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CHAPTER 1 FORM 5

Waves

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Learning Objectives

Understand Waves

Analyse reflection of waves

Analyse refraction of wavesAnalyse diffraction of waves

Analyse interference of waves

Analyse sound waves

Analyse electromagnetic waves

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1.1 Understand Waves

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Learning Outcomes

describe what is meant by wave motion,

recognise that waves transfer energy without transferring matter,

compare transverse and longitudinal wavesand give examples of each,

state what is meant by a wavefront,

state the direction of propagation of waves in relation to wavefronts,

define amplitude, period, frequency, wavelengthand speed of wave,

sketch and interpret a displacement-time graph for a wave,

sketch and interpret a displacement-distancegraph for a wave, classify the relationship betweenspeed, wavelength andfrequency,

describe damping in an oscillating system, and

describe resonance in an oscillating system.

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Session 1

describe what is meant by wave motion,

recognise that waves transfer energy without transferring matter,

compare transverse and longitudinal wavesand give examples of

each.

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WAVES MOTION

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A boy kicks a ball and the ball accidently hits and

breaks the glass window producing noise.

In this case, energy is transferredfrom the source(the

boy) to the receiver(the glass window) by the matter(the ball).

The noise we hear from the breaking glass is due to

the energy transferred to our ears by sound waves.

There are two ways of transferring energy: by the motion of objects. by waves.

WAVES MOTION

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Is a spreading of disturbance from a vibrating

or oscillating motion.

What is wave?

What is wave motion?

The process of transmitting waves.

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Examples of waves

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Mechanicalwaves

Two groups of waves

Electromagnetic

wavesRequire medium forits propagation Do not requiremedium canpropagate viavacuum

Examples: Waterwaves Soundwaves

Example: Electromagneticwaves

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How do waves transfer energy?

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Figure 1.1.2 shows that the cork does not move outwards as the wave

passes. This means that the water itself does not move outwards. Through

wave motion, energy is transferred from the source (the stone) to a receiver(the cork) without involving the transfer of matter (the water).

Energy is transferred from the stone to the cork which does not involve the

transfer of water.

Figure

1.1.2

When we throw a stone into a pond, a ripple spreads out in an expandingcircle from the source of disturbance.

The energy of the stoneis converted to waves. The water wavespropagate on the surface of the water.

A cork floating a distance away will move upand downwhen the ripple

passes it. Thus the ripple transfers energy from the stone to the cork.

The water is not transferred, but the energy of the stone is transferred tothe cork.

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How do waves transfer energy?

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(1)

When you throw a stone

into a pond, it forms circular

waves moving outwardsfrom the pointat which the

stone touches the water.

(2)

When the water wave

moves from one point toanother,the water itself does

not move with it.

So do the ball. The ball

vibrates about its equilibrium

position.

(3)

In this case, water is the mediumwhich

carries the wave.

While the wave propagates through themedium, the wave actually transmits

energy through the medium, but the

particles of the medium itself are not

transported.

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Important!

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Types of waves

Transverse waves Longitudinal waves

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Transverse waves

Transverse waves

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Longitudinal waves

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Example of waves

Example of longitudinal waves

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The most common example of longitudinal waves is soundtravelling in air.

Air particles vibrate to and fro as sound waves propagate in the

air. In figure, the air particles are set to vibrate by the vibration of a

tuning fork.

The movement of the molecules in the air produces compressions

and rarefactions of air molecules. As a sound wave passes through air, the molecules oscillate.

Energy is transferred from one molecule to the next.

Hence,sound waves propagate through the air.

Example of longitudinal waves

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Session 2

state what is meant by a wavefront,

state the direction of propagation of waves in relation to

wavefronts.

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A ripple tank consists of asquare transparent plastic

tray with a lamp on the top.

The tray has sloping sides sothat any wave propagated

will not be reflected back

from the sides (Photograph

1.1.5).

Ripple Tank

Ri l T k

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The ripple tank has a bar suspended with two elastic strings

and the bar is driven by a motor.

When the motor is switched on, the bar will vibrate.

If the bar touches the water surface, straight waves are

produced.

We can see the imagesof waves appearing on

the screen below the

ripple tank(Photograph 1.1.6).

Ripple Tank

Ri l T k

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Similarly a round dipper can be fixed to the bar so that it

touches the water surface.

When the current is switched on, the ripple tank will

produce circular waves(Photograph 1.1.7).

Ripple Tank

Ripple Tank

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Water waves havecrestsand

troughs.

A crestis the highest position of

the wave,

A troughis the lowest position.

In a ripple tank, light rays from

the lamp on top will focus onto

the white screen below.

The bright linescorrespond to

the crests, and the dark linescorrespond to the troughs.

Ripple Tank

The bright lines shown on the screenbelow are wavefronts.

Wavefront

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Wavefronts - is an

imaginarylines

joining all the pointsof the same phase ona wave .

For example,

lines along crestortroughsarewavefronts.

Along the same

wavefront, allparticles of water arevibrating in the samephase.

Wavefront

The lines joining the crestare wavefronts.

The wavefronts of waterin a ripple tank.

