the world communicates - prime education waves and energy the world communicates • year 11 physics...

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Year 11 Physics the world communicates 2.1.1 waves and energy

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2.1.1 waves and energy

the world communicates • Year 11 Physics

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syllabus Students learn to: Students :

1. The wave model

can be used to

explain how

current

technologies

transfer

information

describe the energy transformations

required in one of the following:

– mobile telephone

– fax/modem

– radio and television

perform a first-hand investigation

to observe and gather information

about the transmission of waves in:

– slinky springs

– water surfaces

– ropes

or use appropriate computer

simulations

present diagrammatic information

about transverse and longitudinal

waves, direction of particle

movement and the direction of

propagation

describe waves as a transfer of

energy disturbance that may occur

in one, two or three dimensions,

depending on the nature of the wave

and the medium

identify that mechanical waves

require a medium for propagation

while electromagnetic waves do not

perform a first-hand investigation

to gather information about the

frequency and amplitude of waves

using an oscilloscope or electronic

data-logging equipment

present and analyse information

from displacement-time graphs for

transverse wave motion

plan, choose equipment for and

perform a first-hand

investigation to gather

information to identify the

relationship between the

frequency and wavelength of a

sound wave travelling at a

constant velocity

solve problems and analyse

information by applying the

mathematical model of

v f

to a range of situations

define and apply the following terms

to the wave model: medium,

displacement, amplitude, period,

compression, rarefaction, crest,

trough, transverse waves,

longitudinal waves, frequency,

wavelength, velocity

describe the relationship between

particle motion and the direction

of energy propagation in

transverse and longitudinal

waves

quantify the relationship between

velocity, frequency and wavelength

for a wave:

fv



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2.1.1.1 Waves and energy transfer

The development of our civilisation would have been impossible without effective communication.

The early development of speech and later the written word allowed us to evolve a cohesive

community that was capable of passing ideas and beliefs from generation to generation.

The messenger carrying the information has been supplanted by electromagnetic means of

transmission that allow transfer of data at close to the speed of light. To all intents and purposes, the

sending and receiving of data over satellite links is instantaneous, limited only by the speed of

coding and decoding the information into suitable forms for transmission.

Speech and many modern means of communication utilise waves. There are many different kinds of

waves. The most obvious form of waves are those upon which we surf. Less obvious are sound

waves, and possibly the least obvious are light or electromagnetic waves. In this section we discuss

what waves really are, and their importance in the world around us.

All waves share one thing in common, they provide a means of transferring energy from one point

to another without the physical movement of particles from one point to another.

Ocean waves are generated thousands of kilometres out to sea by the action of wind on the surface

of the ocean. The energy transferred to the surface of the ocean eventually reaches land a few days

later as a breaking wave. However, the water molecules that were originally moved by the wind far

out at sea do not move far from their original positions.

Figure 2.1.1 (1) Energy moves down the string, but the molecules of the string retain their original relative positions.

They pass on their energy to neighbouring molecules, which in turn affect their neighbours. In this

way energy is transferred without mass motion.

If you put energy into a string or rope by shaking one end up and down, the other end of the string

will also begin to move up and down. Energy will have been transferred along the string, but the

molecules of the string will not have moved from their original relative positions.



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In a similar way electromagnetic radiation (which includes light) can be thought of as the transfer of

energy from one place to another by varying electrostatic and magnetic fields. If you could take

hold of an electron in one corner of the room and shake it up and down, you would find that other

electrons at the other side of the room would begin to vibrate a split second later. Energy is

transferred from one side of the room to the other by an electromagnetic wave.

If particles and molecules don't actually move from one place to another when energy is transferred

by a wave, what actually happens to the individual particles? Let's consider what happens if we

drop a rock into a pool. Ripples spread out from the position where the rock entered the pool and

eventually reach the pool's edge. Floating twigs and straw near the centre of the pool are not washed

ashore, instead they begin moving up and down about an equilibrium point. Their vertical motion is

a form of simple harmonic motion. This vertical oscillation is transferred outward from one region

of the pool to the next. As the oscillation builds up in one area it dies away in the preceding area.

The wave is seen to travel out from the pool's centre.

