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© Boardworks Ltd 2003
Energy and Waves : Introduction
Waves carry energy from one place to another.
There are 2 main types of wave you need to know about:
Transverse Waves e.g. light waves
Longitudinal Waves e.g. sound waves
© Boardworks Ltd 2003
Waves : Introduction
TRANSVERSE WAVES
Make a transverse wave using a rope between two people like this :
What type of energy is the wave transferring?
In which direction is the energy moving?
In which direction is each part of the rope moving? [put a marker on the rope to help you see]
kinetic
© Boardworks Ltd 2003
You should have noticed that each part of the wave moves at right angles [90º] to the direction the energy is moving in:
Par
ticle
M
ovem
ent
Energy Movement
Make a labelled diagram of your rope like this :
Waves : Introduction
TRANSVERSE WAVES
© Boardworks Ltd 2003
Crest / Peak
Time
Trough
a
a
Dis
plac
emen
t
+
Waves : Introduction
TRANSVERSE WAVES
© Boardworks Ltd 2003
Wavelength and amplitude
What is the wavelength () of a transverse wave?
What is the amplitude (a) of a transverse wave?
It is the distance between two consecutive points on the wave.
It is the half-height of the wave, the bigger the amplitude of the wave, the more energy it has.
© Boardworks Ltd 2003
LONGITUDINAL WAVES
Make a longitudinal wave using a spring between two people like this :
What type of energy is the wave transferring?
In which direction is the energy moving?
In which direction is each part of the spring moving? [put a marker on the spring to help you see]
kinetic
Waves : Introduction
© Boardworks Ltd 2003
You should have noticed that each part of the wave moves in the same direction as the direction the energy is moving in:
particle movement
energy movement
When you speak to someone, the air vibrates as a longitudinal wave :
LONGITUDINAL WAVES
Waves : Introduction
© Boardworks Ltd 2003
Make a labelled diagram of your spring like this :C
ompr
essi
on
Com
pres
sion
Rar
efac
tion
Rar
efac
tion
LONGITUDINAL WAVES
Waves : Introduction
© Boardworks Ltd 2003
Transverse or longitudinal?
Transverse Longitudinal
sound
p-waves
light
s-waves
water
ultrasound
ultraviolet
soundlight
water p-waves
ultrasound
s-waves ultraviolet
© Boardworks Ltd 2003
Waves : Frequency
The number of waves passing a point each second is called FREQUENCY [ƒ]
Frequency [ƒ] is measured in hertz [Hz]
Frequency [ƒ] = Number of waves passed Time [s]
1 wave per second = 1 Hz
If the wave below passes a point in 1 second, what is it’s frequency?
© Boardworks Ltd 2003
Complete the table
6
10
1
3
5
0.5
Waves : Frequency
Wave Cycles Time (s) Freq (Hz)
2
2
2
© Boardworks Ltd 2003
Waves : wave speed
= 2m
1 second later :
Imagine waves on the sea travelling over a shipwreck:
© Boardworks Ltd 2003
In the diagram, 5 waves pass the shipwreck in 1 second………………….so the frequency = 5 Hz
The wavelength [] = 2 m
This means that the waves travel 10 m in 1 s
……….………………so the speed is 10 m/s
So, in this example, the
frequency X wavelength = speed
5 X 2 = 10
Hz m m/s
This is usually written as :
Waves : wave speed
© Boardworks Ltd 2003
V = x
For the Higher Tier papers, you will need to be able to change the subject of the formula :
= V and = V
Waves : wave speed
© Boardworks Ltd 2003
Formula triangles
v
f
x
Formula triangles help you to rearrange formula. The triangle for the wave equation is shown below:
Whatever quantity you are trying to find cover it up and it will leave you with the calculation required.
So if you were trying to find frequency, f…..
…you would cover f up…
…and you are left with the sum…
f = v
© Boardworks Ltd 2003
Wave Speed – complete the table
Wave ? [Hz] [m] V [m/s]
Water wave
2 1.5
Mexican wave
40 8
Musical note
256 339
Rope
3 0.8
Ultra-sound
35,000 339
3.0
0.2
1.32
2.4
0.01
© Boardworks Ltd 2003
Energy and Waves : A Question
The photograph shows waves travelling across the surface of a pond.1m
1) Is this an example of a transverse or longitudinal wave?
2) Estimate the wavelength of the wave:
3) If the frequency of the wave is 0.2 Hz, calculate the speed of the wave:
transverse
0.15m
0.03m/s
© Boardworks Ltd 2003
Which wave below is a longitudinal wave?
A. Ultraviolet
B. Water
C. Light
D. Sound
© Boardworks Ltd 2003
What are the units of frequency?
A. Metres
B. Hertz metres
C. Hertz/metres
D. Hertz