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KISS Resources for the Australian Curriculum - Science

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Topic 18.9P “Waves & Energy” PhotoMastercopyright © 2013 KEEP IT SIMPLE SCIENCEwww.keepitsimplescience.com.au

KEEP IT SIMPLE SCIENCEPhotoMaster Format

Waves & EnergyYear 9 Physical Sciences

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keep it simple science

Topic 18.9P

KISS topicnumber

Year level designation inNat.Curriculum

Science Understanding StrandB = Biological SciencesC = Chemical Sciences E = Earth & Space SciencesP = Physical SciencesTopic Outline

What is this topic about?To keep it as simple as possible, (K.I.S.S. Principle) this topic covers:

WAVES CARRY ENERGYCharacteristics and measurements of waves. The Wave Equation.

Electromagnetic Waves. Light & colour. Absorption, reflection & refractionof light. Introduction to optical lenses.

MOVEMENT OF HEAT ENERGYConduction, Convection & Radiation. Particle Model and Heat Transfer.

ELECTRICAL ENERGYElectrical Current, Conduction & Circuits. Voltage, Current & Resistance...

Ohm’s Law. Electrical Power & Energy.

Power &EnergyVoltage,

Current &Resistance

ElectricalConduction& Circuits

Heat Transfer & theParticle Model

Conduction,Convection &

Radiation

Wave Measurements& Characteristics

HowHeat Energy

MovesWavesCarry

Energy

ElectricalEnergy

Waves &Energy

ElectromagneticWaves

Light &Colour

Absorption,Reflection &

Refraction

OpticalLenses




What is Energy? Energy is what causes things to change.

There are many different types of energy. Here are just a few:

Energy Type Changes CausedHEAT Change in temperature.

e.g. a stove causes food to get hot.

LIGHT Nerve changes in your eye which allowyou to see things, or chemical changesin the film in a camera.

SOUND Can cause vibrations in your ear whichallow you to hear.

ELECTRICITY Can cause a light bulb to glow and produce light, or a stove element to get hot and produce heat.

RADIO WAVES Can cause electrical vibrations in anantenna for reception of mobile phone, radio & TV programs.

NUCLEAR Energy in the nucleus of atoms which is released by an “atom bomb”, or in a nuclear power station.

An explosion like thisreleases Heat, Light and

Sound energy.

Two More Important Types of Energy

Two types of energy need special attention: (learn these especially!)

Kinetic Energy (KE)KE is the energy of a moving object. Itcauses the object to change its positionby moving.

If the moving object hits something, theKE can cause other changes, such as thedamage done when moving cars collide.

Potential Energy (PE)PE is energy stored in things, and notalways obvious or apparent. There are 3 types:

Gravitational PE is energy stored in anobject in a high position. The energy is notapparent until the object falls down due togravity. As it falls, the energy converts to KE.

Chemical PE is energy stored in chemicals. The energy is not apparent untila chemical change occurs which releasesthe energy. Chemical PE is stored in chemicals like candle wax (can burn torelease heat and light) or in a battery (canmake electricity) or in petrol (can make acar move with KE).

Elastic PE is energy stored in objectswhich have been stretched, compressed ortwisted out of shape. When released, theelastic PE is released, often causing some-thing to move with KE. e.g. When released, a stretched bow makesthe arrow fly.

Chemical PE (in rocket fuel) isconverting into KE (speed) and

Gravitational PE (height)

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We begin with some revision from a previous topic...




Waves Carry EnergyMany types of energy are carried as waves.

Waves carry energy from one place to another. Only the energy moves... substances do NOT move from one place to another.

(A substance might vibrate as energy moves through, but it doesn’t go anywhere.)

Examples of Energy Carried as Waves

Light & Sound WavesWhen you see anything you are detecting light wavescarrying energy to your eyes. Things that glow areemitting (giving out) light waves. Most things that yousee are not emitting light, but reflecting it to your eyes.

When you hear anything, you are detecting soundwaves.

In a lightning storm, the electrical discharge emits lightwaves... so you see the lightning flash. It also createssound waves... so you hear the thunder.

The sound waves travel more slowly that the light, soyou hear the thunder after you see the lightning.

Water Waves

The energy of the wind blowingacross the oceans creates waterwaves. Water waves can carrythe energy for thousands of kilometres across an ocean,

until the waves “break” as theyapproach a coastline.

Before they break, the waterdoesn’t actually go anywhere.The water moves up and down

as huge amounts of energy flow past, but the water itself goes nowhere.

Once the wave begins to break, the water itself surges forward. Now you cancatch it, and now its energy batters the coast causing erosion and movement ofsand.

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Some Waves Need a “Medium”Some waves can only travel through substances. The substance vibrates asthe wave energy passes through. The substance a wave travels through is

called the “medium” for the wave. Without a medium, some waves cannot exist, and the energy cannot move.

Other ExamplesWater waves in the ocean need the water

as a medium. The shock waves of anearthquake need solid rock

as their medium.

When the bell-jar is filled with air, youcan clearly hear the bell ringing.

When most of the air is pumped out,you can barely hear it, but you cansee that it is still ringing.

Sound cannot travel in a vacuum. Itmust have a medium. Sound wavesnot only travel in air, but can movethrough water or solids as well.

Sealed, glass “bell-jar”with electric bell or

buzzer inside.Air can be

pumped outby a vacuum

pump, orallowed backin through a

valve.

Sound Needs a Medium Your teacher might demonstrate this in class.

Some Waves Can Travel Through VacuumNot all waves need a medium to travel through.

Electromagnetic WavesLight waves can (obviously) travel throughempty space from theSun to Earth.

Light does not need amedium to travelthrough.

Light can travel throughsome substances (suchas air, water & glass), but it actually travels best through a vacuum.