Whether we are communicating with our voice, by radio, television, or telephone... we use waves to

do it for us.

Waves are carriers of energy.

Energy is capable of being transformed from one type into another. Consider for example, the

energy transformations in a number of common communication devices.

Mobile Telephone

Mobile phones hardly existed a few years ago. Now it seems that almost everyone has one. They are

mobile devices that operate by transmitting and receiving radio signals, communicating via these

radio waves with an antenna transmitter. A single transmitter services a small geographical area

called a cell (hence cellular phone).

Communication with the ordinary telephone network occurs via radio waves rather than through

wires as it does for a fixed phone. The energy transformations that occur in mobile telephones are as

follows:

( ) Sound energy electrical energy radio electromagnetic energy

electrical energy sound energy

Fax

Here light energy is converted into electrical signals and is then transmitted over the phone lines

where it is converted back into light. The energy conversions are:

light energy electrical energy light energy



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Radio and TV

In radio transmission sound energy is first converted into mechanical energy then electrical energy

in a microphone. This energy is converted into radio waves (electromagnetic energy) for

transmission. Aerials convert this into electrical energy then magnetic and mechanical energy in a

loudspeaker and finally back into sound. The energy conversions can be represented as follows:

sound energy mechanical energy electrical energy

( ) radio electromagnetic energy electrical energy

magnetic and mechanical energy sound energy

In television, in addition to the above conversions, light energy is converted into electrical energy

then into TV (electromagnetic) waves and finally back into light energy.

Fixed telephone

Radio

Fax/modem



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Computer information storage system

Figure 2.1.1 (2) Some of the energy transformations which occur in common devices. Mobile phones, radio and television all transmit and receive signals carried by electromagnetic waves.

Figure 2.1.1 (3) The energy transformations involved in transmitting and receiving a mobile phone call.



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Waves and energy

When an object vibrates in a medium it causes a disturbance that travels away from the source of

vibration through the medium, carrying energy from the vibration with it. For example, a cork

bobbing up and down in the water as shown in figure 2.1.1 (4) will produce water waves while an

oscillating tuning fork figure 2.1.1 (5) will produce sound waves.

Figure 2.1.1 (4) A cork bobbing up and down in the water produces water waves

Figure 2.1.1 (5) A vibrating tuning fork produces sound waves

Waves travel through the medium carrying energy only: they do not take any part of the medium

with them. They cause an oscillation of the particles in the medium as they pass, but every particle

returns to its equilibrium position after each complete cycle of the wave. In this way the particles of

the medium transmit the wave but do not move along with it, and we can think of the wave as

energy moving through the medium.



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Waves are disturbances that transfer energy from one point in a medium to another point. They may

propagate in one, two or three dimensions depending on the type of wave and the medium through

which it is moving.

The best way to understand how waves are formed and how they travel is to consider a single pulse

or wave hump. We can make such a pulse on a horizontal string resting on a table by rapidly

flicking one end of the string up then down. As your hand pulls the end of the string up, adjacent

pieces of the string feel a force that also accelerates them in a vertical direction. They in turn affect

neighbouring pieces of string.

As each succeeding piece of string moves upward, the crest of the pulse moves along the string. By

now your hand has returned to its starting position and the end of the string has also returned to its

original position. As adjacent pieces of string reach the top of their motion they experience a force

pulling them back toward their starting positions.

The source of the pulse is the motion of your hand, and the pulse is transferred down the string

because of cohesive forces (tension) between the particles of the string.

Figure 2.1.1 (6) The movement of a wave depends on energy being passed from particle to particle.



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If you continue to move your hand up and down at the same rate, or if you connect the end of the

string to a mass moving up and down on the end of a spring, the end of the string will move up and

down in a periodic manner. It will return to the same vertical position after a precise interval of time.

A series of pulses will be generated at the end of the string separated by precise intervals of time.

These pulses will move down the string, so that if we take a snapshot at any given time each pulse

will appear separated by precise distances. We now have a simple wave.

Figure 2.1.1 (7) The wave has a sinusoidal shape in space.



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Waves in one, two and the three dimensions

Waves occur in one, two or three dimensions depending on the nature of the wave and the nature of

the medium.

