Waves and Energy

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Waves and Energy
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Waves are everywhere in nature

• Sound waves,
• visible light waves,
• radio waves,
• microwaves,
• water waves,
• sine waves,

• telephone chord waves,
• stadium waves,
• earthquake waves,
• waves on a string,
• slinky waves

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WAVES

• A disturbance that transfers energy from place to
  place
• Waves are created when an energy source causes a
  medium to vibrate.
• Medium is a substance through which a wave can
  travel. A medium can be a solid, liquid or gas.

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Energy
Recall The ability to do work or cause change
2 General Kinds
Potential
Energy due to position stored energy
Kinetic
Energy due to motion
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Major Forms of Energy
Thermal
Mechanical
Chemical
Electromagnetic
Nuclear
Electrical
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Slinky Wave

• Lets use a slinky wave as an example.
• When the slinky is stretched from end to end and
  is held at rest, it assumes a natural position
  known as the EQUILIBRIUM or REST POSITION
• To introduce a wave here we must first create a
  disturbance.
• We must move a particle away from its rest
  position.

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Slinky Wave

• One way to do this is to jerk the slinky forward
• the beginning of the slinky moves away from its
  equilibrium position and then back.
• the disturbance continues down the slinky.
• this disturbance that moves down the slinky is
  called a PULSE.
• if we keep pulsing the slinky back and forth,
  we could get a repeating disturbance.

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Slinky Wave

• This disturbance would look something like this
• This type of wave is called a LONGITUDINAL wave.
• The pulse is transferred through the medium of
  the slinky, but the slinky itself does not
  actually move.
• It just displaces from its rest position and then
  returns to it.
• So what really is being transferred?

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Slinky Wave

• Energy is being transferred.
• The metal of the slinky is the MEDIUM in that
  transfers the energy pulse of the wave.
• The medium ends up in the same place as it
  started it just gets disturbed and then returns
  to it rest position.
• The same can be seen with a stadium wave.

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TRANSVERSE WAVE

• A second type of wave is a transverse wave.
• We said in a longitudinal wave the pulse travels
  in a direction parallel to the disturbance.
• In a transverse wave the pulse travels
  perpendicular to the disturbance.

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Transverse Waves

• The differences between the two can be seen in
  the diagram below.

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LONGITUDINAL WAVE
LONGITUDINAL WAVES

• The wave we see here is a longitudinal wave.
• The medium particles vibrate parallel to the
  motion of the pulse.
• This is the same type of wave that we use to
  transfer sound.

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Longitudinal Waves Continued

• Compression A section of a longitudinal wave
  where the particles are crowded together.
• Rarefaction A section of a longitudinal wave
  where the particles are less crowded than normal.

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Longitudinal
Compression
Rarefaction
The shape of the slinky is a transverse
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Longitudinal Sound Waves
Molecules in the air vibrate about some average
position creating the compressions and
rarefactions. We call the frequency of sound the
pitch.
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Longitudinal or Transverse?
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COMBINATION WAVE

• When waves occur at or near the boundary between
  two media, a transverse wave and a longitudinal
  wave can combine to form a SURFACE WAVE.
• Surface waves look like transverse waves but the
  particles of the medium in a surface wave move in
  circles rather than up and down.

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Types of Waves
Transverse
Particles in a transverse wave move perpendicular
to the direction that the wave itself is
traveling. Ex ocean wave
Longitudinal
Particles move parallel to the direction that the
wave itself is traveling. Ex slinky
Combination
Combination of transverse and longitudinal that
occurs at the surface between 2 mediums. Ex a
wave in water
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Anatomy of a Wave

• Now we can begin to describe the anatomy of our
  waves.
• We will use a transverse wave to describe this
  since it is easier to see the pieces.

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Anatomy of a Wave

• In our wave here the dashed line represents the
  equilibrium position.
• Once the medium is disturbed, it moves away from
  this position and then returns to it

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Anatomy of a Wave
crest

• The points A and F are called the CRESTS of the
  wave.
• This is the point where the wave exhibits the
  maximum amount of positive or upwards displacement

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Anatomy of a Wave
trough

• The points D and I are called the TROUGHS of the
  wave.
• These are the points where the wave exhibits its
  maximum negative or downward displacement.