Light is an electromagnetic (EM) wave.The energy is transmitted by the vibrationof electric and magnetic fields.

There is a whole “family” of other EMwaves which you will study later in thistopic.

The Speed of LightLight waves travel at theamazing speed of300,000 kilometres per second.

That’s just over 3 seconds to travel 1 million km.

Light covers the distancefrom Sun to Earth(150,000,000km) in about

8 minutes. Our fastest space vehicleswould take about a year to travel that far.

We have reason to believe that nothing can evertravel faster than the speed of light.

We should call it the “Speed of EM Waves”because all EM waves travel at this samespeed.




Science Rejects Theories That Don’t WorkTo the scientists of the 18th and 19th centuries it seemed obvious that

light waves must travel through something... there must be a “medium” for light waves.All other types of waves which had been studied (e.g. sound & water waves) needed a

medium and could not travel in a vacuum.

The Theory of the AetherThe “aether” was thought to be a fluidwhich totally filled theUniverse. It had nomass and did notinteract with solidobjects. For example,it had no effect on theplanets movingthrough space.

The only property ofthe aether was that itallowed light wavesto travel throughspace.

In the second half of the 19th century theexistence of the whole family of electro-magnetic (EM) waves was discovered.Light was just one type of a whole spectrum of waves travelling in the aether.

Scientists began experimenting to provethat the aether really existed.

The Michelson-Morley Experiment

This famous experiment in 1887 attemptedto measure certain optical effects whichtheoretically should have occurred undercertain conditions, as light travelled in theaether.

The expected interference patterns did notoccur. The experiment was repeated manytimes, with greater precision each time.Same result... no aether effects.

Arguments went back-and-forth for 20years, but it was finally settled by AlbertEinstein in 1905. His Theory of Relativitygave a new way to think about light andabout space and time. There was nolonger any need for an aether.

Soon, experiments to test Relativity gaveclear, positive results. It was realised thatAether Theory is wrong, so the theory wasquickly rejected.

This is how Science works.

Einstein in 1905

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Scientific Developments Lead to New TechnologiesIn the mid-19th century, scientists were not quite sure what light was.

Earlier theories that light was a stream of particles had been discounted, so it was generally agreed that light was a wave. But what kind?

James Clerk Maxwell(British, 1831-79)

In 1864, Maxwell published histheory of Electromagnetic (EM)Waves.

He showed mathematically howa wave could be made of fluctuating electric and magneticfields and how it would spreadout at the speed of light.

His equations also suggested that thereshould be a whole spectrum of EM wavesat other wavelengths and frequencies. Therace was on to find them!

Radio Waves were discovered andstudied in 1880, and by 1895 the first radio

communications began. By the1930’s the first pictures were trans-mitted by radio signal (television)and radar was being developed.

In World War II, radar developed andextended into the Microwave band.From there we get modern communication networks (& microwave ovens).

In 1895, Wilhelm Rontgen discovered X-Raysand by the early 20th century they were usedfor medical imaging.

Maxwell’s new scientific theories led directly to many new technologies which are vital intoday’s society. Nearly all modern technology comes originally from scientific discoveries.




Please complete Worksheets 1 & 2after this page.

How Energy is Carried by a WaveTo understand how a wave carries energy it is best to start with

something familiar, such as waves in water.

Water WavesIf you look at unbroken “swell” waves inwater, you will see that they are made upof a series of crests and troughs whichmove across the surface.

If you watch a boat as ocean swells go by,you will see that it simply moves up anddown... it is not pushed along.

You cannot surfunbroken swells

because the wavewill not push youforward. You onlyfloat up and downon the crests and

troughs.

As the waveapproaches shore it

begins to “break”. Now the water beginsto tumble forward and you can catch it.However, a “breaker” is NOT a wave in

the scientific sense.

Crest

Trough

A floating boat only moves up anddown as the waves go past.

Energy travels across the surface

In a wave, only the energy travels. The water (or other “medium”) just

oscillates up and down.

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Wave AmplitudeTo a surfer, how big the surf waves are is a very important thing.In scientific terms, this relates to the wave amplitude.

Significance of Wave Amplitude

Surfers tend to measure the height ofa wave as the vertical distance from

trough up to crest.

Notice that the scientific measurement of amplitude is from the

“flat water surface” up to the crest.

In a Sound Wave, the amplitude determines how loud the sound is... its volume.

When you turn up the volume, you areincreasing the amplitude of the soundwaves that you are hearing.

In a Light Wave, the amplitude determinesthe brightness of the light.

When you see 2 lights, but one is brighterthan the other, the difference is the amplitude of the light waves which areentering your eye.

Crest

Trough

Water surface

if there

were no wavesThis distance is the

wave amplitude




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Wavelength and FrequencyThese 2 measurements are very important to describe different waves.

Wavelength (l)The wavelength is a measure of the distance from one wave crest to the next.Wavelength can also be measured fromone trough to the next.

The Greek letter “lambda” (l) is often usedas the symbol for wavelength.

Since wavelength is a distance, it can bemeasured in cm, metres, km, etc.

Some Actual WavelengthsSome radio waves have a wavelength ofseveral kilometres. Light waves have wave-

lengths less than 1/1,000 of 1mm.

Ocean waves typically have wavelengthsof between 20m to 100m. Ripples in a pondhave wavelengths of a few cm.

Sound waves are typically a few cm inwavelength.

Frequency (f)Frequency measures how many waves gopast per second. Symbol “f” is used.

It is very difficult to visualise this with adiagram. Your teacher might demonstratefrequency with a slinky spring.

The unit of measurement is called the“hertz” (Hz) named after a great Germanscientist of the 19th century.

A frequency of 10Hz means that 10 complete waves are going past each second.

Some Actual FrequenciesOcean waves can have very low frequencies of only 0.1Hz, or less. This

means that only 1/10 of a wave passes in 1 second. Another way to think of this isthat it takes 10 seconds (or more) for onecomplete wave to go by.

