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4.2. Travelling Waves

So now we have the definition and key terms out the way we can start looking at wave motion. Moving waves are often called travelling, or progressive waves. Before starting IB you wll have learnt about some of the many different types of waves, such as light, sound, seismic and water waves. You may also have learnt about the 2 main categories we can divide these waves into - longitudinal and transverse. This sections builds upon these key ideas.

Before we start, have a little look at this video by the wonderful Tom Scott. He looks at the sound waves produced by road markings in California, and describes the simple miscommunication that means they sound terrible.

The section has been divided up as follows:

 

Describing Waves

Some of those ideas we looked at in the previous section on Periodic Motion will be very important here. To begin with, Crash Course have an introductory video about some of the main features of travelling waves.

There are many different examples of wave motion, though they all follow the same idea:

All waves transfer energy without transferring matter

There are a few key terms you should be familiar with. Hopefully you have a good grasp on these from GCSE - if not, refresh your memory on this Isaac Physics lesson.

Representation of Waves - Displacement/ Time

We encountered a displacement/ time representation in the previous section. This graph looks at one particular point and how it moves over time (think a rubber duck floating on top of water waves - bobbing up and down). Here, we can measure the time taken for one oscillation my measuring from one peak to the next.

DispTime.png

The purple crosses represent two moments in time. The arrow shows the direction that the duck is moving at each moment in time.

Representation of Waves - Displacement/ Position

This second representation of waves looks very similar to the above. However, this representation is basically a 'snapshot' in time - imagine a photograph taken viewing a wave from the side. Here, we can measure hte wavelength of a wave from one peak to another.

The purple crosses represent two different points on our wave. The arrow shows the direction that these points are moving at that moment in time.

(Note, it is opposite to those shown on the displacement/ time representation)

DispPos.png

The Wave Equation

During the time taken for one oscillation, a wave will travel forwards by a distance of λ. As there are f wavesper second, the wave propagates a distance of fλ in one second. This means that the velocity of the wave wave is given by the following equation:

Wave speed (msˉ¹) = Frequency (Hz) x Wavelength (m)

v=fλ

This is called the 'wave equation' and it is important you are familiar with it. Isaac Physics have a set of practice questions you can have a go at to test yourself.

Video Lessons

IB Physics
Topic 4 Notes
IB-Physics.net
Chapter 4 Summary
IB Revision Notes
Isaac Physics
Waves Concepts
Mr. G
4.2 Teaching Notes
4.2 Student Notes
Physics and Maths Tutor
Waves Definitions
Waves Key Points
Waves Detailed Notes
Waves Progressive & Standing Waves
A Level Resources - content slightly different

Resources

Questions

Cambridge University Press
Topic 4: Add Qs
Topic 4: Add Qs MS
Topic 4: MCQs
CUP Website Link
Freely available online
Dr French's Eclecticon
Waves
Waves Solutions
Link to Dr French's Site
Extension: Pre-University Material
Grade Gorilla
4.2 (Wave Basics) MCQ
Topic 4 (Waves A) End Quiz
Quick IB Specific Mixed MCQs
Isaac Physics
Amplitude and Intensity
Mr. G
4.2 Formative Assessment
Topic 4 Summary Qs
IB Specific Questions
Physics and Maths Tutor
Progressive Waves (AQA 2)
Progressive Waves MS (AQA 2)
Waves (OCR)
Waves MS (OCR)
A-Level Qs: overlapping content
Physics and Maths Tutor
Waves (Edexcel 1)
Waves MS (Edexcel 1)
Waves (Edexcel 2)
Waves MS (Edexcel 2)
A-Level Qs: overlapping content
Physics and Maths Tutor
Waves (AQA 1)
Waves MS (AQA 1)
MCQ Progressive/ Stationary (AQA 2)
MCQ Progressive/ Stationary MS (AQA 2)
A-Level Qs: overlapping content
 

Transverse and Longitudinal Waves

All waves can be characterised as either Transverse or Longitudinal.

Transverse waves include electromagnetic waves, water waves and S-Seismic waves.

For a transverse wave the wave displacement is perpendicular to the direction of wave propagation/ energy transfer. 