Direction of propagation of waves

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The wavefronts of a transverse wave and longitudinal wave

areperpendicularto the direction of propagation of the

wave.

Direction of propagation of waves

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Session 3

define amplitude, period, frequency, wavelengthand speed of

wave,

sketch and interpret a displacement-time graph for a wave,

sketch and interpret a displacement-distancegraph for a wave

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Describing waves

several terms to describe a wave:

Wavelength ()

Amplitude (a) Frequency (f)

Wave speed (v)

Period (T)

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Wavelength ()

The wavelength of a wave is

the distance between twoadjacent points of the samephase on a wave.

The wavelength of an ocean wave can be several metres. The wavelength of the electromagnetic waves used in a

microwave oven is less than a centimetre.

Tsunami waves can have a wavelength up to 161 km.

For example,the distance

between two adjacent crestsor two adjacent troughofthe wave.

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Amplitude (a)

amplitude (a)

The maximumdisplacement of a crest ora trough from theequilibrium position of a

wave.

Amplitude relates to loudness in sound and brightness inlight.

The amplitude of a wave is its maximum displacementfrom its equilibrium position.

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Frequency (f)

The frequency of a wave is the

number of complete oscillationsmade in one second.

For example,

most people cannot hear a high-pitched sound above 20 kHz.

Radio stations broadcast radio waves with a frequency of about 100 MHz.

Most wireless computer networks operate at a frequency of 2.4 GHz.

It is also the number of waves that pass acertain point each second.

SI unit = hertz (Hz).

for waves with very high frequencies:

kilohertz (kHz)

megahertz (MHz)

gigahertz (GHz)

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Wave speed (v)

The speed of a wave is the measurement of how fast a crest ismoving from a fixed point. For example:

the speed of sound waves is about 330 ms-1.

the speed of light is 3.0 x 108ms-1.

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Period (T)

The period of a waveis the time taken to complete oneoscillation.

It is measured in second (s).

The frequencyis the reciprocal of a period of vibration.

The unit for frequency is s which is equal to one hertz.

The characteristicsof a wave form depend on the wavelength,amplitude, velocity, and frequency.

All periodic wave forms have these common characteristics.

The swing of a simple pendulum can be used to illustrate someof these terms.

http://localhost/var/www/apps/conversion/tmp/scratch_6/Pendulum.dochttp://localhost/var/www/apps/conversion/tmp/scratch_6/Pendulum.doc

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Session 4

sketch and interpret a displacement-time graph for a wave,

sketch and interpret a displacement-distancegraph for a wave,

classify the relationship betweenspeed, wavelength and

frequency.

Di l t ti d di l t di t h

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Displacement-time and displacement-distance graphs

The wave motion is due the vibration of particles from their restposition.

The displacement of a particle (from its rest position) at different timesby plotting a displacement-time graph as shown in figure.

From the displacement-time graph, we can find the period of thewave.

For example, in figure, the period (T) is the time measured from

position x to position y.

displacement-time graph

Di l t ti d di l t di t h

http://localhost/var/www/apps/conversion/tmp/scratch_6/WaveGraph.dochttp://localhost/var/www/apps/conversion/tmp/scratch_6/WaveGraph.dochttp://localhost/var/www/apps/conversion/tmp/scratch_6/WaveGraph.dochttp://localhost/var/www/apps/conversion/tmp/scratch_6/WaveGraph.dochttp://localhost/var/www/apps/conversion/tmp/scratch_6/WaveGraph.dochttp://localhost/var/www/apps/conversion/tmp/scratch_6/WaveGraph.doc

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Displacement-time and displacement-distance graphs

Figure below shows the displacement of particles against theirdistance from the source at two different times, at t = 0 and t = T,

where : T is the period of the vibration.

Note that the wavelength is shown by the distance CD.

The displacement-distance graph

D t bt i f

http://localhost/var/www/apps/conversion/tmp/scratch_6/WaveGraph.dochttp://localhost/var/www/apps/conversion/tmp/scratch_6/WaveGraph.dochttp://localhost/var/www/apps/conversion/tmp/scratch_6/WaveGraph.dochttp://localhost/var/www/apps/conversion/tmp/scratch_6/WaveGraph.dochttp://localhost/var/www/apps/conversion/tmp/scratch_6/WaveGraph.dochttp://localhost/var/www/apps/conversion/tmp/scratch_6/WaveGraph.doc

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Data obtain from

Displacement-distance graph are:

Amplitude, aWavelength,

Displacement-time graph are:

Amplitude, aPeriod, T

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Wave speed (v)

http://localhost/var/www/apps/conversion/tmp/scratch_6/WaveSpeed.dochttp://localhost/var/www/apps/conversion/tmp/scratch_6/WaveSpeed.doc

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Session 5

describe dampingin an oscillating system, and

describe resonancein an oscillating system.

http://localhost/var/www/apps/conversion/tmp/scratch_6/Damping.dochttp://localhost/var/www/apps/conversion/tmp/scratch_6/Resonance.dochttp://localhost/var/www/apps/conversion/tmp/scratch_6/Resonance.dochttp://localhost/var/www/apps/conversion/tmp/scratch_6/Damping.doc

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