THERMAL ENERGY

1
Chapter 6
WAVES Dr. Babar Ali
2
CHAPTER OUTLINE
Wave Concept Wave Properties Wave Speed Wave Types Electromagnetic Waves Sound Waves Speed of Sound Doppler Effect Standing Waves Resonance
3
CONCEPT OF WAVES

• A wave is a disturbance that propagates through a
  system transferring energy
• Waves transfer E!
• (courtesy of Dr. Dan Russell, Kettering Univ.)

4
WAVE PROPERTIES
Crest
the highest point on a wave
Trough
the lowest point on a wave
Amplitude
maximum displacement in a wave
height of crest or depth of trough
Amplitude is related to the E of a wave
5
WAVE PROPERTIES
Wavelength (?)
distance between 2 successive crests
Units m or cm
Frequency (f)
of waves produced per unit time.
Units 1/s Hz
Period (T)
of seconds per unit wave.
6
WAVE SPEED

• The speed of a wave can be calculated from its
  frequency (f) and wavelength (?).
• f ? s
• Units

Note, s constant for a given wave
7
Example 1

• A sound wave has a speed of 344 m/s and a
  wave-length of 0.5 m. What is the frequency of
  this wave?

f ??? ? 0.5 m s 344 m/s
f 688 Hz
8
Example 2

• A sound wave has a speed of 344 m/s has a
  frequency of 20 kHz. What is this waves
  wavelength?

f 20 kHz ? ??? s 344 m/s
? 0.017 m
9
Example 3

• An ocean wave passes a point on the pier at the
  rate of 12 waves per minute. What are the
  frequency and period of this wave?

of cycles 12 time 60 s f ??? T ???
f 0.2 Hz
T 5 s
10
LONGITUDINALWAVES

• Waves where direction of particle movement is
  parallel to the direction of wave velocity, are
  longitudinal waves

• Longitudinal waves have areas of high particle
  density (compressions) and of low particle
  density (rarefactions)

• Sound is the most common example of a
  longitudinal wave.

11
TRANSVERSEWAVES

• Waves where the direction of particle movement is
  perpendicular to the direction of wave velocity,
  are transverse waves

• i.e. mechanical waves, electromagnetic waves
  (light, microwave, x-rays, etc.)

12
ELECTROMAGNETICWAVES

• Electromagnetic waves (em waves) are transverse
  waves caused by vibrating electrons (e-)
• Formed through interaction of electric and
  magnetic fields that are perpendicular to one
  another
• In vacuum, ? em waves travel at the same speed
  and differ from each other only in freq.

13
ELECTROMAGNETICSPECTRUM

• The classification of electromagnetic waves
  according to their frequency is called
  electro-magnetic spectrum.
• em waves range from ? rays (short ? , high f) to
  radio (long ?, low f)

14
SOUND WAVES

• Sound is the most common example of a
  longitudinal wave
• In sound waves, the molecules of air transmit the
  vibrations through
  compressions and
  rarefactions

• Sounds waves cannot travel
  in vacuum!

15
SPEED OF SOUND

• Speed of sound is affected by
• wind conditions, temperature, humidity and type
  of medium
• NOT affected by its loundness (amplitude)
• Sound travels faster in liquids and solids
  compared to gases
• Closely packed molecules in solids and liquids
  will transmit the vibrations faster than in gases

• Sound travels faster at higher T
• Faster moving air particles bump into each other
  more frequently and carry vibrations in shorter
  time

16
DOPPLER EFFECT

• Pitch of a sound is the subjective measure of its
  frequency
• High frequency sounds have high pitch, and low
  frequency sounds have low pitch

• The apparent shift in pitch of sound when the
  source is moving relative to the observer is
  called the Doppler Effect

• i.e. The sudden change in pitch of an ambulance
  siren as it goes by an observer is the result of
  the Doppler effect

17
STANDING WAVES

• Waves that are reflected on themselves and appear
  to stand are called standing waves
• Standing waves produce the variety of sounds in
  musical instruments! (Animation) violin

18
RESONANCE

• Every object has natural vibrations caused by
  motion of its molecules
• When a forced vibration matches an objects
  natural vibrations, a dramatic increase in
  amplitude occurs ? This phenomena is called
  Resonance
• Some Examples
• Tacoma Narrows Bridge
• Resonant Bridge

19

• THE END

20
Some Symbols Used

• ? all
• ? there is or there exists
• D Change
• ? infinity
• ? multiplication
• ? proportional to
• ? less than or equal to
• ? greater than or equal to
• ? plus or minus
• ? degrees
• ? therefore
• approximately equal to
• ? decreases
• ? increases

• ? Wavelength
• E Energy
• H Heat
• - Number
• e- - electron
• p - proton