Sound waves have frequencies of 100’s or1,000’s of Hz. Light waves have frequencies measured in billions of Hz.

wavelength

wavelength

Relationship of Wavelength & Frequency

If you shake one end of a “slinky spring” up and down, you can set up a wave pattern.

Now shake it faster, and faster. You are increasing the frequency.Notice that the wavelength gets shorter as frequency gets higher.

wavelength wavelength

wavelength

Shake the spring ina regular way

Shake

faster Even faster

Wave pattern

in spring

Amplitudestays the same

am

pli

tud

e




Please complete Worksheet 3before going on.

Speed, Wavelength & Frequency“Speed”, or velocity means how fast something moves.

The speed of any wave depends on both its wavelength and frequency.

The Wave EquationThere is a simple, mathematical relationship between the wave-length, the frequency and the speed of any wave.

SPEED = WAVELENGTH x FREQUENCY

v = l x f

v = velocity (speed), measured in metres per second (m/s).

l = wavelength, measured in metres (m).(“l” is the Greek letter lambda, equivalent to “L”)

f = frequency, measured in hertz (Hz).

Example CalculationA series of water waves measure 80cm (0.80m) from crest tocrest. Each second, 2.5 complete waves go past.What is the speed of the waves?

SolutionWavelength = 0.80m. Frequency = 2.5Hz.

v = l x f= 0.80 x 2.5= 2.0

The speed of the waves is 2.0 m/s.

Note: If you used l = 80cm, the answer would be 200 cm/s. Thisis exactly the same, but in different units. You should try to usemetres, and m/s whenever possible.

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Electromagnetic (EM) WavesThe EM waves are a “family” of waves which all travel at the “speed of light”.

They have many important properties and uses.

Radio

Microwave

Infra-Red

Light

Ultra-Violet

X-Ray

Gamma

Light can be detected by oureyes, so to us it is the most

important member of the EM family.

Photographic filmand the light

detectors in digitalcameras are

sensitive to lightradiation.

Used for transmitting radioand TV programs & mobile phones.

Also, communication forplanes, ships, military, etc.

Microwaves of some frequencies cancause food to get hot and are used in a

microwave oven. Other frequencies are used in communications, radar & GPS.

Infra-Red is heat radiation. Although wecannot see it, we can feel it

on our skin.Anything glowing “red-hot”

is giving off infra-red.

Mic

row

av

e T

ow

er

Long Wavelength,Low Frequency

Wavele

ng

th g

ett

ing

sh

ort

er

Fre

qu

en

cy g

ett

ing

hig

her

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Short Wavelength,High Frequency

UV causes sunburn and can causeskin cancers. Luckily, most of the UVcoming from the Sun is blocked by

the ozone layer, high up in theatmosphere.

X-Rays are very penetrating.This is why they are used for

medical imaging of bones, andfor detecting weapons, etc in

airline luggage. Excessivedoses of X-rays can be very

dangerous to people.

Gamma Rays are even morepenetrating than X-rays.

They are given off by radio-active substances.They can cause cancer, but are also used to

treat cancer.

Please complete Worksheets4, 5 & 6 before going on.




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Light Waves & ColourVisible light is just a very narrow section ofthe entire “spectrum” of EM waves. Lightincludes a range of frequencies and wavelengths. We see the different frequencylight waves as different colours.

Colours of the RainbowWhat we call “white light” is really a mixture of different light waves of different frequency.If white light (e.g. a beam of sunlight) is passed through a glass prism, the different frequencies are separated to reveal the “spectrum” of whitelight... a rainbow of colours.

Are Colours Real?Each frequency of light is detected separately in our eye and different signals are sent tothe brain. It is in the brain that colours are “seen”. Colour really exists only as the way your brain interprets the information from the eyeregarding the different frequencies of light.

The light waves themselves are not coloured. They are different wavelengths and arevibrating at different rates (different frequencies). Colour is our perception of the different frequencies.

Red

Orange

Yellow

Green

Blue

Violet

white light is a mixture of frequencies

different frequencies spread

out to form a spectrum

Why Things Look ColouredWhy does one thing appear red, and another looks blue?It is all about which frequencies of light bounce off the objects we look at.

White Object Viewed in White Light

White light contains

all the frequencies.

A white object reflects allthe frequencies equally.

All frequencies oflight reach the eye.

The brain interpretsthis as “white”.

white

all

Red Object Viewed in White Light



A red object reflects onlythe red frequencies.

The others are absorbed.

Only the red frequenciesof light reach the eye.

The brain interprets this as “red”.

red

red

Red Object Viewed in Blue Light

White light

A red object reflects onlythe red frequencies, so

blue is absorbed.

A blue filter lets the

blue lightthrough,

but blocksall others. red

blue

Blue Object Viewed in White Light



A blue object reflectsonly blue frequencies.

The others are absorbed.

Only the blue frequenciesof light reach the eye.

The brain interprets this as “blue”.

blue

blue

You may be able to experiment with these situations using a “ray-box” lamp,coloured filters and coloured objects, in a darkened room.

No light is reflected from this object.

The brain interprets this

as “black”.

Please completeWorksheets 7 & 8

after this page.




Absorption, Reflection & RefractionWhen a light wave strikes any substance, any of 3 things can happen.

In fact, all 3 can happen at once.

AbsorptionThe energy of the wave can

be absorbed by the substance.

Where does the energy go?It causes some of the

particles in the substance tovibrate, so it has become

heat energy.

Even substances which aretransparent, like glass orwater, absorb some light.

In perfectly clear water, lightcan only penetrate about

100m. Beyond 100m down inthe ocean it is totally dark.

ReflectionThis means to bounce off.