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Anatomy of a Wave
Amplitude

• The distance between the dashed line and point A
  is called the AMPLITUDE of the wave.
• This is the maximum displacement that the wave
  moves away from its equilibrium.

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Transverse
Crest
Amplitude
Trough
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Anatomy of a Wave
wavelength

• The distance between two consecutive similar
  points (in this case two crests) is called the
  WAVELENGTH.
• This is the length of the wave pulse.
• Between what other points can a wavelength be
  measured?

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Anatomy of a Wave

• WAVE FREQUENCY is the number of waves produced in
  a given amount of time.
• Suppose I wiggle a slinky back and forth, and
  count that 6 waves pass a point in 2 seconds.
  What would the frequency be?
• _______ cycles / second
• _______ Hz
• we use the term Hertz (Hz) to stand for cycles
  per second.

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Frequency
1 wavelength
1 wavelength
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Wave Speed
Distance the wave travels per unit of time. For
example 10cm/sec.

• v ? f OR v d / t
• v stands for velocity or speed
• (Greek symbol for lambda) to represent wavelength
• f stands for frequency
• d stands for distance
• t stands for time

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Speed
Which is traveling faster the top or bottom?
TOP
BOTTOM
1 second
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Wave Speed Problem

• Mac and Tosh are resting on top of the water near
  the end of the pool when Mac creates a surface
  wave. The wave travels the length of the pool and
  back in 25 seconds. The pool is 25 meters long.
  Determine the speed of the wave.
• If the pool is 25 meters long, then the
  back-and-forth distance is 50 meters. The wave
  covers this distance in 25 seconds. Now use v d
  / t.
• v d / t (50 m) / (25 s) ________ m/s

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• The water waves below are traveling with a speed
  of 2 m/s and splashing periodically against the
  Wilbert's perch. Each adjacent crest is 4 meters
  apart and splashes Wilberts feet upon reaching
  his perch. How much time passes between each
  successive drenching?

V d / t is rearranged to solve for time. So, t
d / V
Answer time _________ seconds
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Units
Distance Units mm, cm
Distance Units mm,cm
Amplitude
Wavelength
Wavelength per second Hertz (Hz) 35 Hz
Distance wave travels per unit time. 10 cm/s
Speed
Frequency
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Wave Interactions

• Reflection
• Refraction
• Diffraction
• Interference

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Reflection

• Wave Reflection is when a wave bounces back after
  striking a barrier.
• When sound waves reflect from a surface an echo
  is created.
• Wave reflection from surfaces depends on the
  characteristics of the surface
• Smooth hard surfaces reflect best
• Rough soft surfaces reflect poorly
• Energy not reflected is absorbed or transmitted
  through the material

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Reflection

• Think of arrows pointing in the direction of the
  wave motion
• We can trace the path of these arrows

Angles Equal
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Reflection
Acoustics of room design is very interesting.
Need some reflections to liven the room. Too
many reflections and the sound gets mushy. Look
in a concert hall or auditorium to see the
different sound treatments, such as carpet on
walls or foam on ceilings.
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Refraction

• Wave Refraction is the bending of a wave as it
  passes at an angle from one medium to another.
• Refraction occurs because different parts of the
  wave front travel at different speeds

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Diffraction

• The bending of waves around a barrier or through
  an opening is called diffraction.

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Interference

• Interference is when two or more waves overlap.
• Two Types
• Constructive Interference (Reinforcement)
• Destructive Interference (Cancellation)

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Constructive interference

• Reinforcement when the crest of one wave overlaps
  the crest of another
• Their individual effects adds together, resulting
  in a wave increased in amplitude

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Destructive Interference

• Cancellation happens when crest of one wave
  overlaps trough of another reducing their
  individual effects.

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THE END