Fluids At Rest And In Motion – Surface Tension SS1 Physics Lesson Note

Surface tension is defined as the force per unit length on either side of the imaginary line drawn on the liquid surface at rest. The surface of a liquid behaves as if it was covered with an elastic skin or as if it was in a state of tension.

Surface tension is the force that causes the surface of a liquid to act like a stretched elastic skin. It is a vector quantity and is measured in N/m or Nm¹.

Given below in a table is the surface tension of various liquids:

Demonstration of Surface Tension Phenomenon

Fill a beaker with water and allow the water surface to be completely at rest. Place a dry needle on a small piece of blotting paper and gently place the needle on the water surface without disturbing it. After some time, the paper becomes wet and sinks while the needle remains floating on the water’s surface. Careful observation of the water surface reveals that it acts as a skin, supporting the eight of the needle.

Also, observe the sprout of a tap through which droplets of water gather into a large ball without falling. Careful observation shows the surface of the water like a bag (skin) holding drops. Until the drop is too heavy and breaks off it remains like a bag at the sprout. Other surface tension phenomenons are:

1. A soap bubble forms when a ring is dipped into soapy water.

(b) When a paintbrush is dipped into clean water the hair all comes together, and the hair spreads out when the brush is removed. While in the water surface tension forces, in the film between the hair pull them together tightly.

What Causes Surface Tension?

Intermolecular forces such as the Van der Waals force, draw the liquid particles together. Along the surface, the particles are pulled toward the rest of the liquid. Surface tension is defined as,

The ratio of the surface force F to the length L along which the force acts.

Mathematically, surface tension can be expressed as follows:

T=F/L

Where,

F is the force per unit length

L is the length in which force act

T is the surface tension of the liquid

What is the unit of surface tension?

The SI unit of Surface Tension is Newton per Meter or N/m. 

As we know, surface tension is given by the formula, Surface tension = F/L

We know that F = ma, substituting the value in the equation, we get

 =ma/L

Equating the fundamental quantities into the equation, we get

=MLT-2L-1

Solving further, we get 

MLT-2L-1

Solving further, we get =MT-2

Hence the dimensional formula of surface tension is MT-2

Reduction of Surface Tension

Surface tension can be reduced through one of the following ways:

1. By adding oil to the water
2. By using grease
3. By using kerosene
4. By adding detergent/soap to the water
5. By boiling the water
6. By adding camphor to the water
7. By adding alcohol to the water.

Examples of Surface Tension

1. Rainproof tent materials where the surface tension of water will bridge the pores in the tent material
2. Rainproof tent materials where the surface tension of water will bridge the pores in the tent material
3. Clinical test for jaundice
4. Surface tension disinfectants (disinfectants are solutions of low surface tension).
5. Cleaning clothes with soaps and detergents which lowers the surface tension of the water Washing with cold water
6. Round bubbles where the surface tension of water provides the wall tension for the formation of water bubbles. This phenomenon is also responsible for the shape of liquid droplets.

Example

Compute the surface tension of a given liquid whose dragging force is 7N and the length in which the force acts is 2m.

Solution

Given,

• F = 7N
• L = 2m

According to the formula,

T = F/L

→T=7/2

→T=3.5 N/m

MEANING OF VISCOSITY

Viscosity is the internal friction which exists between layers of the molecules of a fluid (liquid or gas) in motion. The viscosity of a fluid can also be defined as the measure of how resistive the fluid is to flow. It is a vector quantity & measured in pascal-seconds(pa.s). It can be defined mathematically as the ratio of the shearing stress to the velocity gradient in a fluid

Viscosity (ŋ) =           Force

                   Area x Velocity gradient  

Velocity gradient = velocity

                                Length   

W = U +  V

W  U- V = 0        

V = W  U (apparent or effective weight) where V-viscous force, W- weight, U- upthrust

NB: Substances with low viscosity include water, kerosene, petrol, and ethanol. Those with high viscosity are glue, syrup, grease, glycerine etc

Factors Affecting Viscosity

Viscosity varies with material (viscosity is a property of material)

The viscosity of simple liquids

(a) decreases with increasing temperature 

(b) increases under very high pressure

The viscosity of gases 

(a) increases with increasing temperature 

(b) is independent of pressure & density

Effect of Viscosity

Viscosity is responsible for different rates of fluid flow.

Viscosity affects the motion of a body in a fluid.

Application of Viscosity

1. It is used as a lubricant.
2. The knowledge of viscous drag/drag force is applied in the design of ships & aircraft.
3. Used to estimate the enlarged size of particles.

ASSIGNMENT 

1. What is viscosity?
2. What is terminal velocity?
3. State two(2) substances with high viscosity.
4. State Stokes law.
5. What is a viscoelastic substance?