Download Waves Part I - Alliance Gertz

Waves Part I
Phys 4a. Students know waves carry energy from one place to
another.
Phys 4b. Students know how to identify transverse and longitudinal
waves in mechanical media, such as springs and ropes, and on the
earth (seismic waves).
Phys 4d. Students know sound is a longitudinal wave whose speed
depends on the properties of the medium in which it propagates.
The Nature of Waves

What is a wave?
 A wave is a
repeating
disturbance or
movement that
transfers energy
through matter or
space from one
place to another.
Waves transfer energy, not matter. The water
waves below are carrying energy but are not
moving. Waves can only exist as they have
energy to carry.
Categories of Waves
Longitudinal vs. Transverse

One way to categorize waves is on the basis
of the direction of movement of the individual
particles of the medium relative to the
direction in which the waves travel.
Mechanical vs. Electromagnetic
Another way to categorize waves is on
the basis of their ability or inability to
transmit energy through a vacuum (i.e.,
empty space).
 Electromagnetic waves can travel
through a vacuum (empty space)
 Mechanical waves cannot travel through
empty space

Mechanical Waves

Mechanical waves are waves which require a
medium.
 Medium - A form of matter through which the
wave travels (such as water, air, glass, etc.).
 Mechanical waves are not capable of
transmitting energy through a vacuum.
Mechanical Waves

Waves such as light, x-rays, and other forms
of radiation do not require a medium.
 Sound waves are incapable of traveling
through a vacuum
 Examples of mechanical waves: slinky
waves, water waves, and sound waves
Electromagnetic Waves

All light waves are examples of
electromagnetic waves
 An electromagnetic wave is a wave which is
capable of transmitting its energy through a
vacuum (i.e., empty space). Electromagnetic
waves are produced by the vibration of
charged particles.
 Electromagnetic waves, which are produced
from the sun, subsequently travel to Earth
through the vacuum of outer space. Were it
not for the ability of electromagnetic waves to
travel to through a vacuum, there would
undoubtedly be no life on Earth.
Check your understanding
A sound wave is a mechanical wave; not an
electromagnetic wave. This means that
a) particles of the medium move perpendicular to
the direction of energy transport.
b) a sound wave transports its energy through a
vacuum.
c) particles of the medium regularly and repeatedly
oscillate about their rest position.
d) a medium is required in order for sound waves
to transport energy
Transverse Waves
Transverse Waves

Examples: light, ocean, all electromagnetic
waves, and seismic waves
 In transverse waves, the molecules of the
medium oscillate perpendicular to the
direction of propagation
Electromagnetic Spectrum
(Transverse Wave)
Electromagnetic Waves

IN A VACUUM - all electromagnetic waves
move at a speed of 3.0 x 108 meters/sec
 Electromagnetic waves exist with an
enormous range of frequencies. This
continuous range of frequencies is known as
the electromagnetic spectrum
Seismic Waves

What are seismic waves?
– An energy wave which vibrates through the
earth’s crust as the crust bends or breaks.
Seismic waves exist as both transverse
and longitudinal waves. Some travel
through the earth and some travel across
the earth’s surface.
S-Waves (Secondary)

Are transverse waves, or shear waves.
 The ground is displaced perpendicularly to the
direction of propagation.
 S waves can travel only through solids, as fluids
(liquids and gases) do not support shear stresses.
 Their speed is about 60% of that of P waves in a
given material. S waves are several times larger in
amplitude than P waves for earthquake sources.
P-Waves (Primary)





Are longitudinal waves, or compressional waves.
The ground is alternately compressed and dilated in
the direction of propagation.
In solids, these waves generally travel almost twice as
fast as S waves and can travel through any type of
material.
In air, these pressure waves take the form of sound
waves, hence they travel at the speed of sound.
When generated by an earthquake they are less
destructive than the S waves and surface waves that
follow them, due to their bigger amplitudes.
Anatomy of a Seismic Wave
Properties of Waves
Wave
Rest position
Wave

The amplitude is the height of the wave.

The wavelength is the distance from one wave
top, or crest, to the next.

The Crest is the highest point of the wave

The through is the lowest point of a wave

The rest position of a wave is called a node or
nodal line
Frequency of Waves

Frequency is how many waves/second,
in units of 1/s or Hertz (Hz)
Frequency vs. Wavelength
Period vs. Frequency

Frequency is the amount of cycles or waves
per second, measured in Hertz
 A period is how much time it takes for 1 cycle
or wave to pass
 Period and frequency have an inverse
relationship to one another
Check your understanding

Complete the sentence:
– The larger the frequency the _______ the
wavelength.
– Frieda the fly flaps its wings back and forth
121 times each second. The period of the
wing flapping is ____ sec.
– A period of 5.0 seconds corresponds to a
frequency of ________ Hertz.
– As the frequency of a wave increases, the
period of the wave ___________.
Amplitude and Energy of a Wave
Amplitude and Energy

a wave is an energy transport phenomenon which
transports energy along a medium without transporting
matter.
 The amount of energy carried by a wave is related to the
amplitude of the wave.
 The energy transported by a wave is directly proportional
to the square of the amplitude of the wave. This energyamplitude relationship is sometimes expressed in the
following manner.
Wave Speed
Speed = wavelength x frequency
v=fλ
 Units are in meters/second
 Waves carry energy and NOT matter

Check your understanding
What’s the formula for wave speed?
 The sound from a 60 Hz electric razor
spreads out at 340 meters per second,
what is its frequency? Period? Speed?
wavelength?

Mediu
m
Gold
Wavelength Frequency
0.6m
4.2 Hz
Zinc
1.75
2.0 Hz
Nickel
0.95
2.2 Hz
Speed
Longitudinal Waves
Longitudinal Waves

Examples: sound, seismic P-waves
 Longitudinal waves are waves that have
vibrations along or parallel to their direction of
travel; that is, waves in which the motion of
the medium is in the same direction as the
motion of the wave.
 The speed of a longitudinal wave depends
upon the medium through which it travels.
Compressional Longitudinal Wave



On a compressional
wave the area
squeezed together is
called the
compression.
The areas spread out
are called the
rarefaction.
The wavelength is the
distance from the
center of one
compression to the
center of the next
compression.
Sound
Sound is a longitudinal wave
 Sound is a mechanical wave
 Sound is a pressure wave

Sound Waves

Sound waves in air (and any fluid medium)
are longitudinal waves because particles of
the medium through which the sound is
transported vibrate parallel to the direction in
which the sound wave moves
 Sound waves are mechanical waves
because it needs a medium to travel through,
cannot travel through empty space
 Since a sound wave consists of a repeating
pattern of high pressure and low pressure
regions moving through a medium, it is
sometimes referred to as a pressure wave
Check your understanding

A sound wave is different than a light wave in
that a sound wave is
a. produced by an oscillating object and a
light wave is not.
b. not capable of traveling through a vacuum.
c. not capable of diffracting and a light wave
is.
d. capable of existing with a variety of
frequencies and a light wave has a single
frequency.
Review
Waves all carry
energy
Transverse Waves
Longitudinal Waves
-Light, EM waves, Swaves, ocean waves
-Sound, P-waves
-Can travel through
empty spaces
-Matter and energy
travel in perpendicular
direction
-Cannot travel through
empty space
-Matter and energy travel
in parallel direction