In the gif below, the wave is travelling to the right (and transferring energy from left to right), whereas the particles are vibrating up and down. A graphical representation is shown to the right. 

transversewave.gif
transverse1.png

Longitudinal waves include sound waves and P-Seismic waves.

For a longitudinal wave, the wave displacement is parallel to the direction of wave propagation/ energy transfer. 

In the gif below, the wave is travelling to the right (and transferring energy from left to right), while the particles are also vibrating left and right. A graphical representation is shown to the right - it looks similar to the above, but in this case, a positive displacement represents a particle being to the right of its central position. 

longitudinalwave.gif
long1.png

Video Lessons

Chris Doner
Transverse and Longitudinal
IB Specific
Khan Academy
Introduction to Waves
Physics Online
Waves Introduction
Science Shorts
Waves Introduction

Resources

IB Physics
Topic 4 Notes
IB-Physics.net
Chapter 4 Summary
IB Revision Notes
Isaac Physics
Types of Waves
Mr. G
4.2 Teaching Notes
4.2 Student Notes
Physics and Maths Tutor
Waves Definitions
Waves Key Points
Waves Detailed Notes
Waves Progressive & Standing Waves
A Level Resources - content slightly different

Questions

Cambridge University Press
Topic 4: Add Qs
Topic 4: Add Qs MS
Topic 4: MCQs
CUP Website Link
Freely available online
Dr French's Eclecticon
Waves
Waves Solutions
Link to Dr French's Site
Extension: Pre-University Material
Grade Gorilla
4.2 (Wave Basics) MCQ
Topic 4 (Waves A) End Quiz
Quick IB Specific Mixed MCQs
Mr. G
4.2 Formative Assessment
Topic 4 Summary Qs
IB Specific Questions
Physics and Maths Tutor
Longitudinal & Transverse (AQA 2)
Longitudinal & Transverse MS (AQA 2)
Waves (AQA 1)
Waves MS (AQA 1)
A-Level Qs: overlapping content
Physics and Maths Tutor
Waves (Edexcel 1)
Waves MS (Edexcel 1)
Waves (Edexcel 2)
Waves MS (Edexcel 2)
A-Level Qs: overlapping content
Physics and Maths Tutor
Waves (OCR)
Waves MS (OCR)
MCQ Progressive/ Stationary (AQA 2)
MCQ Progressive/ Stationary MS (AQA 2)
A-Level Qs: overlapping content
 

Electromagnetic Waves

You will have come across the electromagnetic spectrum in GCSE (and hopefully are already intimately familiar with the Electromagnetic Spectrum song). You will be familiar with the different parts of the spectrum and perhaps some of their uses, but what exactly do we mean by an 'electromagnetic wave'. The below video by TedEd gives a nice little intro here.

Some of these key points are summarised on the graphic below. It's worth having a general knowledge of some of the rough wavelengths (in particular those for visible light, approximately 400 - 700 nm).

A few pointers that can help remember some of the approximate wavelengths:

Radio waves ≈ metres or km's (as radio waves can diffract round tall buildings and hills for communication, so the wavelengths must be a similar order of magnitude as these objects)

Microwaves ≈ mm's or cm's (as these are the gaps between local hot and cold bits in food heated in the microwave oven)

Infrared 10ˉ⁴ m - 10ˉ⁶ m (wavelengths between red visible and microwaves)

Red visible light 666 nm (666 being the number of the red devil).

Green visible (≈ 555 nm) and blue visible (≈ 444 nm(easy to remember once you know red's wavelength)
UV 10ˉ⁷ m - 10ˉ⁹ m (wavelengths between violet and X-rays)

X-rays ≈ 10ˉ⁹ - 10ˉ¹² m, (X-ray diffraction is used to image atomic structures, because the wavelength is similar to atomic diameters)

Gamma Rays ≈ <10ˉ¹² m (the most energetic waves have the shortest wavelengths)

All parts of the electromagnetic spectrum travel at 3 x 10⁸ msˉ¹ in a vacuum (though may slow down by different amounts when travelling through different media). At this stage, make sure you are confident calculating wavelengths and frequencies using your wave equation. 