Light always reflects at thesame angle at which it hits the

reflecting surface.

More about reflection below.

Shiny, reflecting

surface

Incoming light wave

Reflectedlight wave

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Reflection of LightAll waves can reflect, or bounce off things.

The reflection of sound waves causes echoes. Reflection of radio or microwavesgives us radar . Reflection of light lets us see things.

Shiny & DullSurfaces that are very smooth (at themicroscopic level) cause parallel beams oflight to bounce off uniformly. This appears“shiny”.

Surfaces that are rougher cause lightwaves to bounce off in all directions andscatter everywhere. Since only part of thelight comes to your eye, the surfaceappears non-shiny, or dull.

MirrorsMirrors are the ultimate in shiny. Their sur-face is so smooth that light reflects offalmost perfectly.

If the mirror is flat (a “plane mirror”) it

forms a perfect, back-to-front

reflected image.

Curved mirrors canmagnify the image, or make it smaller.

If the curve ofthe mirror is

more complicated, it can form distorted,

“crazy” images.

Parallel waves ina beam of light

Parallel waves ina beam of light

Waves reflect uniformly.Surface appears

“shiny”.

Light scatters inall directions.

Surface appears“dull”.

Smoothsurface

Rough surface (microscopically)

RefractionThis occurs when the light

penetrates into a transparent substance,such as glass or water.

The light waves changestheir speed and can change

direction.

Refraction will be coveredin more detail later in this

topic.

GlassBlock

Beam

of light




Refraction of LightRefraction can occur to all waves, but is especially important with light.

Refraction occurs when waves enter a new medium, such as light passing from air into glass or water.

Examples of Light RefractionJust look at thisspoon standing in acup of tea. It seems tobe broken at the watersurface. In fact, thespoon is not broken,and is perfectlystraight.

We see the top of thespoon by light rays that travel through airto our eyes.

However, light from the bottom of thespoon travels through water, then glass,then air to our eyes.

As light rays travel from one “medium”into the next, they change directionslightly; they are refracted.

Refraction causes the image of the bottomof the spoon to be offset from the image ofthe top of it, so it looks broken.

Imagine spear-fishing from above the watersurface. You look down into the clear water,and there is dinner. You throw the spearwith perfect accuracy... and you miss!

You are a victim of refraction!You see the fish because of light reflectingfrom it, to your eyes. However, as the lightwaves passed from water into air, theywere refracted and changed directionslightly.

This caused the image of the fish to be off-set a bit. Your spear hit a false image.

Light rays fromfish refract at the

water surface.Image Actualof fish. fish.

Light rays change direction asthey pass from water into air.This makes it look as if thefish is slightly further away.

AIM LOW!

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Experiments with RefractionYou can easily investigate refraction of light with a “ray-box kit”.

The photograph shows a typical experimental investigation.

You can see that the light beams changedirection as they enter the block of clearplastic, and again (opposite direction) asthey come back into air.

Things To InvestigateRotate the plastic block so the light beamsstrike it at different angles.Do the beams always change direction?

Try different shapes of plastic blocks.(curved, triangular, etc)

Look for evidence that when thebeam hits the surface:• some light reflects off it.• some enters and is refracted.• some is absorbed by the plastic.

Clear plastic block

It’s best to work on a flat sheet of whitepaper in a darkened room.

“Light Gate” allows 3 parallelbeams of light out“Ray-box”

lamp

Relection & Refraction are theprocesses which

occur in all OPTICAL devices.From microscopes to cameras, telescopes to contact lenses; allthese involve the bending and

bouncing of light waves.




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Optical Lenses(“Optical” = to do with seeing things)

We use many optical devices to help us see things better.Spectacles and contact lenses correct peoples’ vision. Microscopes magnify tiny

objects, while telescopes make distant objects look closer. Cameras and projectorsfocus images onto film or onto a screen for viewing. All of these rely on

curved lenses which refract light in special ways.

Convex LensesIf you look through a convex lens at close-up things (e.g. the writing on a page) youwill quickly find out that a convex lens is a“magnifying glass”.

Using a ray-box kit, you can see what aconvex lens does to parallel beams of light,by refraction.

Concave Lenses(They go in, in the middle, like a cave)Using a ray-box kit, you can see that aconcave lens does NOT refract light to afocus, but spreads the beams apart.

Only a convex lens can focus an imageonto a screen or film, so they are the mainlenses in cameras, projectors, etc.

Focus, or“Focal Point”

Parallel

beams

of light

Convex Lensseen edge on

Concave Lens

Focusing an ImageWith a Convex Lens

You might try this activity.

Try again with a concavelens... it will not work.

Images can only be focusedon a screen by convex lenses.

Light rays from a candleplaced several metres away.

Upside-down imageof candle forms on paper screen

In a darkened room, hold a convex lens in front of a paper“screen”. Move lens or paper back-and-forth to focus the

image of a distant, lit candle onto the “screen”.

Optic NerveCarries nerve signalsfrom Retina to brain.

The eyeball is filled with a clear jelly-liquid. This

keeps the eye’s shape, and lets light through.

Light Rays

How the Eye WorksOur eyes are optical devices. Inside the eye a convex lens focuses an image on light

sensitive cells in the retina. From there, nerve messages are sent to the brain.

Retina is sensitive tolight, and sends off

nerve messages.

Iris is the “colour” of the eye.It is a ring of muscle which can open or

close the hole in the middle... the “Pupil”. This controls the amount of light entering

the eye.

Lens is transparent & flexible. It refractslight to focus an image on the retina. Theshape of the lens changes according tothe distance of the object being viewed.

Please completeWorksheets 9 & 10before going on.




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The rest of this topic looks at some other ways, apart from waves, thatenergy can move around. We begin by revising a previous topic.

