2. Waves and Sound

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Introduction
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5. Table of Contents
Part A : Definition Definition of Vibration and waves Basic Properties of waves.
- Terminology of waves.
Part B : Types of Waves and Terminology Mechanical waves : Transverse and Longitudinal waves.
Part C : The Wave Equation.
Part D : Common Characteristic of Wave
Part E : Revision
The basics

6. PART A 10.1 What are waves? page232 Definition
A periodic disturbance which travels through a medium from one point in space to the others. OR
A wiggle in space and time is a wave.
What carries waves?
A medium, a medium is the material through which a wave travels.
A medium can be a gas, liquid, or solid.

7. What causes waves?
Waves are created when a source of energy causes a medium to vibrate.

8. PART A Definition of Vibration
A vibration is a repeated back and forth, up and down, from side to side, to and fro, in and out motion in time. OR
A wiggle in time.

9. Each term will be explained….
Terminology of a Wave
A wave is usually described by the following terms :
Amplitude
Wavelength ()
Frequency (f)
Period (T)
Wave velocity (v)
Each term will be explained….

10. Amplitude(A) Definition:
The amplitude is the distance from the midpoint to the crest (or to the trough) of the wave.

11. Wavelength (λ) Definition:
The wavelength is the distance from the top of one crest to the top of the next one. OR
The distance between successive identical parts of the wave.
The distance from successive crest to crest or trough to trough
Unit: meters

12. Frequency(f) Definition:
The frequency is the number of to and fro vibrations per second.
One vibration per second is 1Hz; two vibrations per second is 2 Hz, and so on.
Unit: Hertz (Hz) defined to be one cycle / wave per sec.
Higher frequencies are measured in kilohertz(kHz), and still higher frequencies in megahertz(MHz).

13. Time period(T) Definition:
The period (T) of a wave or vibration is the is the time it takes for a complete vibration --- for a complete cycle.
It is related to frequency by
T = 1/ƒ
Unit : s

15. 10.2 Wave Speed page233 PART C v = ƒ
Definition: The wave velocity is the displacement traveled by the wave in one second ……....
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The speed of periodic wave motion (v) is related to the frequency and wavelength of the waves by --
Wave speed = wavelength/ period
Wave speed= wavelength x frequency
The Wave Equation
v = ƒ

17. Using the Wave Equation Example :
A travelling wave of wavelength 0.6m moves at a speed of 3.0 m/s. What is the period of this wave ?
Now you know  = 0.6 m, v = 3.0 m/s Can you find the frequency of this wave……
By using the wave
equation, v = ƒ = ƒ(0.6) i.e ƒ = 5.0 Hz
Click to see the answer
Then the period of this wave is ???
Period T = 1/ƒ T = 1/5.0 or 0.2 s
Table of
Contents

19. TYPES OF WAVES Waves are classified into two different types according to their natures, how they move means their direction of vibration :
Waves
Longitudinal Waves
Transverse Waves

21. Transverse Waves page234
Definition: A wave in which the medium vibrates in a direction perpendicular (transverse) to the direction in which the wave travels. (page 249)
Examples
Water waves, pulse in a stretched string.

24. 10.3 Longitudinal Waves page 234
A wave in which the medium vibrates in a direction parallel (longitudinal) to the direction in which the wave travels.
Sound, or a spring oscillating up and down, etc.
Examples

28. 10.4 Sound Waves page235
Sound is the propagation of vibrations through a material medium--- a solid, a liquid or a gas.
Molecules in the air vibrate about some average position creating the compressions and rarefactions.
Sound cannot travel in a vacuum (no medium).
We call the frequency of sound the pitch.

30. Speed of Sound Depends on the material of the vibrating medium
Sound can vibrate water, wood (speaker enclosures, pianos), metal, plastic, etc.
Sound speed in dry air is 330 meters/second at 0o C
Faster in warm air, slower in cold
Water 4 times faster, steel 15 times faster
The human ear can normally hear pitches from sound ranging from about 20Hz to about 20,000Hz.
Sound waves of frequencies lower than 20Hz are called infrasonic waves.
Sound waves of frequencies higher than 20,000Hz are called ultrasonic waves.

31. PART E Revision Now you have learned the basic concepts of Waves.
After this lecture, you should be able to answer the following questions :
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Are you ready ?
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32. 1. What is the difference between mechanical and electromagnetic waves ?
2. Can you give some examples of electromagnetic waves ?
3. What is the direction of the particle motion in a transverse wave ?
More questions
4. Is sound a transverse or longitudinal wave ?
5. Is it correct that water waves do not transmit energy from one point to another ?
If you do not have all 5 questions answered correctly, click here and back to part B again if necessary ---->>

33. If you miss any of this questions, go back to the lesson
1. In a wave diagram, can you identify the following terms ? (a) wavelength (b) amplitude.
2. Write down the wave equation.
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of this questions,
go back to the lesson
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34. Waves mechanical Electromagnetic Waves
Now we come to the conclusion of introducing waves.
You should be able to tell the nature of waves, and identify
different types of waves after this lecture.
mechanical
Waves
Electromagnetic
Waves
The study of waves is important in Physics. From the motion of water to the structure of atoms, study of waves applies almost in every category.
THE END OF LECTURE

35. PART B TYPES OF WAVES Waves are classified into different types according to their natures :

36. 1. Mechanical Waves
A material medium is necessary for the transmission for mechanical waves. Mechanical waves cannot travel through vacuum.
Water waves,
Sound
Click for
examples
Vibration of spring