One dimension

As we have seen, a wave travelling down a rope or a spring that has been shaken up and down is

travelling in one dimension Figure 2.1.1 (8) a.

Two dimensions

A wave travelling outwards from where a stone has dropped in to water is travelling in two

dimensions Figure 2.1.1 (8) b. (The arrows indicate the direction of the energy transfer).

Three dimensions

When we speak, the sound spreads out into the space around us in three dimensions. So too, does

the light from a source such as a lamp when it is turned on Figure 2.1.1 (8) c.

Figure 2.1.1 (8) Waves in one, two and three dimensions



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Question 1

Which of the following is an example of a wave?

(A) A ball being thrown across a park.

(B) Air being blown by a fan.

(C) Ripples spreading out on a pond.

(D) A spring being stretched out and held in place.

Question 2

What is the medium of each wave?

(a) A sound wave travelling across a room.

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(b) Ripples spreading out in a pond.

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(c) A Mexican wave at a sports game.

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Question 3

In how many dimensions do the following waves travel?

(a) The light from a candle.

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(b) A wave travelling along a stretched-out spring.

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(c) The shockwave of an explosion.

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Question 4

Musical instruments produce sound waves by causing air particles to vibrate. What is the source of

the vibrations of:

(a) a drum?

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(b) a violin?

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(c) a clarinet?

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(d) a trumpet?

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Question 5

It is possible to transform the energy carried by waves into different forms of energy.

Explain why this is useful.

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2.1.1.2 Types of waves

We can classify waves in two ways: by the direction of vibration of the medium and by the type of

medium.

Direction of vibration of medium

Waves can be classified by the direction in which the medium moves in relation to the direction in

which the wave carries energy through the medium. The two most common types of waves are:

Longitudinal waves: In longitudinal or compression waves the medium moves back-and-forth

parallel to the wave's velocity. This type of wave can be produced in a stretched spring as illustrated

in figure Figure 2.1.1 (9) The regions where the spring is more tightly bunched are called

compressions, while the regions that are less tightly bunched are called rarefactions. Sound waves

are longitudinal waves that propagate through gases, liquids and solids.

Transverse waves: When transverse waves travel through a medium, the medium moves back-and-

forth perpendicular to the wave’s velocity. Figure 2.1.1 (11) shows how a stretched spring is

distorted as a transverse wave travels along it. Other examples of transverse waves include waves

on stretched strings, water waves and electromagnetic waves (e.g. light, radio, TV waves).

Transverse waves can only travel through materials in which transverse or shear forces can be

transmitted. For this reason transverse mechanical waves cannot travel through gases or within the

body of a liquid.

Transverse waves cause the medium to be displaced perpendicularly to the wave’s velocity (e.g.

light and water waves).

Longitudinal waves cause the medium to be displaced parallel to the wave’s velocity (e.g. sound).



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Figure 2.1.1 (9) Longitudinal pulse in a stretched spring

Figure 2.1.1 (10) Longitudinal waves (a) in a slinky spring (b) in a gas



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Figure 2.1.1 (11) : Transverse wave in a stretched spring

Figure 2.1.1 (12) A transverse wave



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Electromagnetic and gravity waves

The waves discussed so far have transferred energy by mechanical means. Adjacent particles

interact by means of contact forces, pressing against their neighbours or pulling on them.

A positively or negatively charged particle is affected by changing electrostatic or electromagnetic

fields. Moreover, we can create a disturbance in a group of charged particles simply by waggling

one of the particles up and down. This motion creates a varying electromagnetic field which moves

outward from the charge that has been accelerated. Electromagnetic waves vibrate in two

dimensions and travel in a third dimension. We can detect the progress of this electromagnetic wave

by studying the motion of other charged particles in its path. X-rays, light, heat radiation and radio

waves are all forms of electromagnetic waves. Electromagnetic waves can travel through a

vacuum—they do not need to be transmitted through a medium.

In a similar way, if we move a mass anywhere in the universe, that motion will have an effect on all

of the other masses in the universe. The effect would normally be very small. However, in cases in

which stars are orbiting each other or a star explodes, the changing distribution of material can be

thought of as a source of gravity waves. It is expected that these waves would cause other masses in

their path to vibrate. Attempts are being made to detect gravity waves using massive cylinders of

metal and looking for minute changes in their length as the gravity wave passes through them.

