Tension force generated in (applied by) the liquid surface is called surface
tension force
Explanation of surface tension on the basis of intermolecular forces
Explanation of surface tension on the basis of energy
A piece of stone can be of random shape because solids don’t have surface tension. But a piece of water (water
drop) is in spherical shape. Since there is tension in surface of water. So the water surface act like a tight
membrane (tight bag). To minimize its surface area, the water drop takes spherical shape. For small drop
gravitational pressure (?gh) is negligible so the small drop is almost spherical. But in big drop gravitational
pressure (?gh) is considerable so the big drop has oval shape.
Potential energy stored due to surface tension force is called surface energy.
To understand this, suppose a thin film of soap solution is formed between the
fix frame and the slider. Both front and the back surface will pull the slider with
If we fill water in glass tube, the surface becomes concave in shape, if water is filled in silver tube, the surface
becomes horizontal and if Hg is filled in glass tube, the surface becomes convex.
Shape of liquid surface is called meniscus. At point of contact, angle between the tangent to the liquid surface
and solid surface submerged in liquid is called angle of contact
The force of attraction between the molecules of the same substance is called cohesive force. The cohesive
forces is effective if distance between molecules is less than 10–9 m. If distance between molecule is greater than
10–9 m then cohesive force is negligible. The sphere drawn around a particular molecule as centre and range of
cohesive forces (10–9 m) as radius is called sphere of influence (sphere of molecular activity). The centre of
molecule is attracted by only the molecules lying inside the sphere of influence.
Due to stretched rubber, the air The water surface also acts like a stretched rubber.
inside gets compressed. So pressure So due to tension in the surface the water inside get
of air inside will be greater than compressed
pressure of air outside
Applying force balance
Pressure excess inside a liquid bubble kept in air
CAPILLARY ACTION :
A glass tube of very small diameter is called capillary
1. The oil in a lamp rises in the wick by capillary action.
2. The tip of nib of a pen is split up, to make a narrow capillary so that the ink rises upto
the tip of nib continuously.
If capillary tube of insufficient length is used
SOME OTHER APPLICATIONS OF SURFACE TENSION
