Surface Tension
Cohesive force: The
force of attraction between the molecules of the same material is called
cohesive force. Cohesive force is maximum in solids. Because of that solids
have fixed shape and cohesive force is negligible in gases.
Adhesive Force: Force
of attraction between the molecules of different substance is called adhesive
force. Due to adhesive force anybody sticks to other.
Surface Tension: Surface
tension is property of a liquid by virtue of which it has the tendency to have
the area of its free surface minimum as if it ware under tension like a
stretched elastic membrane.
Surface
tension of a liquid is measured by the normal force acting per unit length on
either side of an imaginary line drawn on the free surface of liquid and tangential
to the free surface.
So, if a
force F acts on an imaginary line of length l, then surface tension,
T = F/l.
► 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/meter2.
► Surface tension of liquid decreases with the increase of
temperature and becomes zero at critical temperature.
Capillary tube: A
tube having very narrow (fine) and uniform bore is called a capillary tube.
Capillarity: If a
capillary tube is dipped in a liquid, liquid ascends or descends in the
capillary tube. The phenomenon is called capillarity.
► The height by which liquid ascends or descends in
capillary tube depends upon radius of the tube.
The
capillarity depends on the nature of liquid and solid both. The liquid which
wets the wall of tube rises in tube and the liquid which does not wets the wall
of the tube descends in the tube. For example, when a glass capillary tube is
dipped in water, water rises in the tube and the shape of water meniscus in
concave, similarly when a glass capillary tube is dipped in to mercury; mercury
descends in the tube and shape of mercury meniscus in convex.
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