Prentice Hall Physical Science
Waves – CH 17 and 18
17.1 Mechanical Waves
A.
What are Mechanical Waves?
a mechanical waves is a disturbance that carries energy from one place to another
mechanical waves require a medium which is a material that the wave moves through
mechanical waves are created when a source of energy causes a vibration to travel through a
medium
most do not transport matter from one place to another
B.
Types of Mechanical Waves
1.
Transverse Waves – causes the medium vibrate at right angles to the direction the wave is
traveling
crest is the highest point above the resting position
trough is the lowest point below the resting position
it looks like a typical wave (fig 2, p. 501)
2.
Longitudinal Waves – causes the medium to vibrate parallel to the direction the wave is
traveling
compression is the area where the particles of the medium are close together
rarefaction is the area where the particles of the medium are spread out
fig 3, p. 502
3.
Surface Waves – a wave that travels along the surface separating two media
it moves almost a combination of transverse and longitudinal waves and the result is
motion in a circle (fig. 4, p. 503)
when ocean waves approach the shore they act differently because of a difference in
friction on the top and bottom of the wave
17.2 Properties of Mechanical Waves
A.
Frequency and Period
a period is the time it takes for a wave to complete one cycle (crest to crest or trough to trough)
frequency is the number of complete wave cycles that pass a certain point in a given amount of
time
it is measured in cycles per second or hertz (Hz)
figure 5 p. 504
one cycle per second = 1 Hz; six cycles per second = 6 Hz
B.
Wavelength
it is the distance between a point on one wave and the same point on the next cycle of the wave
crest to the next crest or trough to the next trough
increasing frequency decreases wavelength
C.
Wave Speed
wave speed = wavelength * frequency
wavelength is measured in meters and frequency in hertz so wave speed is m/s
EX 1: The waves in a pool have a wavelength of 0.20 m and a frequency of 2.8 Hz.
What is the speed of these waves?
K:
λ = 0.20 m
f = 2.8 Hz
unk: s = ?
s=λ*f
s = (0.20 m)(2.8 Hz)
s = 0.56 m/s
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D.
EX 2:
A student moves the end of a soft spring back and forth to make waves. The
waves travel at 1.8 m/s and have a wavelength 1.2 m. What is the frequency of the wave?
K: s = 1.8 m/s
λ = 1.2 m
unk:
f=?
f = s/λ
f = (1.8 m/s)/1.2 m
f = 1.5 Hz
Amplitude
amplitude is the maximum displacement of the medium from rest (usually the distance from
rest to the top of the crest or the bottom of the trough)
the more energy the wave has, the higher its amplitude
in a longitudinal wave, the more the medium is compressed or spread out, the more energy it
has
17.4 Sound and Hearing
A.
Properties of Sound Waves
1.
Speed
sound travels faster in solids, slower in liquids, and slowest in gases because the closer
together the particles are, the faster the sound can travel
it also depends on the density of the material and how elastic the medium is
2.
Intensity and Loudness
intensity is the rate at which energy flows through a given area (essentially how much
energy a wave has)
the more intense a wave is, the louder the sound is
the more intense the wave is, the higher its amplitude
intensity is measured in decibels (dB)
fig. 15, p. 515
3.
Frequency and Pitch
the frequency of a sound depends on how fast the source of the sound is vibrating; the
faster the source vibration, the higher the frequency of the sound
pitch is how you hear frequency; high frequency = high pitch and low frequency = low
pitch
B.
The Doppler Effect
it is the change in sound frequency because the source of the sound is moving
as the sound’s source moves toward you, you hear a higher pitch because the source is
“catching up to” the sound and the sound waves get shoved together
as the sound’s source moves away from you, you hear a lower pitch because the source is
moving away from the sound and spreading the waves out
18.1 Electromagnetic Waves
A.
What are Electromagnetic Waves?
electromagnetic waves are transverse waves formed from a vibrating electric field and a
vibrating magnetic field at right angles to each other (fig. 2, p. 533)
they are produced when an electric charge vibrates or accelerates
electromagnetic waves do NOT need a medium to travel; they can travel through a vacuum
(empty space)
the transfer of energy by electromagnetic waves is called electromagnetic radiation
B.
The Speed of Electromagnetic Waves
all electromagnetic waves travel at the same speed in a vacuum, regardless of the observer’s
motion
the speed of light in a vacuum is 3.00*108 m/s
C.
D.
Wavelength and Frequency
the wavelength and frequency of electromagnetic waves varies
because the speed of electromagnetic waves is constant, when wavelength increases, frequency
decreases and when wavelength decreases, frequency increases (inversely proportional)
EX 1: What is the frequency of an electromagnetic wave with a wavelength of 2.0 m?
K:
s = 3.0 *108 m/s
λ = 2.0 m
unk: f = ?
f = s/λ
f = (3.0 * 108 m/s)(2.0 m)
f = 1.5 * 108 Hz
Intensity
the brighter a light is, the more intense it is and the more energy its wave is carrying
18.2 The Electromagnetic Spectrum
different frequencies of electromagnetic radiation make different types of waves
the whole electromagnetic spectrum is made of all of these types of waves; there are six types
A.
Radio Waves
longest wavelength, lowest frequencies
radio and TV use radio waves
microwaves are the shortest wavelength radio waves
RADAR stands for RAdio Detection And Ranging – it send a radio wave out which bounces off of
an object and back to the detector which gives information about the object like its speed
B.
Infrared Rays
higher frequencies and shorter wavelengths than radio waves
used as heat sources and to discover areas of heat differences
heat (infrared) lamps are used to keep food warm
C.
Visible Light
all the light you can see: ROYGBIV
red has the longest wavelength (750 nm) and violet has the shortest (400 nm)
D.
Ultraviolet
Higher frequency and shorter wavelength than violet light
used by your body to make vitamin D
can damage skin
can be used to kill microorganisms
E.
X-rays
higher frequency and shorter wavelength than ultraviolet
they can penetrate matter that light cannot
used to take pictures of the inside of things
F.
Gamma Rays
shortest wavelength and highest frequency giving them the most energy
overexposure can be deadly
used to treat cancer and inspect tools