Lesson Notes By Weeks and Term - Senior Secondary School 1

SUBJECT: PHYSICS

CLASS:  SS 1

DATE:

TERM: 3rd TERM

 
WEEK 8

TOPIC: CAPILIARITY

CONTENT: Definition of Capillarity

        Cohesion and adhesion

        Application of capillarity

Capillarity is defined as the tendency of liquids to rise or fall in narrow capillary tubes.

Cohesive force is the force of attraction between molecules of the same substance

Adhesive force is the force of attraction between molecules of different substance or it refers to the force which makes molecules of different substance to attract.

Application of Capillarity

Adhesion of water to glass is stronger than the cohesion of water, hence, when water is spilled on a clean glass surface, it wets the glass. The cohesion of mercury is greater than its adhesion to glass, hence, mercury spilled 0n glass forms small spherical droplets.

EVALUATION

• Differentiate between cohesion and adhesion

1. Explain the rise of water in a glass capillary tube using kinetic theory

Reading Assignment : Read more on capillarity – New school physics (pg104-105)

WEEKEND ASSIGNMENT

1 Differentiate between cohesion and adhesion

2.Explain the rise of water in a glass capillary tube using kinetic theory

WEEK 9

TOPIC: ELASTICITY

CONTENT : Definition of elasticity

          Hook’s law

          Tensile stress, tensile strain and young modulus

Elasticity is the tendency of a material to regain its original size or shape after deformation or after it has been compressed or extended.

Hook’s law states that provided the elastic limit is not exceeded, the extension in an elastic material (wire) is proportional to the force applied i.e Fαe

 F = Ke…………………………………………1

Where K is force constant, stiffness or elastic constant

Force constant is the amount of force that causes a unit extension. It is the ratio of force to extension of an elastic material.

K → force constant, stiffness or elastic constant

A →proportional limit

L →elastic limit

B →yield point

OL → elastic deform

BC→plastic deform 

Hook’s law applies up to the elastic limit. For load beyond L the wire (material) stretch permanently. The point where small ↑ in load produces large extension is known as yield point. 

Breaking point is the point where the wire cannot withstand any further increase in load.

Yield point : it is the minimum stress/load acting on an elastic material beyond which plastic deformation sets in.

Elastic limit is the maximum load (force) which a body can experience and still regain its original size.

EVALUATION

1. Define (a) elastic limit (b) elastic constant (c) yield point (d) breaking point
2. State Hooke;s law of elasticity

Tensile  stress  is the force acting on a unit CSA of a wire/rod or force per unit CSA of a wire or rod.

Tensile stress = Force/Area …………………….2

Tensile strain is the extension per unit length

Tensile strain = extension/Original length ……………………..3

Young modulus can be defined as the ratio of tensile stress to tensile strain

Young modules = tensile stress/ tensile strain…………………..4

EVALUATION

1.Define young modulus of elasticity

2. A spiral spring extends from a length of 10.01cm to 10.10cm when a force of 20N is applied on it.Calculate the force constant of the spring

Reading Assignment : Read more on elasticity – New school physics (pg 93-96)

WEEKEND ASSIGNMENT

1.The SI unit of tensile stress is ….. (a) N/m (b) Nm (c) N/m2 (d) m2

2.The SI unit of tensile strain is ….. (a) N/m (b) Nm (c) N/m2 (d) none of the above

3. The SI unit of tensile strain is ….. (a) N/m (b) Nm (c) N/m2 (d) none of the above
4. The SI unit of force constant, K is ….. (a) N/m (b) Nm (c) N/m2 (d) m
5. Hooke’s law states that (a) F α A (b)F α e  (c) E α F (d) E α A

 THEORY

• A wire is gradually stretched by loading it until it snaps (a) sketch a load- extension graph for the wire

• indicate on the graph the elastic limit(E), yield point (Y) and breaking point (B)

 WEEK 10

TOPIC : ENERGY STORED IN AN ELASTIC MATERIAL

CONTENT : Force in  a bar

Energy stored in a wire

Energy stored per unit area

Force in a bar

When a bar is heated and then prevented from contracting as it cools, a considerable force is exerted at the end of the bar. Given a bar of a young modulus E, a cross sectional area A, a linear expansivity of magnitude α and a decrease in temperature of Ó¨,then

E = stress/strain = F/A/e/L

=F/A х L/e

E = FL/Ae  

 F = EAe/L ………………………….1

Recall, Linear expansivity α  = change in length/original length х temperature change

α = e/L х Ó¨

e = αLÓ¨

F = EAαeLÓ¨/eL

F = EAαÓ¨ ……………………………2

EVALUATION

1. Show that F = EAαÓ¨ when a bar is heated and then prevented from contracting as it cools
2. A steel rod of cross sectional area 2cm2 is heated to 100˚C and then prevented from contracting when it cooled to 10˚C. find the force exerted on the steel = 12 х 10-6/K and young modulus is 2 х1011N/m2

ENERGY STORED IN A WIRE

The application of force on any wire provided the elasticity limit is not exceeded is proportional to the extension provided.

Consequently, the force in the wire has increase from zero to F

Average force = f + O/2 = f/2

Recall work done = Average force х distance

W = f/2 x e = ½ fe ………………………….3

Substitute eqn 1 into 3

W = EAe2/2L……………………………….4

ENERGY STORED PER UNIT VOLUME

WV = ½ X Stress X Strain…………………..5

This implies that Wv = Ee2/2L2

Substance which lengthens considerable and undergo plastic deforemation until they break are known as ductile substance.  E.g lead, coper, wrought iron.

Substance which break after the elastic limit is reached are known as brittle substance e.g glass and high carbon steel. It should be noted  that brass, bronze and many alloys appears to have no yield point. By this we mean that this material increase in length beyond the elastic limit as the lead is increased without the sudden appearance of a plastic stage.

EVALUATION

1. A uniform steel wire of length 4m and are of cross section 3x10-6m2 is extended by 1mm. Calculate the energy stored in the wire if the elastic limit is not exceeded (young modulus = 2x1011n/m2)
2. A spiral spring is compressed by 0.02m. Calculate the energy stored in the spring if the force constant is 400n/m2

Reading Assignment : Read more on elasticity – New school physics (pg96-98)

WEEKEND ASSIGNMENT

1. A steel rod of cross sectional area 2cm2 is heated to 100˚C and then prevented from contracting when it cooled to 10˚C. find the force exerted on the steel = 12 х 10-6/K and young modulus is 2 х1011N/m2
2. A uniform steel wire of length 4m and are of cross section 3x10-6m2 is extended by 1mm. Calculate the energy stored in the wire if the elastic limit is not exceeded (young modulus = 2x1011n/m2)