EMspectrum2.png

Electromagnetic waves are transverse waves, in which the displacement is perpendicular to the direction of wave travel, but what exactly is the displacement we are talking about in this case? EM waves are not matter waves (as sound and seismic waves are), so the displacement is not caused by moving particles. Instead, EM waves are made up of fluctuations in the Electric and Magnetic Fields (which we will look at in subsequent chapters). These field vectors are oriented perpendicular to one another as shown below, and it is these fluctuations that are what we perceive as light, radio waves or X-rays. As these are fluctuations in the electric and magnetic fields, they do not require a medium to travel through, so our EM waves are able to pass through the vacuum of space.

EM-Wave.gif

Video Lessons

Resources

IB Physics
Topic 4 Notes
IB-Physics.net
Chapter 4 Summary
IB Revision Notes
Isaac Physics
Electromagnetic Waves
Mr. G
4.2 Teaching Notes
4.2 Student Notes
Physics and Maths Tutor
Waves Definitions
Waves Key Points
Waves Detailed Notes
Waves Progressive & Standing Waves
A Level Resources - content slightly different

Questions

Cambridge University Press
Topic 4: Add Qs
Topic 4: Add Qs MS
Topic 4: MCQs
CUP Website Link
Freely available online
Grade Gorilla
4.2 (Wave Basics) MCQ
Topic 4 (Waves A) End Quiz
Quick IB Specific Mixed MCQs
Mr. G
4.2 Formative Assessment
Topic 4 Summary Qs
IB Specific Questions
 

Sound Waves

Sound waves are a longitudinal wave - and a type of matter wave (therefore they need a medium to travel through). Sound waves are vibrations of matter (e.g. a guitar string, air molecules, an ear drum), that transmit energy. A greater amplitude of vibration is perceived as a louder volume, whereas a higher frequency of vibration is perceived as a higher pitch. Humans are able to hear roughly sounds between 20 Hz and 20 000 Hz (though this upper limit decreases with age, have a go at working out your 'hearing age' here). 

phet-logo-trademarked.png

 PHET have a simulation  that allows you to look at a few properties of sound. Change the pitch and amplitude and look at the effect on the waves. You can also remove the air from the speaker and listen to the effect on the sound. The two source interference is something we will later look at in Section 4.4.

It's worth making sure you are familiar with a few experiments to measure the speed of sound. THere is the classic GCSE favourite of banging together two bits of wood and measuring the time taken for sound to get to you with a stopwatch - Note, this is RUBBISH.

 

A much better way of doing this is digitally with an oscilloscope. Hopefully you've had a go at using one of these in your classroom, otherwise there is a simulation here. This video explains a little bit about how the speed of sound can be measured using two microphones connected to an oscilloscope.

Video Lessons

Resources

IB Physics
Topic 4 Notes
IB-Physics.net
Chapter 4 Summary
IB Revision Notes
Mr. G
4.2 Teaching Notes
4.2 Student Notes
Physics and Maths Tutor
Waves Definitions
Waves Key Points
Waves Detailed Notes
Waves Progressive & Standing Waves
A Level Resources - content slightly different

Questions

Cambridge University Press
Topic 4: Add Qs
Topic 4: Add Qs MS
Topic 4: MCQs
CUP Website Link
Freely available online
Grade Gorilla
4.2 (Wave Basics) MCQ
Topic 4 (Waves A) End Quiz
Quick IB Specific Mixed MCQs
Mr. G
4.2 Formative Assessment
Topic 4 Summary Qs
IB Specific Questions
 

Additional Resources

IB Questions

A question by question breakdown of the IB papers by year is shown below to allow you to filter questions by topic. Hopefully you have access to many of these papers through your school system. If available, there may be some links to online sources of questions, though please be patient if the links are broken! (DrR: If you do find some broken links, please contact me through the site)

Questions on this topic (Section 4) are shown in light blue.

Use this grid to practice past IB questions topic by topic. You can see from the colours how similar the question topic breakdown is year by year. The more you can familiarise yourself with the IB question style the better - eventually you will come to spot those tricks and types of questions that reappear each year.