How Heat Energy MovesWe cannot see or hear heat energy, but we can feel it because ofreceptors in our skin. Heat energy is responsible for the measurementwe call “temperature”.

Heat energy can “flow” (or transfer) from a hotter zone to a coolerzone in three ways:

Conduction Conduction is the main way that heat energy moves through solids,but heat can also conduct through liquids & gases. Some solid substances allow heat energy to flow through quickly. These are saidto be “good conductors” of heat. (example: metals)

Other substances are poor conductors because heat flows throughthem slowly. If they are very poor conductors, they are called “insulators”. (examples: air, wool, foam plastics)

Convection Convection is the main way that heat energy moves through liquids and gases. Hot fluid rises and flows in a “convection current”. This will be explained in more detail soon.

RadiationRadiation is the only way that heat energy can move through empty space, such as when the heat of theSun reaches the Earth. Heat radiation is carried by Infra-Red waves (I.R.), which are Electromagneticwaves and travel at the speed of light. I.R. waves can also travel through air, and many other transparentsubstances. Anything glowing “red-hot” is giving off I.R. heat radiation.

Experimenting With Heat EnergyInvestigating ConductionAn experiment you may have done, orseen demonstrated:

This allows you to compare the rate of heatconduction through different metals.

If a copper rod was used, you probably foundits toothpicks fell off first.

All metals are good heat conductors, but copper is really excellent.

Two (or more) different-metal rods, held byclamps, heated at one end by a bunsen.

Toothpicks stuckon with grease

or wax.

As heat conductsthrough the metals, the

grease melts andthe toothpicks

drop off.

Heat Conduction in the Kitchen

Look at the features of a typical saucepan,and relate them to the conduction of HeatEnergy.

Copper bottom forrapid heat conduction

from the stove

The body is made ofstainless steel for

strength, hygiene, easycleaning and nice

appearance.

To avoid being burned, the parts that a person needs to touch

are made of insulating plastic.




Conduction & the Particle ModelHow can conduction be explained?

You need to be reminded of some details of the Particle Model of Matter.

Particles in a solid are vibrating, but areheld in place by forces of attraction.

Heat energy makes the particles vibrate faster. What

we call “temperature” is really a measure of theamount of movement

of the particles.

Heat Added

He

at

The diagram below represents the particles in a metal rod. Imagine what happens if this is heated at one end,

as in the experiment in the previous page.

He

at

He

at

At the end where heat is applied, the particles begin vibrating more andmore. They bump into their neighbours and cause them to vibrate more.

Later still, this “daisy-chain” of vibrations is causing particles to jigglewhich are well away from the heat source. The heat energy is being carried

(“conducted”) through the metal rod.

Heat ConductionConduction is very slow, but is the only way that heat can move in most

solids. Conduction can also occur in liquids and gases, but in these “fluids” another, faster form of heat transfer usually takes over... Next page

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Investigating Convection Currents

You may have done an experiment similar to this:The coloured dye makes any movement of thewater visible.

The hotter water (directly above the bunsen)flows upwards, then spreads across the top, thensinks down the sides to complete a circuit.

This is a convection current, which moves heatquickly through gases (e.g. the atmosphere) and liquids (e.g. oceans).

Beaker of water.

Drop of food colouringdye, which makeswater currents visible.

Tripod & Gauze.

Bunsen.

convectioncurrents in

water.

Convection & the Particle ModelHow can convection currents be explained? Back to the Particle Model of Matter...

If you have already studied the topic about “Plate Tectonics” you will know that convection currents caneven flow in “semi-solids” like the hot, dense rocks of the Earth’s mantle. While slow, these currents

carry heat energy through the Earth much faster than heat conduction ever could by itself.

Heat Added

Particles in a liquid may be quite closetogether, but are not fixed in position.They move around among each other.

When heated, the particlesmove faster. They collide fasterand harder and push each other

away. This causes the volumeof the liquid to increase.

The liquid expands and occupies more space.

Notice that the particles DO NOT expand. They forceeach other a bit further apart so the substanceexpands, but the particles stay the same size.

You also need to be reminded of the

concept of density:

Density = Mass Volume

From this, if the volume increases, but mass remains the same, then the

density must get lower.

Density & FlotationYou are reminded that things float in a liquid if they areless dense than the liquid around them.

Convection CurrentsSo, when a liquid is heated, (see diagram) the heatedzone expands and becomes less dense. This “parcel” ofliquid floats in the surrounding liquid, so it rises to thesurface.

At the surface it spreads sideways. Now it loses heat tothe air... by radiation, conduction or by setting-off convection currents in the air. One way or the other, thesurface liquid loses heat and cools.

Meanwhile, freshly heated convection currents haverisen up behind. The cooling surface liquid is nowdenser than the surrounding liquid, so it begins to sink.

Soon, there is a constant flow of liquid transferring heat throughout. A “convection cell” is formed whenthe currents form a continuous cycle.

1.Heated

“parcel” ofliquid

expands &becomes

lessdense.

4.Cooledsurfaceliquidsinks

because itis nowdenser

than thesurround-ing liquid.

3. Surface liquid cools as itloses heat to the air.

2. Heated,lower

densityliquid

floats, soit flows

upwardsas a con-vectioncurrent.

Liquidtransferring

heat byConvection

Convection Currents inthe air are responsible for

many weather eventssuch as thunderstorms

and cyclones.

Please completeWorksheet 11.




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Electrical EnergyThe most useful form of energyfor our society is electricity. It isso useful because:

• electricity can be produced inone place (e.g. in a power station)and moved instantly to whereverthe energy is needed.

• electricity can be easily converted into many other forms of energy, as needed.e.g. it is easily converted to heat, light, etc.

What is Electricity?Electricity is a flow of electrons. Electrons can flowthrough some substances (electrical conductors),but not through other things (electrical insulators).