Medium of wave

Seismic waves, sound waves and water waves are all mechanical waves.

Mechanical waves require the energy associated with the wave to be passed from particle to particle

within the material through which the wave appears to move. These waves must have a medium in

which to propagate—they cannot travel through a vacuum

Longitudinal or compression waves can travel in any material whether it is solid, liquid or gas.

However, it is impossible for a transverse wave or a torsion wave to travel through liquid or gas.

Have you ever tried to twist water?

Electromagnetic waves rely on the interaction of electric and magnetic fields to carry energy, and

unlike mechanical waves do not need a material medium for the transfer of energy. They can travel

through a total vacuum.

Gravity waves result from rapid changes in position of massive bodies. These waves are also

transverse waves and like electromagnetic waves do not require a medium through which to

propagate.



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First-hand investigation: waves You should investigate various waves in slinky springs and ripple tanks in class.

For the slinky, if the spring was laid out flat on the ground and one end was held stationary while

the other end was moved, the things to be studied could include:

1 the effect of the direction of movement of your hand;

2 the effect of tension (that is, how much you stretch the spring) on the speed;

3 what happens when you move your hand faster;

4 what happens when the wave gets to the stationary end.

For the ripple tank, this could include:

1 the effect of using a straight edge to generate a wave;

2 the effect of dipping the straight edge faster into and out of the water;

3 the effect of dripping water into the tank;

4 the effect of the depth of water.

You can probably think of other activities.

The investigation carried out by two students may look something like the following. (This is only

one way in which it could have been recorded and is the most formal. The way in which you do it

will probably be determined by what your teacher wants!)

Experiment: waves in one and two dimensions

Aim:

To investigate one-dimensional waves in a slinky spring and two-dimensional waves in a ripple

tank.

Method:

A slinky spring was placed on the ground and stretched to a length of about 5 m. The effect of

moving one end in different directions was noted. The effect of stretching it different amounts was

also noted as was the effect of moving the end faster. Finally, what happened when it hit the end,

was recorded.

Next a ripple tank was set up. Waves were generated using the edge of a ruler and by dripping water

from an eyedropper. Placing a sheet of glass in the tank changed the depth of water.



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Results:

Slinky: By ‘flicking’ one end at right angles to the slinky a single pulse moved down the slinky.

Continuous movement set up a transverse wave that travelled along the slinky. Grasping a number

of coils in one hand and then releasing them along the length of the slinky created a longitudinal

pulse.

Stretching the slinky more increased the tension and the speed of the pulse (or waves).

Moving the end faster resulted in more waves being generated and the distance between each wave

was lessened. (The wavelength decreased.)

When a pulse reached the end, it 'bounced back' on the opposite side to the side that it came down

on. That is, if it travelled down on the left-hand side, it reflected back on the right-hand side.

Ripple tank: In the ripple tank, waves were produced when the straight edge of a ruler was dipped

into the water. They travelled away from the edge in a straight line. By moving the ruler faster in

and out of the water, the waves were closer together.

Drops from an eyedropper falling into the tank set up circular waves that travelled outwards from

where the drop hit the water.

The waves travelled slower in the shallow water.

Question 6

Complete the table by identifying four types of mechanical and electromagnetic waves.

Mechanical Electromagnetic



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Question 7

Recall two differences between mechanical and electromagnetic waves.

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Question 8

Compare the motion of particles in a transverse wave and in a longitudinal wave.

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Question 9

Classify the following waves as transverse or longitudinal.

(a) A ripple in a pool of oil

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(b) A sound wave travelling underwater

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(c) A radio wave broadcast from a satellite

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Question 10

Are gravity waves mechanical waves? Explain your answer.

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Practice Questions

Question 11

Describe three of the energy transformations involved in using a mobile phone.

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Question 12

Identify:

(a) Wave X

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(b) Wave Y

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(c) Arrow A

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(d) Arrow B

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(e) Arrow C

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(f) Arrow D

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the world communicates - prime education waves and energy the world communicates • year 11 physics...

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