Surface Tension
Cohesive and adhesive force
The force of attraction between similar molecules is called cohesive force eg:- Force between water molecules.
The force of attraction between dissimilar molecules is called adhesive force eg:- Force between water and Glass molecules.
Surface Tension
The property by virtue of which liquid surface behaves like a stretched member and tends to occupy the minimum surface area is called surface tension.
Mathematically
Surface tension is the force per unit length of an imaginary line drawn in the plane of liquid surface action at the right angle of this line.
If 'f' be the force acting on the imaginary line of length 'l' then
T= f/L
Note
a. In the case of an object
i. when the object 'l' is on the surface of the liquid
T=f/l
ii. When an object of length 'L' is inside the liquid
T=F/2l
b. In case of a ring;
i. when a ring of perimeter 2π on the surface of the liquid
T=F/2πR
ii) When a ring of '2πr' is inside the liquid;
c. In the case of a rectangle
i) When the rectangle of perimeter '2(l + b)' on the surface;
ii) When the rectangle of '2(l + b)' is inside the liquid;
Surface Energy
The amount of workdone against surface tension per unit change in surface area is called surface energy.
i.e.,
Let us consider a rectangle frame ABCD of wire in which BC can be slid horizontally. Suppose it is dipped in a soap solution such that a thin layer is deposited over an area. if BCD is due to surface tension (T) the layer tends to minimize surface area. Suppose, force 'F' is applied perpendicularly to BC towards outwards. So, the membrane is extended by a distance 'x' horizontally. Such that force is given by;
F = t.2l [ ∵ Since the soap film has two surfaces in contact with air, so total length of wire = 2l ]
So, workdone by this force 'F';
W = F x d
= F x X
W = t.2l.X
Hence,
This surface energy is equal to surface tension.
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