Surface Tension
Cohesive Force : The force of attraction between the
molecules Of same substance is called eohesive force.
Cohesive force is maximum in This iS why solids
have a fixed shape. Cohesive force is negligible in case of
Adhesive Force: Force Oi attraction the
molecules of different gubßtanceg is called adhesive force,
Due to adhesive force, one body sticks to other.
Surface Tension : Surface tension is the property Of a
liquid by virtue of which it has the tendency to have the area
of its free surface minimum as if it were under tension like
a stretched elastic membrane.
A hquid drop attains spherical due to surface
tension. since, for given volume, sphere has minimum
surface area. Surface tension of a liquid is measured by
the normal force acting Fh.•r unit length on either side of
an imaginary line drawn on the free surface of liquid and
tangential to the free surface.
if a force F acts on an imaginary Line of length then
surface tension, T =F/ I.
Work done in increasing the surface area Of a liquid
by unity under isothermal condition is equal to surface
tension of liquid, According to this definition, unit of
surface tension is joule/ meter:-
Surfacetensionof a liquid decreases with the increase of
temF*rature and becomes zero at critical ternl»rature.
Capillary tube: A tube having very narrow (fine) and
uniform bore is called a capillary
Capillarity : If a capillary is dipped in a liquid,
liquid ascends or descends in the capillary tube. This
phenomenon is called capillarity,
The height by which liquid ascends or decends in a
capillary upn the radius of the
Ille capillarity on the nature of liquid and
solid both, The liquid which wets the wall of tube rises in
the tube and the liquid which not wet the wall of tube
de«ends in the tube For example, when a glass capillary
is dipped in water. water rises in the tube and of
water meniscus is concave, similarly when a glass capillary
is in mercury, mercury decends in the and
of mercury meniscus is convex.
Illustrations Of capillarity
1. A piece of blotting paper soaks ink because the pres Of
the blotting palkr serve as capillary tubes.
2. The oil in the wick of a lamp rises due to capillary action
of threads in the wick.
3. The root hairs of plants draws water from the soil
through capillary action.
4. To prevent loss of water due to capillary action, the soil
is loosened and split into pieces by the farmers.
5. If a capillary is dipped in water in an artificial
satellite, water rises up to otherend of because of
its zero apparent we•ght. how lcmg the tube may
6. Action of towel in soaking up water from the body is
due to capillary action of cotton in the towel.
7. Melted wax, in a candle up to wick by capillary
action.

If a clean and dry needle is kept slowly on the surface
Of water, it floats due to surface tension.
The addition Of detergent or soap decrease the surface
tension of water and thus increaæs the cleaning ability.
Bubbles of soap solution are big because addition of
soap decreases surface tension of water.
When kerosene oil is sprinkled on water, its surface
tension decreases, As a result the larva of mosquitoes
floating on the surface of water die due to sinking.
Warm soup is tasty because at high its
surface tension is low and consequently the soup
spreads on all parts Of the tongue.
If the liquid surface is curved, there is always an excess
pressure on the concave side.
Excess pressure in a liquid drop = 2T/R
Excess pressure in a soap bubble =4T/R
T= surface tension, R = radius