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Definition: For a liquid in
contact with a solid, the angle between the surface of the solid and the
tangent drawn to the free surface of the liquid at the point of contact
measured from the side of the liquid is called the angle of contact.
Characteristics of the Angle of Contact:
Capillary Tube: A tube with a very fine bore is called
a capillary tube.
Applications of Capillarity:
to Find Surface Tension of a Liquid by Capillary Rise Method
Principle: The height, h, to which a
liquid rises in the tube and the radius of the tube are carefully
measured. Surface tension is calculated using the appropriate formula.
Apparatus: A clear capillary tube is dipped in a liquid and
clamped vertically. A bent index pin is attached to the capillary tube
and adjusted so that its tip just touches the water surface outside the
Measurement of h:
A travelling microscope with known least count
is focused on the liquid meniscus in the capillary. The horizontal
cross-wire is made tangential to the liquid meniscus and the microscope
reading is noted.
The beaker of water is removed. The microscope is focused on the tip of
the index pin and the reading is noted. The difference between these two
readings gives the capillary rise.
Measurement of r:
tension is calculated using the formula T =
2 cos q
where r is the radius
of the capillary tube, h is the capillary rise, d is the density of the liquid, g is the
acceleration due to gravity and q is the angle of contact.