Surface tension is the property of a liquid because of which
the surface behaves like a stretch the elastic membrane. Surface tension is due
to the inter molecular force of attraction between the molecules of the liquid.
The molecular force is always an attractive force. The molecular force of
attraction between the similar kind of molecules is called a cohercive force
and the molecular force of attraction between the different kinds of molecules
is called as an adhesive force. Both of them are always an attractive forces.
Each molecule can influence the surrounding molecules up to
certain distance and this particular distance is called as a molecular range.
Taking the molecule as the centre and the molecular range is a radius if a sphere is drawn,then that is called a sphere of influence. Within the sphere of
influence, the given molecule can influence the surrounding molecules.
Surface tension is defined as a tangential force acting per
unit length at right angles to the either side of the imagined line drawn on
the free liquid Surface of the liquid in
the equilibrium state.
To understand the direction of the surface tension force,we
shall imagine a line on the surface of
the liquid. At right angles to the
either side of the line if you draw a tangential line, it gives the direction
of the force and the direction of the surface tension. This is treated as a scalar physical quantity.
A small needle is able to float on the surface of the liquid
because of the surface tension force.
A small insect is able to walk on the surface of the water
due to the surface tension. Here the weight of the small insect is able to be
compensated by the force due to the surface tension in the opposite direction ,
therefore it is not sinking down.
When we are using the
length in the definition of the surface tension we shall write the free length
of the surface. Under different circumstances the length of the free surface
will change and correspondingly the force due to the surface tension also
change as shown below. When you are dealing with the wire it will how two free
surfaces where as when we are dealing with a closed body like a metal body it
will have only one free surface.
Problem and solution
When a wire of length l
and a cross-sectional radius r is
floating on the surface of the liquid, find the minimum surface of the wire
such that it may not sink ?
While we are solving this problem, we shall consider that the weight is acting in the downward direction and is compensated by the force due to
the surface tension. Therefore at the equilibrium state we can just equate this two forces.
Problem and solution
A liquid is contained in a vertical Tube of a semicircular
cross-section find the ratio of the force of surface tension on the circular part
as well as the flat part?
In solving this problem we have to count the surface length
of the liquid bit carefully. The 1st case it is half of the
circumference of the circle it is in the second case it the flat part and is nothing but the diameter of the circle.
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