Electrical Conductors InsulatorsAll Metals, (Copper Plastics, cotton, is especially good) wood, airgraphite, salty water. pure water.

Notice that generally (but some exceptions) the thingsthat are good heat conductors are also good electrical conductors, and heat insulators are alsoelectrical insulators.

Next, more revision from a previous topic as we investigate society’s most useful form of energy...

This equipment is for testing the electrical conductivity of a variety of objects or substances.

The alligator clips are attached to the test object,then the power is turned on. If the bulb lights up, itmeans that electricity is flowing through the entirecircuit. Therefore, the test object is a conductor.

If the bulb does not light, then electricity is not getting through. Therefore, the test object is not a

conductor... it is an insulator.

ACoff

on

DC

PowerPack

Light

Bulb

Wires with alligator

clips

Substance to be tested for conductivity

- +

Conductor or Insulator?

Electrical CircuitsAn electrical circuit always contains 3 parts:

Power Source such as a battery, “power pack” or mainspower.

Energy Converters such as light bulbs, heaters or motors.

Electrical wires(good conductors) which connect theparts.

Complete Circuit. For electricity to flow at all, there must be a complete circuit (anunbroken chain of conductors) from the negative (-ve) terminal to the positive (+ve) terminal.

If there is any break in the circuit (e.g. a wire not connected properly) the electrons cannot get through and the whole circuit stops working.

ACoff

on

DC

- +

Electrons come from the negative terminal and flowaround the circuit to the +ve terminal.




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If you could see the atomic world inside a copper wire, you’d see that the atoms of copper can lose electrons so easily that there are billions of “loose electrons”

hanging around between the atoms.

These electrons are not going anywhere, but can easily jump from atom to atom.

Every battery or other power source produces an electric field. The field of a battery is producedby a build-up of electrical charges due to chemical reactions. The electric field of “mains” power

is produced by magnetic effects in a generator at a power station.

When the wire becomes part of a circuit, the electric field instantly reaches through the wire andexerts a force on every electric charge. The atoms within the copper wire cannot move, but the

“loose” electrons immediately gain energy from the field and begin flowing in the wire.

This flow of electrons is the electric current.The amount of “push” that the electric field can give the electrons is called “voltage”.

What Makes the Electrons Flow?

Copper atom

Loose electron

Measuring Voltage & CurrentCurrentThe flow of electrical current can be measured by a special device called an“ammeter”.

Sketch

Symbol used in a circuit diagram

The unit of current is an ampere, oftenabbreviated to “amp”, symbol “A”.

1 amp of electrical current involves billions and billions of electrons flowing ina circuit.

VoltageThe “push” in an electrical circuit can bemeasured by a “voltmeter”.

Sketch

Symbol used in a circuit diagram

The unit of voltage is a volt, symbol “V”.

1 volt is a very small electrical “push”. A car battery supplies 12V and mains electricity is 240 V. This is a very dangerous level.

AV

AV




Please complete Worksheet 12before going on.

Voltage & CurrentOne way to get an understanding of electrical voltage and current

is to use an analogy; a comparison to a more familiar substance... water.

Imagine a water tank supplying water to a gardenfountain. In the diagram at right, notice how muchwater is spraying from the fountain, and how far it

squirts into the air. WaterTank

Fountain

Fountain

WaterTank

The analogy to electricity is simple:The water pressure is like VOLTAGE. Higher voltage = more push.

The water flow is like CURRENT of electricity. More current = more electrons flowing.

If the voltage is higher, it pushes more electrical current through the circuit.The combination of voltage and current determines the energy delivered.

Now imagine exactly the samewater tank, same fountain, samesize pipes, but the tank has been

raised onto a tower.

More Pressure = More FlowRaising the water tank higher creates morewater pressure. More pressure forces morewater to flow... there is a greater current of

water in the pipe.

The combination of higher pressure andgreater water flow means more energy is

carried by the water. There’s more of it, and itsquirts higher into the air.

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Another Factor... Resistance to FlowContinuing the water analogy, imagine 2

water pipes of different diameter.

They are connected to the same watersupply and the pressure in the pipes isexactly the same. Will the same amount

of water flow?

No! The narrow pipe has more resistanceto water flow, so less water current flows.

Sa

me

Pre

ss

ure

Larger pipe, less resistance to flow...

Narrow pipe,

more resistance to flow...

more current

less current

OR, imagine 2 pipes of equal diameter, but one is rusted or half filled with sediment and“gunk”. It is partly clogged and water cannot flow through it as easily as through aclean pipe. The “gunk” causes resistance to the flow.

Different wires, different light bulbs, etc in an electrical circuit have different amounts ofelectrical resistance. If there is more resistance, less current can flow. If there is lessresistance, more current can flow. (For the same amount of voltage “push”.)




Building a CircuitWhen you put together an electrical circuit, it needs to be done correctly.

Ammeter measurescurrent flow throughthe main circuit loop.

Voltmeter must be placed in a“parallel” side-branch circuit.

Electrons must flow into the meters at their (-ve) terminal and out from their (+ve) terminal.

This simple circuit contains a power source& light bulb, with an ammeter and voltmeter

to measure the current and voltage.

A

V

ACoff

on

DC

A

V

Sketch of this Circuit and Explanation of Correct Construction.

CircuitDiagram

Diagrams likethis are used toclearly & simply

represent a circuit.

Main Circuit.Electricity

flows throughammeter and

bulb.

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Voltage, Resistance & CurrentYou have probably already figured out that there is a simple relationship

between voltage, current & resistance involved in an electric circuit.

What happens to the CURRENT if you...

Increase the Voltage?(and the Resistance stays the same)

Set up a simple circuit as shown and thenwatch what happens as the voltage isincreased by turning-up the power packvoltage setting.

