayat Shikshan Sanstha’s , Yashavantrao Chavan Institute of Science Satara Presentation on SURFACE TENSION Shri. JadhaV s. k.

2. Class- XII
Physics - I
SURFACE TENSION

3. Surface Tension Surface tension is the property by virtue of which
the free surface of a liquid behave like a stretched
elastic membrane under tension and tries to have minimum possible area.
Examples :
1) Rain drops are spherical in shape due to surface tension
2) Insects can walk on the surface of water due to surface tension

4. Surface Tension : The tangential force per unit length acting at right
angle to an imaginary line drawn in the free surface
of a liquid is called Surface Tension.
T =
T = F/L
Unit - N/m or J/m2 - S.I. dyne/cm or erg/cm2 - CGS
1 N/m = 103 dnye /cm Dimensions - [M1L0T-2]
FFF F L

5. Application 1) The force required to lift a thin wire or needle of
length l from the surface of water or liquid is
F = T x 2L
2) Force required to lift a thin rectangular plate from
the surface of water is
F = 2T (L + b)
where l is length and b is breadth.
3) Force required to lift thin circular plate from the surface of water is
F = T x 2π r,
where r - radius of the plate.
4) Force required to lift a ring from surface of water is
F = 2π T (r1 + r2)

6. Intermolecular force :
The force of attraction or repulsion between two molecules of substance is called intermolecular force.
Cohesive force :
The force of attraction between the molecules
of two same substance is called as Cohesive force

7. 2) Adhesive force : The force attraction between two molecules of different substance is called adhesive force.

8. Range of Molecular Attraction : The maximum distance up to which the intermolecular force of attraction is effective is called range of molecular attraction.
Sphere of influence :
An imaginary sphere drawn with a molecule as centre and the range of molecular attraction as radius is called the sphere of influence.
Surface film
The thin layer of the liquid whose thickness is equal to range of molecular attraction.

9. Surface Tension on basis of molecular TheoryP Q P’ Q’ A

10. Surface energy—
The potential energy per unit surface area is called surface energy per unit area of surface.
Units--
S.I. — joule
CGS – erg
Dimensions—

11. Relation between surface tension and surface energy— P P’ A D F F’ L B Q Q’ C

12. .
Work done = P.E. = Surface Energy = T.dA .
dA = 2Ldx

13. Characteristics of angle of contact :
The angle between the tangent drawn to the liquid surface at point of contact and solid surface measured on the side the liquid is called the angle of contact. (θ)
Characteristics of angle of contact :
It is constant for a given liquid- solid pair
The liquid, which completely wets solid it is zero
e.g. pure water in contact with glass.
The liquid which partially wets the solid it is acute
e.g. Kerosene in contact with glass.

14. Characteristics of angle of contact :
4) The liquid which does not wet’s the solid, it is obtuse e.g. Mercury in contact with glass. 5) It increases with increasing temperature 6) A water proofing agent (powder) increases the angle of contact. 7) It decreases with adding insoluble or partially soluble impurity. 8) It increases with adding soluble impurity. 9) It depends upon medium above the free surface.

15. Different forces acting on molecule A.
Angel of contact on the basis of molecular forces.
Molecular Forces
SOLID
Air A p Q Q
LIQUID
Liquid
Different forces acting on molecule A.

16. A P R Q Q T θ Case 1. Liquid which partially wets the solid SOLID Air

17. T A P θ Q Q R Case 2. Liquid which does not wets the solid. SOLID Air

18. A P Q Case 3.Liquid which completely wets solid. R T SOLID Air LIQUID

19. Shape of liquid drop Liquid which Partially wets the Solid
Liquid which doesn’t wet the Solid
T3sinθ T3 T3
air
air
Liquid
Liquid θ θ T1 T2
T3cosθ
solid
solid
For equilibrium of the drop,

20. This equation gives following cases
If and
cosθ is positive
θ is acute
ex. kerosene in contact with glass
Case 2 : If
cosθ is negative
θ is obtuse
ex. mercury in contact with glass
Case 3 : If
cosθ = 1
θ is nearly equal to zero
Ex. water in contact with glass
Case 4 :
If or
cosθ > 1
which is impossible
liquid is spread over the solid surface and drop shall not be formed.

21. Drops and bubbles
The shape of liquid drops and bubbles are spherical due to surface tension
Expression for excess pressure :
The excess pressure inside the drop is (Pi-Po)
The work done during the expansion
But,
For Soap Bubble P0 Pi r dr

22. Pressure difference exist across free surface of liquid
Case-1 For plane surface
Case-2 For concave surface
Case-3 For convex surface
pressure difference on two sides of plane surface is zero while pressure on concave side is greater than pressure on convex side of liquid surface T T T T

23. Explanation of capillarity or capillary action
The rise or fall of a liquid in a capillary tube is called Capillarity
PA > PB (1)
PC = PD (2)
From equation (2) and (3)
PA = PD
From equation (1) and (4) A C B D

24. Capillarity : Applications :
I) A blotting paper absorbs ink or rag cloth absorbs water due to capillarity
II) Ink rises in a pen or oil rises up the wick of a lamp due to capillarity
III) From the roots of tree Sap rises through the stem due to capillarity
IV ) Water under the soil rises to the surface through capillaries formed in the soil
V ) Farmers plough their field in summer these capillaries are broken and water is retained
Tcos
Tcos T T
Tsin
Tsin h  

25. Factor’s affecting on Surface tension
1) Effect of temperature on S.T. :
The surface tension decreases with increasing temperature.
In some exceptional cases the S.T. of the liquid increases with increasing temperature.
e.g. molten Cu & Cd
Application :
It is easier to wash clothes in hot water than
ordinary water because S.T. of hot water is less
than cold water which help to clean clothes

26. Application 2) Effect of impurity on Surface tension
i) S.T increases with highly Soluble impurity.
e.g. NaCl
ii )S.T decreases with weakly or insoluble impurity.
Application
a) Soap or any detergents are used for washing clothes purposes because they reduces S.T. and help to remove dust particles from dirty clothes
b) Tooth paste used for cleaning the teeth which reduces S.T. and helps cleaning teeth

27. ..
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