Waves Intro

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Waves Intro

• Chapter 25

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Vocabulary

• Wave
• Vibratory disturbance that propagates (moves)
  through a medium
• Medium
• Material through which a wave propagates
• Pulse
• Single disturbance

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Waves

• Waves transfer energy from one place to another,
  not mass

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Wave Types

• Two main types
• Transverse
• Motion of the disturbance is perpendicular to the
  direction of the wave propagation
• Longitudinal
• Motion of the disturbance is parallel to the
  direction of the wave propagation

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Transverse Waves

• Motion of the disturbance is perpendicular to the
  direction of the wave propagation
• Example Light

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Longitudinal Waves

• Motion of the disturbance is parallel to the
  direction of the wave propagation
• Example Sound

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Surface Waves

• Combination of transverse and longitudinal waves
• Example Water

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Water Waves (surface)
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Wave Characteristics

• Amplitude, A (m)
• Displacement away from equilibrium point
• Wavelength, ? (m)
• Length of 1 wave cycle
• Period, T (s)
• Amount of time for 1 wave cycle

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Wave Characteristics (cont)
T (s)
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Wave Characteristics (cont)

• Frequency, f (Hz or s-1)
• Number of cycles per second
• Inverse of period
• Speed, v (m/s)
• How fast wave is traveling
• Related to frequency (period) and wavelength

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Equations

• f frequency (Hz)
• T period (s)
• v speed (m/s)
• ? wavelength (m)

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Light

• Light is also called electromagnetic radiation
• Light is a combination of fluctuating electric
  fields and magnetic fields that are perpendicular
  to each other

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Electromagnetic Spectrum
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Electromagnetic Spectrum

• R Radiowave
• M Microwave
• I Infrared
• V Visible
• U Ultraviolet
• X X-Rays
• G Gamma
• C Cosmic

Wavelength Decreases
Frequency Increases
Energy Increases
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Light (cont)

• Transverse Wave
• Travels through vacuum
• Color is based on frequency
• Green Light 5.6 x 1014 Hz
• Speed of light in a vacuum (air also)
• c 3 x 108 m/s

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Sound

• Longitudinal Wave
• Needs a material (medium) to move
• Pitch is based on frequency
• Concert A 440 Hz
• Speed of Sound in air is dependent on Temp
• v 331 m/s at STP

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Wave Speed

• Waves must follow the kinematic equation
• The speed of waves depends upon the material that
  the wave travels through

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Wave Speed

• Sound can not travel in a vacuum, light can
• Light travels fastest in a vacuum, slower in all
  other materials
• Sound travels faster in more dense materials

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Phase Difference

• Two points are considered in phase when they
  are at the same point in a wave cycle
• The amount of in or out of phase is measured in
  degrees

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Phase Difference Examples

• What point is in phase with A?
• B and D are how far out of phase?
• Name two other points in phase with each other.

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Wave Motion

• Waves propagate in all directions without barriers

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Wave Fronts

• Line that represents waves that are all in phase,
  usually crests

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Principle of Superposition

• When two waves meet, they combine together
  briefly, then go their separate ways
• Crest crest bigger amplitude
• Trough trough bigger amplitude
• Crest trough lower amplitude

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Interference

• Constructive Interference
• When 2 waves interfere with resultant wave having
  larger amplitude
• Destructive Interference
• When 2 waves interfere with resultant wave having
  smaller amplitude

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Simulation Examples

• http//www.surendranath.org/Applets/Waves/TWave02/
  TW02.html

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Interference Example

• Two point sources (green dots)
• What do the red dots represent?
• What do the blue dots represent?

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Sound Beats

• Interference produced when two sounds interact
• Frequency of beats is equal to difference of
  frequencies of two sounds
• Concept used to tune pianos
• Demo

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Doppler Effect

• Change in frequency due to moving wave source or
  observer
• Example
• Example

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Doppler Effect

• When distance between source and observer is
  decreasing, frequency increases
• Blue Shift
• When distance between source and observer is
  increasing, frequency decreases
• Red Shift

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Sonic Boom

• When moving object exceed the speed of sound, air
  builds up into a shock wave

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Sonic Boom
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Standing Waves

• Occurs when two waves traveling in opposite
  directions in the same medium, with the same
  amplitude and same frequency
• Resultant wave appears to be standing still
• Demo

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Nodes and Antinodes

• Nodes
• Points of maximum destructive interference
• Antinodes
• Points of maximum constructive interference

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Nodes and Antinodes
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Nodes and Antinodes
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Video

• YouTube Video
• How does this work?

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Resonance

• Natural Frequency
• Particular frequency that every elastic body will
  vibrate at if disturbed
• Resonance
• Vibration of a body at its natural frequency
  because of the action of a vibrating source of
  the same frequency

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Real Life

• Microwaves produce waves that have the same
  frequency as the vibrational frequency of water
  molecules
• UV rays have the same frequency as certain
  chemicals in human skin, causing sun burns
• Google Tacoma Narrows Bridge

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Harmonics

• Fundamental Frequency(1st Harmonic)
• Lowest frequency possible
• 2nd Harmonic
• 2x frequency of 1st Harmonic (Octave higher)

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Closed Pipe Harmonics (Lab)

• 1st Harmonic L 1/4?
• ? 4L
• 3rd Harmonic L ¾ ?
• ? 4/3L
• 5th Harmonic L 1 1/4 ?
• ? 4/5L

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Open Pipe Harmonics

• 1st Harmonic L ½?
• ?2L
• 2nd Harmonic L ?
• 3rd Harmonic L 1 ½ ?
• ?2/3L