Waves

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Waves
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Chapter 17 Assignments

• 506/2-4
• 507/910
• 527-530/1-10,11,13,14,18,19,21,26,27, and
  Preparing for the FCAT.
• Complete workbook pages for Chapter 17.

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Types of Waves

• Objectives
• Waves transfer energy.
• Distinguish between mechanical waves and
  electromagnetic waves.
• Explain the relationship between particle
  vibration and wave motion.
• Distinguish between transverse waves and
  longitudinal waves.

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Waves transfer energy

• A wave is a disturbance that carries energy
  through matter or space
• Does the matter move with the wave?
• No.

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Comparisons of Waves
Type of Wave Mechanical Mechanical Electromagnetic
Form Longitudinal Transverse Modeled as transverse
Medium Solids, liquids, and gases Solids and liquids None required
Travel as Compression and Rarefactions in matter Up-and-down movement of matter Oscillating electric magnetic fields
Examples Sound waves, some earthquake waves Water, rope, some e-quake waves Visible light, radio, X rays
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Comparing Mechanical and E-M

• Mechanical waves need a medium, E-M waves do not
• Sound is an example of an mechanical wave
• Light is an example of Electromagnetic waves
• A Medium is a substance like air (a gas), water
  (a liquid), or metal (a solid)
• Light waves can travel without a medium present -
  how sunlight gets to earth!

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Transverse Waves

• In a transverse wave the particles vibrate
  perpendicular to the wave energy

vibration
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Transverse examples

• Light -E-M spectrum
• Visible
• Invisible
• (TV, radio, X Ray, etc.)
• Water waves

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Longitudinal

• The particles vibrate parallel to the direction
  of the wave. (Sound is an example.)

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Longitudinal examples

• Sound

Wave motion
Particle motion
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Frequency, f, in hertz (Hz)
Greater frequency means a higher pitch. Which of
these would have a higher pitch? Also, one hertz
is one cycle per second or 1/sec Humans can hear
from about 10 to 10,000 Hz.
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Speed of Sound

• is fastest in solids.
• That is why train robbers in the old movies put
  their ear to the train tracks so that they could
  hear if a train was coming.
• Velocity frequency x wavelength
• V f l
• Units v in m/s
• f in 1/s, cycles/second, or Hertz, Hz
• l in m

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Velocity Problem

• A wave on a rope has a wavelength of 2.0 m and a
  frequency of 2.0 Hz. What is the speed of the
  wave?
• V f l
• V 2.0 s-1 x 2.0 m s-1 1/s
• V 4.0 m
• s

V f l
Add this to your circle sheet ?
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If a wavelength is 100mm and its frequency is 10
Hz, find its velocity.

• V f l 10 Hz x 100m 1000m/s
• Period, T, the time for one wave to pass Period
  is the opposite of frequency
• Period 1 / frequency
• So the period, T of a 5 Hz (or 5/second) wave is
• T 1 .20 second
• 5

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Characteristics of Waves
amplitude
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Comparisons

Type of Wave Mechanical Mechanical Electromagnetic
Form Longitudinal Transverse Modeled as transverse
Description Compressions rarefactions Sine wave shaped movement of matter Oscillating e-m fields
Measure of Wavelength Distance between 2 successive compressions Distance between 2 successive crests Distance between 2 successive crests
Measure of amplitude Distance from rest to max. compression Difference in height between crest rest Diff. Between max field zero.
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Amplitude

• The height of a wave is related to loudness for
  sound waves.
• The unit of loudness for sound is a decibel.

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Other properties of waves

• Reflection - when a wave bounces off an object
  like a mountain causing an echo
• Refraction - when a wave goes from one medium to
  another it changes speed and direction

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Interference - when 2 waves pass through the
medium at the same time.
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Resonance

• When sound from an instrument causes something
  nearby to vibrate. Like a drum causing the
  strings in a piano to vibrate.

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Doppler Effect

• The apparent change in frequency when a source of
  a sound moves toward or away from you - like an
  ambulance siren.
• Its pitch (frequency) appears to change.

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More information available at

• http//www.glenbrook.k12.il.us/GBSSCI/PHYS/Class/s
  ound/u11l2d.html