LESSON 5: WAVE ENERGY50 5-1 Wave On

Transverse and Longitudinal Waves5-1 WAVEON

Content on Wave Energy There are two types of waves: transverse and longitudinal. An ocean wave is atransverse wave because the medium (the ocean) moves at right angles to thewave. Just like the string you tied to a doorknob, the ocean moves up and down,but the wave moves toward the shore. The crest of a transverse wave is the highpoint, and the trough is the lowest downward point (see Figure B).

What Wave?Crest Crest

Have you ever done the wave at a baseball game? That undulating motion pro-duced by spectators standing, one after another, in sequence reminds us of themotion of waves in nature. Waves are difficult to see because most of them areinvisible. But like the wave at a baseball game, the energy in waves moves fromone place to another.

If you drop a marble into a pool of water, you create a wave. The marble haskinetic energy because it is moving, and it transfers some of this energy to thewater particles, causing them to move. The particles of water transfer this ener-gy to adjacent particles, and the wave moves outward from the center.

Trough Trough

FIGURE B

A DisturbanceA wave is a disturbance that moves energy through matter or space. The mat-

ter or space through which a wave moves is described as the medium. Air is a medi-um for sound waves. Light waves do not need a medium because they can travelthrough a vacuum. As a wave travels through a medium, the medium does notmove along with the wave. Only the energy is transmitted. To see this, tie a stringto a doorknob, and move it up and down to form a wave (see Figure A). The stringis not moving toward the doorknob, but energy is moving along the string. The wavemoves from your hand to the doorknob. But the medium, a string in this case,moves up and down.

In longitudinal waves, the medium moves back and forth in the same direc-tion as the wave travels. Ask a friend to hold one end of a Slinky®. When you pushyour end of the Slinky® in and out, you are creating a longitudinal wave. Particlesof the medium are compressed or pushed together in areas called compressions.As a compression moves forward, it leaves behind a space containing fewer par-ticles, called a rarefaction. Sound waves travel in this manner.

Wave Characteristics

There are many types of waves: sound waves, light waves, radio and televisionwaves. All waves have some common characteristics.

Amplitude. This characteristic refers to the maximum distance molecules ofthe medium are displaced from their original position. If you push the Slinky® verygently, the waves you create will have a small amplitude. However, if you shove theSlinky® forward with a lot of force, you can create waves with large amplitudes.

Wavelength. All waves are measured by wavelengths. A wavelength is thedistance between two consecutive crests or between two successive identical waveparts. Wavelength is usually measured in meters or centimeters. However, wave-lengths of light are measured in millionths of a meter (micrometers).

Frequency. The number of waves that pass a point per unit oftime is calledfrequency. For example, if 1000 complete waves passed a point in one second, thewaves would have a frequency of 1000 cycles per second. Wave frequency is mea-sured in units called hertz (Hz). A frequency of 1 Hz is equal to one wave per sec-ond. Higher frequencies are measured in kilohertz (kHz), and extremely high

FIGURE A

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Sounds are produced by vibrating objects that cause the surrounding medium,air, to vibrate. The speed of sound in air is about 340 meters per second. You candetermine how far you are from a bolt of lightning by counting the number of sec-onds between the time you see the lightning and the time you hear the thunder.Multiply the number of seconds between the lightning and the sound of thunderby 340 meters, and you know how far you are from the storm. Thunder is a com-pression of air molecules caused by the heat of the lightning bolt. Light travelsabout a million times faster than sound, so the light from the lightning bolt reach-es your eyes almost instantly.

Sound has two important components.Intensity. The amount of energy in a sound wave is referred to as that

wave's intensity. Intensity determines the loudness of a sound, and is measuredin units called decibels. The decibel scale begins at zero, the point at which wecan just hear. The sound of a rocket engine is deafening and is 200 on the decibelscale. All sounds with intensities near 120 decibels can cause pain to humans.Sounds above 85 decibels can damage hearing. Loud music, especially playedthrough earphones, damages many people's hearing.

Pitch. Sound waves also have pitch, which is determined by how fast themolecules of the medium are vibrating. The frequency of a sound wave deter-mines the pitch of a sound. High-frequency sound waves have high pitches. Ahigh note by a female singer may be about 1000 Hz, and a male bass may sing at70 Hz. Humans can hear sounds between 20 Hz and 20,000 Hz. Ultrasonicsounds are those waves whose frequencies are above 20,000 Hz. Dogs can hearpitches up to 25,000 Hz, and porpoises can hear them up to 150,000 Hz. Somebats produce their own ultrasonic frequencies, which they use for navigation.

The Doppler Effect

Have you ever listened to the sound of a siren approaching you? The pitch of thesiren gets higher as it approaches you due to the Doppler effect. When there ismotion between the observer and the source of sound, the frequency of soundwaves changes. As the sound approaches the observer, the waves are compressedcloser together, and waves reach the observer sooner than they would have if thesource of sound had not been moving. Therefore, sound waves that reach theobserver seem to have a higher pitch because their frequency has increased. Asthe source of sound moves away from the observer, the sound waves are fartherapart, and the pitch drops.

JV~72. Complete these sentences with the correct words.

A.

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B.

c.D.

a. The highest point on a transverse wave is the ,

b. The lowest point on a transverse wave is the .

c. The squared is proportional to the amount-of energy thewave carries.

d. The distance from one crest to the next crest is the '

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e. _____________ is a measure of the number of waves that pass a pointin a given amount of time.

Part B; Use the five illustrations of transverse waves below to answer the following questions.

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1. Waves V and W have the same . but wave V has twice the____________ of wave W.

2. Waves Wand X have the same • but wave X has twice the.s.: of wave W.

3. Wave shows a steady frequency but a changing amplitude.

Wave shows a steady amplitude but a changing frequency .

o , Waves and ' have a low amplitude and a steady' frequency,