# Lesson Notes By Weeks and Term - Senior Secondary 1

Viscosity

Term: 1st Term

Week: 11

Class: Senior Secondary School 1

Age: 15 years

Duration: 40 minutes of 5 periods each

Date:

Subject:      Physics

Topic:-       Viscosity

SPECIFIC OBJECTIVES: At the end of the lesson, pupils should be able to

1. Explain the meaning of Viscosity
2. Carry out an experiment to Determine the Terminal Velocity of a Steel Ball Falling in a Fluid
3. State the factors Affecting Viscosity
4. List the effects of Viscosity
5. State the Applications of Viscosity

INSTRUCTIONAL TECHNIQUES: Identification, explanation, questions and answers, demonstration, videos from source

INSTRUCTIONAL MATERIALS: Videos, loud speaker, textbook, pictures

INSTRUCTIONAL PROCEDURES

PERIOD 1-2

 PRESENTATION TEACHER’S ACTIVITY STUDENT’S ACTIVITY STEP 1 INTRODUCTION The teacher reviews the previous lesson on workdone in lifting a body and falling bodies Students pay attention STEP 2 EXPLANATION He explains the meaning of viscosity and carries out an experiment to determine the terminal velocity of a steel ball falling  in a liquid Students pay attention and participates STEP 3 DEMONSTRATION He states and explains the factors affecting viscosity and the effects of viscosity. He further states and explains the application of viscosity Students pay attention and participate STEP 4 NOTE TAKING The teacher writes a summarized note on the board The students copy the note in their books

NOTE

VISCOSITY

Viscosity is the internal friction which exists between layers of the molecules of 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

Length                                          2

W=U+V

W – U- V = 0       ……………………………………………………………3.

V = W – U (apparent or effective weight) Where: V-viscous force, W- weight, U- upthrust

NOTE: Substances with low viscosity include water, kerosene, petrol, ethanol etc.

Those with high viscosity are glue, syrup, grease, glycerin etc.

Experiment  to  Determine  the   Terminal  Velocity  of  a  Steel  Ball  Falling Through a Fluid

Aim: To determine the terminal velocity of a steel ball falling in through a jar of glycerin

Apparatus: steel ball, cylindrical calibrated jar, glycerine

Diagram:

Procedure: Set-up the apparatus as shown above &gently drop the steel ball in the jar of glycerin

Observation: It will be observed that the ball is accelerating in the liquid. Also the time taken for the ball to move from A-B will be different from B-C and so on. A time will be reached when the ball will be moving at a constant speed or velocity. It is that point that terminal velocity is experience. Graph:

Conclusion: Terminal velocity is attained when W = V + U. At a point when the ball is moving at a constant speed through the glycerine.

Precaution:

1. The steel ball should be dropped gently on the liquid
2. Experiment should be done under constant temperature
3. Avoid error of measurement when taken the reading.

NB: Terminal velocity is the maximum velocity of an object when the viscous force due to motion of the object equals the apparent (effective) weight of the object in the fluid where there is no longer net force on the object.

Drag force is the force that keeps the object continuously moving after the terminal velocity has been attained.

Stokes’ Law state that at the terminal velocity, the upward frictional force

(F) = 6ΠŋrV

Where F- Frictional/Drag force, ŋ- viscosity, r- radius of sphere, Vt- Terminal velocity

FACTORS AFFECTING VISCOSITY

1. Viscosity varies with material
2. The viscosity of simple liquids (a) decreases with increasing temperature (b) increases under very high pressure
3. The viscosity of gases (a) increases with increasing temperature (b) is independent of pressure & density

EFFECTS OF VISCOSITY

1. Viscosity is responsible for different rate of fluid flow
2. Viscosity affect motion of body in fluid

APPLICAION OF VISCOSITY

1. It is use as a lubricant
2. The knowledge of viscous drag/drag force is applied in the design of ship & aircraft
3. Use to estimate the enlarge size of particles

NB: A liquid is said to be VISCOSTATIC if its viscosity does not change (appreciably) with change in temperature

EVALUATION:   1. What is viscosity?

1. State two application of viscosity
2. Mention three viscous liquid that you know
3. Derive the dimension of viscosity
1. State two effect of viscosity
1. Describe an experiment to determine the terminal velocity of a steel ball falling in a fluid

CLASSWORK: As in evaluation

CONCLUSION: The teacher commends the students positively