Increase the Resistance?(and the Voltage stays the same)

Start with the same circuit as at the left,then increase the resistance by addinganother light bulb as shown. Leave thepower pack setting as it was, so the voltage remains the same.

Watch the ammeter readingand bulb brightness.

A

ACoff

on

DC

If the voltageis increased, the currentincreases.

(For the sameresistance.)

If the resistanceis increased, the currentdecreases.

(For the samevoltage.)

A

ACoff

on

DC

You do not need a voltmeter

for these circuits.

Please completeWorksheet 13.



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Please completeWorksheets 14 & 15

after this page.


Ohm’s LawThe relationship between Voltage, Currentand Resistance can be decribed mathematically as well as in a general,descriptive way.

This mathematical relationship was firstdiscovered in the 1830’s by a Germancalled George Ohm. It is known as “Ohm’sLaw” in his honour.

Mathematically, Ohm’s Law is often written as:

Voltage = Current x Resistance, V = I x R

but it is more meaningful if written as:

Current = Voltage I = VResistance R

I = electrical current, in amps (A).V = voltage, in volts (V).R = resistance, in ohms (W).

The “ohm” unit is named in honour of George Ohm.

The symbol “W” is omega, a Greek letter for “O”.

It is used because symbol “O” or “o” could be

confusing.

Written in the form I = V/R, the Ohm’s Lawequation tells you that the amount of current flowing in a circuit depends onboth the voltage “pushing” and on theamount of resistance trying to stop theelectricity.

More voltage more current.

More resistance less current.

Example Calculation

An electrical circuit has a resistance of6.0W. What current would flow if connected to a 12V car battery?

Solution:I = V = 12 = 2.0 A.

R 6.0

The current would be 2.0 amp

Ohm’s Law by ExperimentOhm’s Law can be “re-discovered” and tested by making your own measurements

with an electrical circuit similar to that shown below.

For each combination of voltage and resistance, youcan measure the current flow on the ammeter andcheck if it agrees with Ohm’s Law. (Use Voltmeterreadings for voltage, NOT power pack settings.)

Typical Results:Voltage Resistance Current Actual

(V) (W) Calculated Current7.8 10 0.78 A 0.8 A

12.0 6 2.0 A 1.9 A

You will find that the results agree with Ohm’s Law,with some experimental error.

by Ohm’s Lawcalculation

Ammeterreading

Different resistors, of known resistance value, can be used.

For each one, you can vary the voltage setting.

Solid-state resistor,or resistance coil.

A

V

ACoff

on

DC




Converting Electrical EnergyThe great value of electricity in our society is that it can be easily

converted into other forms of energy such as light, sound, heat or kinetic energy.

Examples of Electrical Energy Changes

Light BulbElectricity Light energy

FanElectricity Movement (K.E.)

ToasterElectricity Heat

TelevisionElectricity Light Energy + Sound Energy

Microwave OvenElectricity

MicrowavesHeat

Power ToolsElectricity

Kinetic Energy

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Designing Practical Electrical CircuitsIn a home, office or school, electrical circuits are designed to have a number of items in

one circuit. For example, there may be 10 separate lights in one circuit, but each onecan be switched on or off separately. How is this done?

It turns out there are 2 different ways to arrange items in an electrical circuit.

Series CircuitsThis diagram shows 3 light bulbs arranged“in series” in a simple circuit.

Circuit Diagram

3 light bulbsin series

ACoff

on

DC

In this circuit the electricity must flowthrough all the bulbs, one after the other.




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Parallel CircuitsIn this circuit, the current can divide into 3 parts.

Some current flows through each bulb, but a particularelectron only goes through one of the bulbs, not all 3.

These bulbs are arranged “in parallel”.

Imagine you are an electron flowing out of the power pack. Youcan go to any light bulb and flow through it, then return to the

power pack, but you can only go through one bulb.

ACoff

on

DC

Designing Practical Electrical Circuits (cont.)

Advantages of Parallel CircuitsParallel electrical circuits offer many advantages compared to series circuits.

• Full delivery of power to each device in the circuit. (e.g. brighter lights)• Each device can be switched on or off independently.• If one device “burns-out” all others continue to operate.

CircuitDiagram

If all the parts are available, youcould build the circuit at left.

It demonstrates the features andadvantages of connecting

multiple devices in parallel.

Switch closed - light on

Switch open - light off

Switch closed - light on

ACoff

on

DC

2. Now build the parallel circuit shown above. Use the same bulbs and the same power pack setting as in the series circuit.a) Turn on the power and note the brightness of thebulbs.

b) Turn power off and remove one bulb from itssocket. Turn power back on.

c) Experiment with one or more switches in variouslocations.

Get the picture?

An Experiment You Might Do1. Build the series circuit shown in the previouspage. Set power pack as instructed by your teacher.

a) Turn on the power and note the brightness of thebulbs.

b) Turn power off and remove one bulb from its socket. Turn power back on.Do the other 2 bulbs work if one is missing or“burned-out”?

c) Add one or more switches to the circuit.Is it possible to switch one light on, while the othersstay off?

A series circuit has a much higher resistance so less current flows and less power can be delivered to each

device. Multiple devices in series cannot be independentlyswitched on or off... one off, all off.

Parallel Circuits are always used in practice.



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Please completeWorksheet 16

before going on.


The amount of energy (of any kind) converted per second is called “power”.Power is measured in “watts” (W).

The “watt” unit is named in honour of James Watt (1736-1819) who worked with steamengines, not electricity. He developed the concept of “Power” and contributed greatly tothe “Industrial Revolution” which led to our modern, mechanised, technological world.

How Much Electrical Power?The higher the “wattage” of any electrical appliance, the more electricalenergy it uses per second... this means more “power”.

The amount of power used depends on both the voltage and the current.

