“Molecular Theory for Surface Tension” MHT-CET Class 12 Physics Crash Course | Chapter Mechanical Properties of Fluids
In this session G.S. Khairnar discussed the theory and MCQ’s of “Molecular Theory for surface Tension”. The session will be useful for all the aspirants of MHT-CET 2022. The session will be in Marathi and English.
Molecular Theory for surface Tension:-
Hello students, in this article we are going to discuss the important property of fluid due to which insects can walk on the pond of water, drops of liquid becomes spherical in shape and so on. The property of liquid which we are going to discuss is termed as ‘surface tension’. Before starting with actual discussion for surface tension, let’s define some terms as follows,
- Intermolecular force of attraction:-
Between any two molecules of a liquid, there exists a force of attraction. This force is called as the intermolecular force.
The intermolecular force is a short range; i.e.it effective over a very short distance, (about 10-9 m) beyond this distance, the force is negligible.
- Types of intermolecular force:-
(1) Cohesion and Cohesive force:
Force of attraction between two molecules of the same substance is called cohesion, and the force of attraction is called the cohesive force.
E.g. i) Attraction between two water molecules. ii) Attraction between two mercury molecules.
(2) Adhesion and Adhesive force:
Force of attraction between two molecules of the different substance is called adhesion, & the force of attraction called adhesive force.
E.g. Attraction between molecules of water & glass.
- Range of molecular attraction:
The maximum distance between two molecules up to which intermolecular force are effective is called range of molecular attraction.
- Sphere of influence:
An imaginary sphere drawn round a molecule as a center, with a radius equal to the range of molecular attrition is called the sphere of influence of those molecules.
- Surface film:
The liquid layer at it free surface having thickness equal to range of molecular force is called as surface film.
Let’s learn the concept Molecular theory for surface tension …!
Consider three molecules A,B,C of same liquid in the container such that A lies well inside the liquid, B just below the surface and C exactly at the surface of liquid, as shown in fig.
Molecule A: The sphere of influence of molecule A lies completely inside the liquid. As a result this molecule get attracted by neighbouring liquid molecules equally in all directions and therefore net cohesive force is zero.
Molecule B: Major part of the sphere in influence of molecule B is inside the liquid while small part is outside. As a result molecule get attracted be more liquid molecules with in liquid and by few air molecules outside the liquid.
Thus, net cohesive force on molecule ‘B’ is in downward direction.
Molecule C: Exactly half sphere of influence of molecule C is inside the liquid, there is large number of liquid molecules in lower half while few air molecules in upper half of sphere of influence. Molecule C is acted upon by net resultant force is downward force its magnitude is maximum.
Thus molecules lies in surface film of liquid experience maximum downwards force. Every system tries to have minimum potential energy for equilibrium.
To have minimum potential energy, molecules at liquid surface tend to reduces area to minimum value. Thus, liquid surface behave as a stretched membrane. This tendency of liquid surface is known as surface tension.
Now we shall define the concept of surface tension…….!
It is defined as the force per unit length.
The force acting per unit length of imaginary line drawn on free liquid surface at right angles on either sides of the line.
If L is the length of line & F is the force acting on it, the surface tension (T) is given by,
Molecular theory for Surface Tension:-
Let’s solve the following MCQ’s……………!
Q.1) The force of attraction between two molecules of identical substance is…….
a) cohesive force
b) adhesive force
c) dissipative force
d) frictional force
Answer
a) cohesive force
Q.2) The force of attraction between two molecules of different substance is…….
a) cohesive force
b) adhesive force
c) dissipative force
d) frictional force
Answer
b) adhesive force
Q.3) The range of intermolecular force is…….
a) 10+9 m
b) 10+12 m
c) 10-12 m
d) 10-9 m
Answer
d) 10-9 m
Q.4) The resultant force acting on the molecule lying exactly on free surface of liquid is…
a) Adhesive force, directed inside the liquid
b) Adhesive force, directed outside the liquid
c) Cohesive force, directed inside the liquid
d) Cohesive force, directed outside the liquid
Answer
c) Cohesive force, directed inside the liquid
Q.5) Resultant force acting on the molecule lying completely inside the liquid is…
a) Zero
b) Adhesive force, directed outside the liquid
c) Cohesive force, directed inside the liquid
d) Cohesive force, directed outside the liquid
Answer
a) Zero
Q.6) Tendency of free surface of liquid to behave as stretched membrane is known as…
a) Viscosity
b) Pressure
c) Surface tension
d) Potential energy of liquid
Answer
c) Surface tension
Q.7) Square plate of side 4 cm is dipped in water, force acting on the plate due to surface tension is (T=0.08 N/m)…
a) 13 mN
b) 13 kN
c) 13 N
d) 13 dyne
Answer
a) 13 mN
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