PHYSICS S.S ONE 3RD WEEK 4

Topic:                    FLUID AT REST AND IN MOTION               

Sub topic:

Reference materials:

(1) ESSENTIAL PHYSICS, TONALD PUBLISHERS, EWELUKWA (2017)

(2) NEW SCHOOL PHYSICS, AFRICAN FIRST PUBLISHERS, ANYAKOHA M.W (2011)

(3) INTERNET

Instructional materials: NEEDLES, FILTER PAPER, WATER AND BOWL

Entry behavior: The students have been taught crystal solid

Behavioural objective: At the end of the lesson the students should be able to:

I.                    Explain surface tension

II.                  Mention the effect of surface tension and how to be reduced

III.                Explain the capillary action of water and mercury

IV.                State the unit of viscosity

V.                  State the effect and application of viscosity

CONTENT

FLUID AT REST AND IN MOTION

SURFACE TENSION

This is the effect of force on the surface of a liquid which makes it behave like a stretched elastic skin.

Or it is a tangential force on surface of a liquid acting perpendicularly per unit length across any line on the surface.

Effects of surface tension;

Because of surface tension,

1.       Steel needle when carefully placed on top of water floats, despite its greater density.

2.       Some birds and insects can walk on the surface of water.

3.       Some drops of water from the top are in form of a spherical shape.

4.       Soap film inside the cotton loop when broken makes or forms a circle as shown below


Make a ring of thin wire. Tie a thread loosely across the middle as shown in (i). Dip the ring in soap solution or liquid detergent so that a film forms across it. Break the film on one side of the thread. The thread pulls tight, forming a circle as shown in (ii).This because surface tension stretches molecules on the liquid surface farther apart than normal.

Explanation on surface tension:

Surface tension is due to molecules on liquid surface being slightly further apart like those in a stretched wire. Therefore experience attractive forces from their neighbours in liquid surface. The forces  stretch the molecules on the surface, making it behave like a stretched elastic skin.

Reduction of surface tension;

Surface tension can be reduced by;

1.       Increasing the temperature of the liquids

2.       Addition of detergents or soap solution.

Experiment to demonstrate surface tension

-       Some water is poured in a clean trough

-       It is then left to settle and a filter paper (blotting paper) is placed on the water surface.

-       A pin smeared with Vaseline is carefully placed on top of the filter paper as shown below.


After sometime, the filter paper will absorb water and sink while the pin will remain floating on the water surface.

CAPPILARITY/CAPPILLARY ACTION:

This is the rise or depression of a liquid in a capillary tube.

The rise of water in a capillary tube is because the cohesive force between the water molecules is less than the adhesive force between molecules of glass and water. It is also for this reason that water spreads over glass surface

                                         Water spreads


                                                          Glass surface

When similar capillary tubes are dipped in mercury, each surface is depressed below the outside level of the beaker and the surface curves down wards as shown below.

Mercury is depressed more in narrow tube than in a large one. This is because cohesive forces between molecules of mercury are greater than adhesive forces between molecules of mercury and glass. It is also for this reason that mercury does not wet glass but forms droplets on glass as shown.

Mercury droplets


 

FORCE BETWEEN MOLECULES

Cohesion/Cohesive force

This is the force of attraction of molecules of the same substance e.g water- water molecules, mercury – mercury molecules

Adhesion/Adhesive force;

This is the force of attraction between the molecules of different substances e.g water – glass molecules.

Behavior of liquids on the surface;

When water is dropped on a glass surface it wets it and spreads out in a thin surface because adhesive force between the water molecules and glass is greater than the cohesive force between water molecules.

 


When mercury is dropped on a glass surface it forms spherical droplets or large flatten drop because cohesive forces between mercury molecules is greater than adhesive forces between mercury and glass.

 Spherical droplets of mercury

Large flattened drop                                                                                                                               

 

 

Application of capillarity;

 

1.       The rise of oil in a lamp wick upwards.

2.       Absorption of water in a towel.

3.       The rise of water in mineral salts in plants

4.       Action of a blotting paper.

Disadvantages of capillarity;

House bricks and concrete are porous. Capillary action is likely to draw water upwards from

The ground through them, making the building dump (wet).This problem is overcome by putting

Waterproof layer made from plastic that is placed in the layers of bricks at the bottom of the house.

VISCOSITY

Viscosity is the internal friction between layers of a liquid or gas (fluids) in motion. Viscosity decreases with temperature. Viscosity is denoted by η (eta) measured in NSm-1 and a vector quantity.

 Thus, η =

Classification of fluid according to their viscous properties

I.                     When frictional force is low: low viscosity e.g water, kerosene, ethanol or petrol etc

II.                   When frictional force is large: high viscosity eg grease, syrup, glue, glycerine.

TERMINAL VISCOSITY (OR SPEED)

 

 


                                                                                                                                                                 

 

 

 


considering the diagram above, a body falling in a container of liquid (e.g glycerine). There are three forces acting on the body which are upthrust ‘U’ of the liquid on the body acting upward, the viscous force ‘V’ opposing its motion and weight ‘W’ acting downward. So

Mg – V – U = ma where a is acceleration. At a point the body moves in uniform velocity that is called TERMINAL SPEED, where the viscous speed is proportional to the velocity V, also the acceleration ‘a’ = 0.

So from the equation  mg – V – u = ma

                                       Mg = V + U

EFFECT OF VISCOSITY

1.       It is responsible for the different rates of flow of fluid.

2.       It affect motion of bodies in fluids.

APPLICATION OF VISCOSITY

I.                    Viscous is used as lubricant.

II.                  The  knowledge of viscous drag is applied in the design of ships and aircraft.

PRESENTATION

Step I: The teacher explains surface tension.

Step II: The teacher explains the effect and how to reduce surface tension.

Step III: The students demonstrate surface tension experimentally

Step IV: The teacher explains capillarity and is application

Step  V: The teacher defines viscosity and states its effect

Step VI: The students gives examples of viscous substance.

EVALUATION

The teacher assesses the students by asking these questions:

I.                    Explain surface tension

II.                  Mention the effect of surface tension and how to be reduced

III.                Explain the capillary action of water and mercury

IV.                State the unit of viscosity

V.                  State the effect and application of viscosity

 ASSIGNMENT

    Read about solar collector 

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