the wave model sinusoidal waves sound and light waves energy & intensity

Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley.

• The wave model

• Sinusoidal waves

• Sound and light waves

• Energy & Intensity

• The Doppler effect and shock waves

Chapter 15Traveling Waves and Sound

Topics:

Sample question:

The large ears of this bat give us a clue to how it locates its prey: with sound. How do bats use sound to “see” the world around them?

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Reading Quiz

1. Which of the following is a longitudinal wave?

A. sound wave

B. water wave

C. light wave

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1. Which of the following is a longitudinal wave?

A. sound wave

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Answer


Reading Quiz

2. When the particles of a medium move with simple harmonic motion, this means the wave is

A. a sound wave

B. a sinusoidal wave

C. a standing wave

D. a harmonic wave

E. a transverse wave

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2. When the particles of a medium move with simple harmonic motion, this means the wave is

B. a sinusoidal wave

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Answer


Reading Quiz

3. We measure the sound intensity level in units of

A. watts

B. joules

C. candelas

D. decibels

E. hertz

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3. We measure the sound intensity level in units of

D. decibels

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Answer


Types of Waves

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Copyright © 2007, Pearson Education, Inc., Publishing as Pearson Addison-Wesley. Slide 15-3

Wave demonstrations

Transverse Waves

Longitudinal Waves

1. How does a piece of the spring move?

2. Which way does it oscillate?


What factors determine wave speed?

Wave speed is determined by the medium

• Related to a = Fnet / m

• Does not depend on how hard or how fast the wave generator moves


Waves on Strings and in Air

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Snapshot and History Graphs

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Checking Understanding

The graph below shows a snapshot graph of a wave on a string that is moving to the right. A point on the string is noted. Which of the choices is the history graph for the subsequent motion of this point?

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The graph below shows a snapshot graph of a wave on a string that is moving to the right. A point on the string is noted. Which of the choices is the history graph for the subsequent motion of this point?

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Answer


The graph below shows a history graph of the motion of one point on a string as a wave moves by to the right. Which of the choices below shows a snapshot graph of the motion of the string?

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The graph below shows a history graph of the motion of one point on a string as a wave moves by to the right. Which of the choices below shows a snapshot graph of the motion of the string?

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Answer


A wave travels back and forth on a guitar string; this is responsible for making the sound of the guitar, as we will see. As the temperature rises, the tension in a guitar string decreases. How does this change the speed of a wave on the string?

It is possible to use sound to probe the air temperature of a region of the atmosphere. A source emits a pulse of sound which is then measured by a distant microphone. A measurement of the time between the emission and the reception of the pulse allows a determination of the temperature of the intervening air. Explain how this technique works.

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Examples


A particular species of spider spins a web with silk threads of density 1300 kg/m3 and diameter 3.0 µm. A typical tension in the radial threads of such a web is 7.0 mN. If a fly lands in this web, which will reach the spider first, the sound or the wave on the web silk?

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Example


Sinusoidal Waves

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Sound and Light Waves

The speed of sound varies with the medium.

Light and other electromagnetic waves in vacuum and in air move at the same speed, 3.00 x 108 m/s.

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For this sinusoidal wave:

1. What is the amplitude?

A. 0.5 m

B. 1 m

C. 2 m

D. 4 m

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1. What is the amplitude?

A. 0.5 m

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Answer




2. What is the wavelength?

A. 0.5 m

B. 1 m

C. 2 m

D. 4 m

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2. What is the wavelength?

C. 2 m

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Answer




3. What is the frequency?

A. 50 Hz

B. 100 Hz

C. 200 Hz

D. 400 Hz

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3. What is the frequency?

B. 100 Hz

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The new generation of cordless phones use radio waves at a frequency of 5.8 GHz. What is the wavelength of these radio waves?

A speaker emits a tone of a particular frequency. Suppose the air temperature increases. What happens to the wavelength of the sound?

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Examples


Energy and Intensity

As waves spread out, so does the energy of the wave.

The intensity decreases.

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The Decibel Scale

Sound intensity level is measured in decibels.

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The Doppler Effect

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If you are standing 2.0 m from a 100 W lamp, what is the intensity of the radiation on your skin? How does this compare with the intensity of sunlight, approximately 1000 W/m2 at the surface of the earth?

Suppose it was so quiet outside that you could detect a sound at the threshold of your perception, 0 dB. Now suppose that someone was playing a stereo with the volume cranked up all the way. How far away could you detect the sound from the stereo?


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Additional Clicker Questions

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A snapshot and a history graph for a sinusoidal wave on a string appear as follows:

What is the speed of the wave?A. 1.5 m/sB. 3.0 m/sC. 5.0 m/sD. 15 m/s


A snapshot and a history graph for a sinusoidal wave on a string appear as follows:

What is the speed of the wave?

B. 3.0 m/s

Answer


Additional Examples

A 5.0 kg block is hung from the ceiling on a 2.0-meter-long metal wire with a mass of 4 g. The wire is “plucked” at the very bottom, where it connects to the block. How long does it take the pulse to reach the ceiling?

The intensity of sunlight is approximately 1000 W/m2 at the surface of the earth. Saturn is about 10 times as far from the sun as the earth. If the earth were moved to the distance of Saturn, what would be the intensity of sunlight at the surface?

Suppose you are powering a spacecraft with a 1.0 m2 array of solar cells with an efficiency of 12%. Above the earth’s atmosphere, where the intensity of sunlight is approximately 1300 W/m2, what is the maximum power you could get from the solar cells? How much power could you get if your spacecraft was nearing Neptune, 30 times as far from the sun as the earth?

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the wave model sinusoidal waves sound and light waves energy & intensity

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