18th August 2020

# JSS 1 : PHYSICS : WEEK 2

NEW APPROACH

WEEK/TERM: THIRD TERM/WEEK 2

DATE: 05.05.2020

CLASS: J.S.S 1

SUBJECT: PHYSICS

TOPIC: SURFACE TENSION

REFERENCE: PHYSICS FOR SENIOR SECONDARY SCHOOL BY METROSE (PLC)

BRIEF EXPLANATION: Fluid is at rest when it is not in motion that is, it is stationary. When fluid is at rest, one intermolecular force that is recognizable is surface tension.

NOTES FOR STUDENTS (LESSON ONE)

Surface tension is an inter-molecular force (force that exists between the molecules of a substance) that causes the surface of a liquid to behave as if it is covered by an elastic skin. It is a vector quantity and is measured in N/M or NM-l. thus surface tension = -l

Intermolecular forces in surface tension are the two listed below:

1. Cohesive force or force of cohesion: This is a force that exists between molecules of the same substance. For instance cohesion exist among molecules of water.
2. Adhesion or adhesive force: is the force of attraction existing among molecules of different substances.

Explanation of why water wets glasses but mercury does not.

Water is able to wet the surface of glass because the force of adhesion is greater than the force of cohesion. Similarly, mercury does not wet glass because the force of cohesion is greater than the force of adhesion.

As a result, when mercury spills on glasses, it does not wet the glass but instead forms globules or spherical droplets on the glass.

LESSON TWO

Effects of surface tension or experiments to show there are forces acting along the surface of a fluid.

1. Insects like ants and water skaters are able to hulk across water surface without sinking, due to surface tension.
2. Small metallic objects like razor blade and needles float on water when carefully placed on water because of surface tension.
3. Soap bubbles may be used to demonstrate the existence of surface tension.
4. The rise and fall of liquid in a narrow tube.
5. When water is allowed to drop slowly from a water tap, the shape of each small drop is spherical.
6. When a brush is dipped inside water or paint and is brought out, all the strands cling together.

How to reduce surface tension

Surface tension can be reduced through one of the following ways

1. By boiling the water
2. By adding detergent or soap to the water
3. By adding oil to the water
4. By adding alcohol to the water
6. Adding camphor to the water.

Exercises/Assignment

1. What is surface tension?
2. Surface tension is a _____________ quantity and is measured in ____________
3. Write the formula for calculating surface tension.
4. Explain why water can wet a glass but mercury cannot.
5. Write down 3 effects of surface tension.
6. Write down 3 methods of reducing surface tension.
7. Explain why it is possible for insects like ants to walk on the surface of water without sinking.
8. Differentiate between force of adhesion and force of cohesion.
9. Write one application of surface tension