Physics Year 3

Summary

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Chapter 3 Energy 3

Chapter 5 Expansion 5

Chapter 8 Measuring heat 6

Chapter 11 Pushes & Pulls 7

Chapter 13 Pressure 9

Chapter 14 Forces 10

Chapter 15 Turning Forces 11

Chapter 16 Work, Energy and Power 12

Chapter 17 Machines 13

Chapter 23 Reflection 14

Chapter 24 Curved mirrors 17

Chapter 25 Refraction 18

Chapter 26 Lenses 19

Chapter 30 Static electricity 20

Chapter 31 circuits 20

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Chapter 3 Energy

Energy can exists in different forms:

Chemical energy (food) Thermal energy (heat) Light energy Sound energy Elastic potential energy Gravitational potential energy Kinetic energy (movement) Electrical energy Nuclear energy

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Renewable sources of energy are :

Solar energy Biomass Wind energy Wave energy Hydro-electric energy Tidal energy Geothermal energy

You need 40km2 of solar panels to equal the amount of energy produced by a modern power station.

You need 20km of floats to equal the amount of energy produced by a modern power station.

You need 2000 windmills to equal the amount of energy produced by a modern power station.

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Chapter 5 Expansion

When objects get hotter they grow bigger. We say the expand.

When objects cool down they get smaller. We say they contract.

A bi-metallic strip is made of two metal strips with different expansion coefficients (often brass and iron) which are placed side by side and then riveted or welded together.

A bimetallic strip is used in a fire alarm or as an Electrical thermostat.

When it gets too hot the bi-metallic strip will bend and make contact with the contact and completes the circuit causing a current to flow and the alarm to sound.

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Bi-metalic strip

contact

Fire alarm

Chapter 8 Measuring Heat

All moving molecules of an object have internal energy. Its internal energy increases. Like all other forms of energy, internal energy is measured in a unit called joule (J). A math produces 2000J = 2KJ to burn up completely. 1,000J=1KJ , 1,000,000J=1MJ.

Your body needs a total of 10 MJ per day. While sleeping you use about 4KJ every minute.

The specific heat capacity of a substance is the amount of energy that is used to raise the temperature of 1Kg of the substance by 1℃ .

Energy needed=specific heat capacity ×mass × change∈themperature

joules J /kg℃kg℃

Material Specific heat capacity J/Kg C ̊�Water 4200Meths 2500Paraffin 2200Ice 2100Aluminium 880Sand 800Copper 380Mercury 140

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Chapter 11 Pushes and Pulls

Pushes and pulls are forces. Whenever we are pushing or pulling, lifting or bending, we are exerting a force.

Forces can :

Change the speed of an object Change the direction of an object Change the shape of an object

Weight is a very common force. Weight is the force of gravity due to the pull of the Earth.

If an object is taken to the Moon, it weighs about one-sixth as much, because the Moon is smaller than the Earth.

In outer space objects become weightless.

Hooke’s Law : The extension of a spring is proportional to the force pulling it (up to the limit of proportionality).

Forces are measured in units called newtons (N)7

A mass of 1kg (here on Earth) weights almost 10 newtons.

Weight on Earth=mass ×10

newtons kg N /kg

The mass of an object is the amount of matter in it. It is measured in kilograms (kg). the mass of a book is around 1kg.

We say masses have inertia, a reluctance to start moving.

Newton’s First Law : Every mass stays at rest or moves at constant speed in a straight line unless a resultant force acts on it.

Chapter 13 Pressure

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Pressure(N /m2)=force (¿newtons)

area(¿ square metres)

A force acting over a small area gives a larger pressure.

The pressure exerted on us by the atmosphere is 100,000 newtons per square metre.

A mercury barometer :

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Chapter 14 Forces

Friction is a very common force. Whenever one surface slides over another, friction always tries to oppose the movement. Friction is often nuisance, because it wastes energy.

Ways to reduce friction :

Polishing the surface Lubricating with oil Separate by air Rolling instead of sliding Streamlining

Newton’s 3rd Law : The action force and the reaction force are equal and opposite.

Forces are shown in a Free-body force diagram :

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Chapter 15 Turning forces

Moment of a force = force(N) x perpendicular

In equilibrium : total anti-clockwise moment = total clockwise moment.

The centre of gravity is the point through which the whole weight of the object seem to act. An object can be stable, unstable or neutral equilibrium depending on the position of its centre of gravity. To be more stable an object needs a low centre of mass and a wide base.

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Chapter 16 Work, Power and Energy

1 joule is the power needed to move a force of 1 newton through a distance of 1 metre.

work done=force× distancemoved

( joules)(newtons)(metres)

The principle of Conservation of energy : energy can be changed from one form to another, but it cannot be created or destroyed.

Work done=Energy transfered

Efficiency=usefull energy outputtotalenergy input

×100%

Change∈Epotential (J )=weight (N )× ∆ height (m)

Change∈Egravity (J )=mass (kg ) × g× ∆ height (m )

( g=10 on earth )

Eelastic=average force× distance

E kinetic=12

×mass × speed2

Power=work donetimetaken

Chapter 17 Machines

Some machines are :

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Levers ramps

pulleys

Machines are less than 100% efficient because of friction.

Chapter 23 Reflection

Angle I

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normal

Angle R

The angle with which the ray hits the object is called the angle of incidence

The middle dotted line is the normal

The angle with which the ray reflects from the object is called the angle of Refraction

If an object reflects light the angle of incidence equals the angle of reflection.

If you want to draw the outgoing ray :

1 draw an normal ;

2 measure angle I ;

3 draw angle R the same amount of degrees as angle I.

A periscope :

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A pheriscope are 2 mirrors at an angle of 90 degrres wich let you look straight but than at an higher level.

An image in a plane mirror

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Draw the reflection of an object in a mirror :

1 draw the image of the object

2 make 2 rays from the eye to the imaged object (behind the mirror dotted lines)

3 finish it by connecting the not dotted lines with the non-mirrored object

Chapter 24 Curved mirrors

There are two kind of curved mirrors :

A mirror that curves in called a concave mirror.

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Parallel rays of light are reflected through the focal point of a concaved mirror.

A mirror that curves outwards called a convex mirror.

Parallel rays of light are reflected so that they seem to come from the focal point of a convex mirror.

Chapter 25 refraction

If a ray of air goes in to another substance it is refracted.

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Refractive index = sin (angle I )sin(angle R)

Refractive index petrol = 1.45

Refractive index diamond = 2.43

Chapter 26 Lenses

There are two kind of lenses :

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A lens that curves in called a concave mirror.

Parallel rays of light are refracted so that they seem to come from the focal point of a concaved lens.

A lens that curves outwards called a convex mirror.

Parallel rays of light are refracted through the focal point of a convex lens.

Chapter 30 Static energy

Like electric charges repel whereas unlike electric charges repel.

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Chapter 31

For an electric current to flow, there must be a complete circuit, with no gaps.

The symbols for components of a simple circuit :

There are series and parallel circuits :

resistance=p . d .(V )

current (I )

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