Waves

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Waves

• A disturbance in a medium that transfers energy
  and momentum

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To produce a Wave

• A vibration (disturbance)
• A medium a substance to travel through.

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Examples of Waves

• Sound
• Light
• Water

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There are two types of waves
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• Transverse the individual wave particles move
  perpendicular to the velocity of the wave.

Examples Electromagnetic waves (light waves,
radio waves, microwaves, x-rays) Wave on a string
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Longitudinal the individual wave particles move
parallel to the velocity of the wave.
Examples Sound Waves
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Parts of a wave
Frequency The number of wave cycles in 1
second.
Units 1/s Hertz (Hz)
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Wave Interference The combination of two or
more waves.

• Constructive interference Two waves combine to
  make a bigger wave.
• Destructive interference Two waves combine to
  make a smaller wave.

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wavelength (velocity)(Period)
wavelength (m)
Period (s)
wavelength (m)
frequency (hz)
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The Wave Equation
v velocity of the wave (m/s) ? wavelength
(m) f frequency (1/s Hz)
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Example1 A sound wave has a frequency of 256
Hz. What is the wavelength? The speed of sound
is 340m/s.
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Example 2 A radio wave has a frequency of
96.9MHz. What is the wavelength? The speed of
light is 3.0 x 108 m/s.
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Standing Waves on a String
The velocity of a wave on the string depends on
the mass per length of the string and the tension
in the string.
v velocity of the wave (m/s) FT Tension in the
string (N) m mass of the string (kg) L length
of the string (m)
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The fundamental frequency 1st Harmonic.
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The fundamental frequency 2nd Harmonic.
L
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The fundamental frequency 3rd Harmonic.
L
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The fundamental frequency 4th Harmonic.
L
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The fundamental frequency 5th Harmonic.
L
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Summary
fnnf1
fn nth harmonic n 1, 2 ,3, .. f1 1st
harmonic (fundamental frequency)
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Conditions for interference
d path difference L2 L1
Constructive Interference d 0, ?, 2?, 3? . d
n? n 0, 1, 2, 3,
Destructive Interference d ?/2, 3?/2, 5?/2
. d (n ½)? n 0, 1, 2, 3, .
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Sound Waves

• The speed of sound in air at room temperature is
  340m/s.
• The speed of sound increases with increasing
  temperature.
• The speed of sound in water is 1500m/s.
• The speed of sound in aluminum is 5100m/s.

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Physics
HumanPerception
Pitch
frequency
loudness
Intensity/Amplitude
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Pressure fluctuations in air due to a vibrating
tuning fork.
Applet
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Frequency range of the human ear.
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Ultra Sound
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Human Ear
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Sound intensity and the decibel scale
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Beats

• Beats occur when two sound waves have slightly
  different frequencies interfere with one another.
• The number of beats per second is called the beat
  frequency.
• The beat frequency is determined by subtracting
  the two frequencies.

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Standing Sound Waves in a Tube

• The wave travels at the speed of sound (340m/s)
• Open ends must have an antinode
• Closed ends must have a node.
• A pressure wave is set up in the tube.
• A tube open at both ends acts just like the
  string.
• A tube closed at one end only has odd harmonics.

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Open Tube
Just like the string
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Closed Tube
Odd Harmonics
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Resonance occurs when the driving frequency
matches the natural frequency, resulting in large
amplitude vibrations. Here are some examples of
resonance

• Pushing someone on a swing.
• The Tacoma Narrows bridge.
• Breaking a wine glass with a sound wave
• Earthquakes totally destroying some buildings and
  not damaging others.

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The Doppler Effect is a change in frequency
(pitch) due to the relative motion of the sound
source and observer.

• As the sound and listener approach each other the
  frequency is higher.
• As the sound and listener move away from each
  other the frequency is lower.
• The Doppler effect also occurs with light
  producing the red and green shift of distant
  stars.
• Doppler radar is used to track weather systems

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