Efficiency of Power UseWe use electricity to produce other forms of energy we

want, such as light or heat. However, some electrical appliances are very inefficient at converting the energy. For example, a light bulb only converts about 10% of the

electrical energy used into light. The rest iswasted by being converted into unwanted

heat. Compact fluorescent lights and L.E.D.lights are much more efficient.

A modern compact-fluoro light rated at 15W producesabout the same amount of light energy as an old-fashioned

100W bulb light. Same amount of light, but much lesspower consumption = more efficient. That’s why the old-

style light bulbs are being phased-out.

Electrical Power

Mathematically, Power = Voltage x Current

P = V x I

P = electrical power, in watts (W).V = voltage, in volts (V).I = electrical current, in amps (A).

Example CalculationAn electric toaster has resistance of 80W.If connected to a 240V circuit:a) use Ohm’s Law to calculate the current flow.b) calculate its “wattage”, or power consumption.

Solution:a) I = V/R b) P = V x I

= 240 / 80 = 240 x 3.0= 3.0 A = 720 W

Current is 3.0 amps. Power is 720 watts.




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From these examples, and knowing abouttoast, you can probably figure out that 1 jouleof energy must be a very small amount. It is sosmall that it is often more convenient to usekilojoules (kJ) as the unit of energy.

1 kJ = 1,000 J, so the toasters both use 36kJ ofenergy to make a piece of toast.

Scientists tend to measure energy in kJ, but in everyday life, another energy unit is commonly used when dealing with electricity...

Measuring EnergyEnergy (of any kind) is measured in “joules” (J).

This unit is named for the English scientist James Prescott Joule (1818-89) who contributed greatly to our modern understanding of energy.

The connection between energy and power is very simple: sincepower is the amount of energy converted per second, then the amountof energy used is equal to the power multiplied by the time involved.

So, mathematically, Energy = Power x time

E = P x tand remember that P = V x I, so

E = V x I x tE = electrical energy, in joules (J).P = power, in watts (W).V = voltage, in volts (V).I = electrical current, in amps (A).t = time, in seconds (s).

Electrical Energy

Example Calculation 1The electric toaster (previous page) was found tohave a power rating of 720W. To make toast ithas to be switched on for 50 seconds. How much energy is used to make the toast?

Solution: E = P x t = 720 x 50 = 36,000 JThe toaster uses 36,000 joules of energy.

Example Calculation 2A different model toaster uses 5.0A of current at240V and takes just 30 seconds to make toast.How much energy is used?

Solution: E = V x I x t = 240 x 5.0 x 30 = 36,000 J

This toaster also uses 36,000 joules of energy.(It has a higher power, so it works faster, butuses exactly the same amount of energy.

Kilowatt-HoursAlthough the joule (or kJ) is the correctunit for energy, you need to also be familiar with another unit commonly usedin electricity supply accounts.

If an electrical appliance has a power rating of 1,000W (= 1kW) and it runs for 1 hour, then it will use 1kilowatt-hour(kWh) of energy.

The kWh is the unit shown on your electricity bill. At the time of writing, itcosts about 25-30 cents for 1kWh of electrical energy.

The kWh is a much larger unit than ajoule. In fact, 1kWh = 3,600,000J (or 3,600 kJ)

Mathematically, the formulaEnergy = Power x time

E = P x tcan still be used, but with different units.

E = electrical energy, in kilowatt-hrs (kWh).

P = power, in kilowatts (kW).

t = time, in hours (h).

Example CalculationOne cold day, Fred had his electrical heater runningfor 8 hours. Its power rating is 1,500W (= 1.5kW).Fred has to pay 26.5 cents for his electricity.a) How much energy was used?b) What did that add to Fred’s next power bill?

Solution:a) E = P x t = 1.5 x 8 = 12 kWh.

b) Cost = 12 x 26.5 = 318c = $3.18.




Please complete Worksheet 17.

Impacts of Electricity...Our widespread use of electricity has resulted in some huge benefits

to people, and some terrible damage to the environment.

... on SocietyThe scientific understanding of electricity has led to theinvention of thousands oftypes of electrical appliances, devices andtechnologies. Theimpacts of these onhumans has been hugelybeneficial.

Electrical lights, heating,air-con, refrigerators,washers, etc. make ourlives more comfortable, easy and convenient. Electrical tools andmachinery save time and make ourjobs easier.

Electricity powers our phones andcomputers and make possible ourcommunications (e.g. TV, internet),financial and trade systems and entertainment.

... on the EnvironmentWhile the impacts of electrical technologieson people have been beneficial, the impactson the natural environment have all beennegative.

Most electricity is generated from burningcoal, which releases huge amounts of CO2into the atmosphere.

We now believe this is a major cause of the“Greenhouse Effect” causing “GlobalWarming” and climate change.

Even “greenhouse-friendly” power productioncan cause problems:• Dams for hydro-electricity destroy river ecosystems.

• Wind generator turbines kill thousands of birds.

• Nuclear power involves the risk of accidentsand the serious problem of nuclear waste disposal.

Electricity.Is it now a

necessary evil?

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How Society Influences ScienceNot only does scientific development have an impact on human society,

but the reverse is sometimes true... social factors can influence the development and acceptance of scientific ideas and new technologies.

Development of Electric CarsIn the early history of motor cars there weremany attempts to make and sell electricallypowered cars.

At a time when petrolwas cheap and peopledidn’t know about environmental damage,the electric cars did notsurvive commerciallybecause petrol engineswere cheaper and morepowerful.

Now, society’s attitudes have changed.More people are concerned about environmental damage & climate change.

Although expensive and lesspowerful, more and moreelectrically-powered cars

(and “hybrids” with a small petrolengine plus electrical motors powered by rechargeable batteries)are now being developed and sold.These new technologies are nowwelcome in our greenhouse-

conscious world.

1919 Electric Car in

a Museum

kiss resources for the australian curriculum - science...

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