Physics Chap 16 - Notes on waves (chapter 16) PDF

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Notes on waves (chapter 16)...

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Lauren Cox, Physics 209

Bonus Assignment

11/8/18

Chapter 16: Waves and Sounds 16.1 The Nature of Waves !

- Two features of all waves ! 1. A wave is a traveling disturbance ! 2. A wave carries energy from place to place !

- For a wave, such as water, there is no bulk movement of water but a disturbance on the surface of the water !

- Two basic types of waves: transverse and longitudinal ! • Transverse wave: one in which the disturbance occurs perpendicular to the direction of travel of the wave!

‣ Examples: radio waves, light waves, & microwaves ! • Longitudinal wave: one in which the disturbance occurs parallel to the line of travel of the wave!

‣ Example: sound waves! • Water is neither a transverse wave or a longitudinal wave but has components from both !

16.2 Periodic Waves !

- Periodic waves: waves that consist of cycles or patterns that are produced over and over again by the source!

• Longitudinal and transverse waves are periodic waves! - A wave is a series of many cycles ! - Parts of the wave! • Amplitude (A): the maximum excursion of a particle of the medium in which the wave exists from the particle's undisturbed position!

‣ distance between a crest (highest point on the wave pattern) and the undisturbed position!

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Lauren Cox, Physics 209

Bonus Assignment

11/8/18

‣ distance between a trough (lowest point on the wave pattern) and the undisturbed position!

• Wavelength (λ): the horizontal length of one cycle of the wave! ‣ horizontal distance between two successive crests, two successive troughs, or any two successive equivalent points on the wave!

• Period&T: time required for one complete up/down cycle, just as it is for an object vibrating on a spring!

‣ Frequency (f) is related to period T (often measured in hertz)! f

=

1 ! T

- A simple relation exists between the period, the wavelength, and the speed of any periodic wave!

v =

λ = f λ! T

Example: !AM and FM radio waves are transverse waves consisting of electric and magnetic disturbances traveling at a speed of&3 x 108 m/s. A station broadcasts an AM radio wave whose frequency is&1230 x 103 Hz (1230 kHz on the dial) and an FM radio wave whose frequency is&91.9 x 106 Hz (91.9 MHz on the dial). Find the distance between adjacent crests in each wave.

16.5 The Nature of Sound !

- Sound is a longitudinal wave that is created by a vibrating object! • Sound can be created or transmitted only in a medium (a gas, liquid, or solid)! ‣ The particles of the medium must be present for the disturbance of the wave to move from place to place!

‣ Sound cannot exist in a vacuum - To see how sound waves are produced and why they are longitudinal, consider the vibrating diaphragm of a loudspeaker:!

• When the diaphragm moves outward, it compresses the air directly in front of it; This compression causes the air pressure to rise slightly!

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Lauren Cox, Physics 209

Bonus Assignment

11/8/18

‣ Region of increased pressure is called a&condensation,&and it travels away from the speaker at the speed of sound!

• After producing a condensation, the diaphragm reverses its motion and moves inward!

‣ The inward motion produces a region known as a&rarefaction,&where the air pressure is slightly less than normal!

- When the condensations and rarefactions arrive at the ear, they force the eardrum to vibrate at the same frequency as the speaker diaphragm!

• The vibratory motion of the eardrum is interpreted by the brain as sound! - Each cycle of a sound wave includes one condensation and one rarefaction, and the&frequency&is the number of cycles per second that passes by a given location!

• A sound with a single frequency is called a&pure tone • The brain interprets the frequency detected by the ear primarily in terms of the subjective quality called&pitch

- Experiments have shown that a healthy young person hears all sound frequencies from approximately 20 to 20,000 Hz (20 kHz)!

• Sound waves with frequencies below 20 Hz are said to be&infrasonic • Sound waves with frequencies above 20 kHz are referred to as&ultrasonic - Pressure amplitude&of the wave: the magnitude of the maximum change in pressure, measured relative to the undisturbed or atmospheric pressure!

• &The pressure fluctuations in a sound wave are normally very small! - Loudness&is an attribute of sound that depends primarily on the amplitude of the wave: the larger the amplitude, the louder the sound!

16.7 Sound Intensity !

- The amount of energy transported per second by a sound wave is called the&power&of the wave and is measured in SI units of joules per second&(J/s) or watts (W)!

- The&sound intensity&(I) is defined as the sound power&that passes perpendicularly through a surface divided by the area&of that surface:! Page 3 

Lauren Cox, Physics 209

I =

Bonus Assignment

11/8/18

P ! A

• The unit of sound intensity is power per unit area (W/m2)! - the smallest sound intensity that the human ear can detect is about 1 x 10-12 W/m2; this intensity is called the&threshold of hearing

- Spherically uniform radiation:! p I = ! 4π r 2 • From this we see that the intensity of a source that radiates sound uniformly in all directions varies as&1/r2 !

‣ Example: if the distance increases by a factor of two, the sound intensity decreases by a factor of&22= 4! Example: 12 x 10-5 W of sound power passes perpendicularly through the surfaces labeled 1 and 2. These surfaces have areas of&A1 = 4.0 m2 and!A2 = 12 m2. Determine the sound intensity at each surface and discuss why listener 2 hears a quieter sound than listener 1.

16.8 Decibels !

- The&decibel&(dB) is a measurement unit used when comparing two sound intensities! - Intensity level β (expressed in decibels) is defined as follows:!  = (10d B)log( β

I )! Io

• An intensity level of zero decibels does NOT mean that the sound intensity I is zero; it means that I = Io!

- Experiment shows that if the intensity level increases by&10 dB,&the new sound seems approximately twice as loud as the original sound!

16.10 Applications of Sound in Medicine !

- When ultrasonic waves are used in medicine for diagnostic purposes, highfrequency sound pulses are produced by a transmitter and directed into the body!

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Lauren Cox, Physics 209

Bonus Assignment

11/8/18

• As in sonar, reflections occur! - When ultrasound is used to form images of internal anatomical features or foreign objects in the body, the wavelength of the sound wave must be about the same size as, or smaller than, the object to be located!

- Neurosurgeons use a device called a&cavitron"ultrasonic"surgical"aspirator (CUSA) to remove brain tumors once thought to be inoperable!

• Ultrasonic sound waves cause the slender tip of the CUSA probe to vibrate at approximately 23 kHz.&!

- Another application of ultrasound is in a new type of bloodless surgery, which can eliminate abnormal cells, such as those in benign hyperplasia of the prostate gland!

• This technique is known as HIFU (high-intensity&focused&ultrasound).&! - The Doppler flow meter is a particularly interesting medical application of the Doppler effect!

• This device measures the speed of blood flow, using transmitting and receiving elements that are placed directly on the skin !

• The transmitter emits a continuous sound whose frequency is typically about 5 MHz.&

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