General

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Waves
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Waves

• A wave carries energy from one place to another.

A wave is one way of transmitting signals from
one place to another.
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Words used to describe waves
There are several words used to help describe
waves. Here are the ones that have to be learned!
Crest The highest point on a wave
Trough The lowest point on a wave
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• Amplitude

This is how high the crest of a wave goes above
the centre line.

• The amplitude is a measure of how much energy a
  wave has.

Amplitude is measured in metres (m).
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Wavelength

• The wavelength of a wave is represented by l
  (Greek letter lambda).

wavelength, l
It is the distance between one point on a wave to
the same point on the next wave.
wavelength, l
wavelength, l
Wavelength is measured in metres (m).
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Transverse waves

• Transverse particle motion is perpendicular to
  the direction of energy transfer.
• Particle Motion
• Energy Transfer
• Examples water waves, waves in ropes, light
  waves
• Transverse waves can be drawn with a sine curve
  or using wavefronts

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Transverse waves

• Transverse waves can be drawn with a sine curve
• Or using wavefronts Wave fronts represent the
  crests

ray
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Sound Waves

• What type of wave is a sound wave?
• Longitudinal disturbance is in the same
  direction as the propagation of the wave.
• What causes sound waves?
• Something vibrates
• Forwards high pressure
• Backwards low pressure

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longitudinal waves

• Longitudinal waves are harder to visualise but if
  we imagine a sound wave passing through air, the
  particles might look something like this
• Direction of disturbance is the same as direction
  of energy travel.
• In space no one can hear you scream Why?

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In space no one hears you scream

• Sound needs particles to vibrate
• It can travel through a solid, liquid, or gas.
• Not in a vacuum

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Frequency

• The frequency of a wave is represented by the
  letter f.

It is the number of waves in one second.
Frequency is measured in Hertz (Hz).
The frequency of waves can be calculated from
number of waves

frequency
number of seconds
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Period

• Period (T, seconds) the time taken for one
  complete cycle
• Frequency 1 f 1
• Period T
• Example A pendulum completes 10 swings in 8s.
  What is its frequency?
• There are 10 swings in 8 seconds so each swing
  takes 0.8s.
• Period 0.8s
• F1/T
• F1/0.8
• F1.25Hz

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Wave speed

• The wave speed or wave velocity is represented by
  the letter v.

This is how fast the crests are moving or how
quickly the energy is transferred from one place
to another.
Wave speed is measured in metres per second (m/s).
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The Wave Equation

• A particular wave has a frequency of 2Hz and a
  wavelength of 4m, what can this tell us about how
  fast it travels?

A frequency of 2Hz means that there are 2 waves
every second.
If each wave is 4m long.
The energy has travelled 2 x 4 8m in one second.
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8 m/s

So the speed

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This is the same answer that we would get if we
multiplied the frequency by the wavelength!
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The Wave Equation

• This means that we can calculate the wave speed
  from the following equation

wave speed
frequency
wavelength

x
or
f
x
v

l
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Examples

• 1. How fast do the waves travel across a pool if
  the frequency of the wave generator is 5Hz and
  the wavelength of the waves is 3m?

l
f
x

v

x
v
5
3
m/s
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v
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• 2. Waves travelling at 24m/s have a frequency of
  4Hz. What is the wavelength of the waves?

v

l
f
24

l
4
m
6

l
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• 3. What is the frequency of a wave which is
  travelling at 10m/s and has wavelength of 2.5m?

v

f
l
10

f
2.5
Hz
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f