Page 137 - Physics

Mechanical properties of matter
Mechanism of capillarity
Capillarity occurs because
of the intermolecular forces
between the liquid and the
surrounding solid surfaces. If
the diametre of the opening
is sufficiently small, the
combination of surface tension
and adhesive forces between
the liquid and the container
act to propel the liquid through
the opening. Note that, surface
tension is due to cohesive
forces. That is, capillarity is
caused by both adhesion and
cohesion.
A common apparatus used to
demonstrate capillarity is the
capillary tube. The capillary
tube is a narrow tube with a
fi xed length. When the lower
end of the tube is placed
in water, adhesion occurs
between the liquid and the
tube walls. This pulls the
liquid column along the tube
wall. The height of the liquid
column increases until the
weight of the liquid column
is suffi cient to overcome the
cohesive and adhesive forces
that propel the liquid.
Factors affecting capillarity
Capillarity is mainly affected
by the following factors:
Physics Form 1 Final.indd 131
The diametre of the tube or opening
The height of the liquid column is inversely proportional
to the diametre of the tube or opening. The weight
of the liquid column is proportional to the square of
the tube’s radius. It follows that a narrower tube will
draw a longer liquid column than a wider tube. For
example, the water in the glass capillary tube will rise
to a height approximated by 0.3 where d is the diametre
d
(in centimetre) of the tube. Thus, if a glass tube has a
diametre of 0.5 mm, water will rise to a height given
as; h = 0.3 cm = 6 cm.
0.05
Tubes of different diameters will result to water columns
of different heights, as shown in Figure 6.14.
Water
Figure 6.14: Capillary rise in tubes of different diameters
Nature of the Liquid
Capillarity also depends on the nature of the liquid
since it is governed by cohesive and adhesive forces.
For example, when the same capillary tube is dipped
in water and mercury, the mercury level in the tube
falls while the water rises, as shown in Figure 6.15.
Water Mercury
Figure 6.15: Capillary rise and fall
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