12/16/12  

1  

Chapter 15: Traveling Waves and Sound ¨  Key Terms:

¤ Wave model ¤ Mechanical waves ¤  Electromagnetic waves ¤  Transverse ¤  Longitudinal ¤  Sinusoidal ¤ Amplitude ¤ Wavelength ¤ Wave fronts ¤ Doppler Effect

Types of Waves

¨  Mechanical: waves that involve the motion of a substance through which they move (medium) ¤  Ex: water waves, wave on a string, sound

¨  Electromagnetic: waves of an electromagnetic field; require no medium ¤  Ex: light, radio waves, microwaves, x-rays

¨  Matter: small particles also act like waves (wait for chapter 28!)

Shapes of Waves

¨  Transverse – particles move perpendicular to wave direction

¨  Longitudinal – particles move parallel to wave direction

12/16/12  

2  

Sinusoidal Waves

¨  The type of wave produced by some source that oscillates with simple harmonic motion. ¤ Ex: FM radio waves are produced by electrons

oscillating back and forth in SHM in an antenna

¨  Amplitude ¨  Wavelength

Wave Speed

¨  The speed at which waves move depends on the medium.

¨  If a wave travels in a string, the linear density of the string and the tension of the string determine wave speed.

Sample Problem #1 (ex. 15.1, page 481)

¨  All spiders are very sensitive to vibrations. An orb spider will sit at the center of its large, circular web and monitor radial threads for vibrations when an insect lands. Assume that these threads are made of silk with a linear density of 1.0 x 10-5 kg/m under a tension of 0.40 N. If an insect lands 30 cm from the spider, how long will it take for the spider to find out?

12/16/12  

3  

Speed of Sound

¨  The speed of a sound wave depends on the medium it is traveling through and the temperature of the medium.

Speed of Light

¨  The speed of all electromagnetic waves in a vacuum is…

c = 3.00 x 108 m/s

¨  This speed will change in other mediums, although it is essentially the same in air.

Sample Problem #2 (ex. 15.3, page 482)

¨  During a thunderstorm, you see a flash from a lightning strike. 8.0 seconds later, you hear a crack of thunder. How far away did the lighting strike?

12/16/12  

4  

Sample Problem #3 (ex. 15.4, page 486)

¨  A boat is moving in the ocean. The waves have 2.0 meters between the top of the crests and the bottom of the troughs. The period of the waves is 8.3 s and their wavelength is 110 m. ¤ Sketch the wave and label all known quantities. ¤ Determine the speed of the ocean wave.

Sample Problem #4 (ex. 15.6, page 488)

¨  Humans can hear sounds in the range of 20 Hz to 20 kHz. What are the wavelengths of sound waves at the limits of human hearing and at the midrange of frequency 500 Hz? ¤ Notes sung by humans and the keys near the center of

a piano keyboard are close to 500 Hz.

Sample Problem #5 (ex. 15.7, page 488)

¨  Ultrasound waves are waves with a frequency well above our range of hearing. Ultrasound waves can travel through human tissue at 1540 m/s, and reflections of their small wavelengths can be used to create an image of the body’s interior. To make a sufficiently detailed ultrasound image of a baby in its mother’s uterus, a physician has decided that a wavelength of 0.50 mm is needed. What frequency is required?

12/16/12  

5  

Sample Problem #6 (ex. 15.8, page 489)

¨  The wavelength of microwaves in a microwave oven is 12 cm. What is the frequency of the waves?

Wave Fronts

¨  Lines that locate the crests of a wave.

Power, Intensity, Energy

¨  Energy is transferred as waves travel.

¨  Power is the rate of energy transferred.

¨  Intensity is the ratio of power to area affected by a wave. ¤ Waves focused on a smaller area have a greater

intensity.

12/16/12  

6  

Loudness

¨  Loudness is related to the intensity of a sound wave.

¨  The decibel scale is used to describe the loudness of sounds. ¤ An increase of 10 dB means the sound is ten times

louder. ¤ An increase of 20 dB means the sound is 100 times

louder.

Loudness

Sound Decibels Hearing

threshold 0

Rustle of leaves 10 Conversation 60 Rock Concert 110

Pain Threshold 120 Jet Engine 130

12/16/12  

7  

The Doppler Effect

¨  The change in apparent frequency of a wave due to relative motion between the source of the wave and an observer.

Sample Problem #7 (ex. 15.13, page 496)

¨  You observe the siren of a police car to produce a sound with frequency of 550 Hz, and then later a sound of 450 Hz. Describe the possible motions of the police car and yourself.

12/16/12  

8  

Red Shift and Blue Shift

¨  The frequency of light waves can also shift due to relative motion between source and observer.

¨  This is seen in the red shift of light emitted from distant galaxies.