1 Chapter 6 WAVES Dr. Babar Ali

2 CHAPTER OUTLINE  Wave Concept Wave Concept  Wave Properties Wave Properties  Wave Speed Wave Speed  Wave Types Wave Types  Electromagnetic Waves Electromagnetic Waves  Sound Waves Sound Waves  Speed of Sound Speed of Sound  Doppler Effect Doppler Effect  Standing Waves Standing Waves  Resonance Resonance

3 CONCEPT OF WAVES  A wave is a disturbance that propagates through a system transferring energy  Waves transfer E! (courtesy of Dr. Dan Russell, Kettering Univ.)

4 WAVE PROPERTIES Crest:the highest point on a wave Trough:the lowest point on a wave Amplitude:maximum displacement in a wave height of crest or depth of trough Amplitude is related to the E of a wave

5 WAVE PROPERTIES Wavelength (λ):distance between 2 successive crests Frequency (f):# of waves produced per unit time. Units = m or cm Units = 1/s = Hz Period (T):# of seconds per unit wave.

6 WAVE SPEED  The speed of a wave can be calculated from its frequency (f) and wavelength (λ). f × λ = s  Units: Note, s = constant for a given wave

7 Example 1: A sound wave has a speed of 344 m/s and a wave- length of 0.5 m. What is the frequency of this wave? f = ??? λ = 0.5 m s = 344 m/s f = 688 Hz

8 Example 2: f = 20 kHz λ = ??? s = 344 m/s λ = m A sound wave has a speed of 344 m/s has a frequency of 20 kHz. What is this wave ’ s wavelength?

9 Example 3: An ocean wave passes a point on the pier at the rate of 12 waves per minute. What are the frequency and period of this wave? # of cycles = 12 time = 60 s f = ??? T = ??? f = 0.2 Hz T = 5 s

10 LONGITUDINAL WAVES  Waves where direction of particle movement is parallel to the direction of wave velocity, are longitudinal waves  Longitudinal waves have areas of high particle density (compressions) and of low particle density (rarefactions)  Sound is the most common example of a longitudinal wave.

11 TRANSVERSE WAVES  Waves where the direction of particle movement is perpendicular to the direction of wave velocity, are transverse waves  i.e. mechanical waves, electromagnetic waves (light, microwave, x-rays, etc.)

12 ELECTROMAGNETIC WAVES  Electromagnetic waves (e&m waves) are transverse waves caused by vibrating electrons ( e - )  Formed through interaction of electric and magnetic fields that are perpendicular to one another  In vacuum,  e&m waves travel at the same speed and differ from each other only in freq.

13 ELECTROMAGNETIC SPECTRUM  The classification of electromagnetic waves according to their frequency is called electro-magnetic spectrum.  e&m waves range from γ rays (short λ, high f) to radio (long λ, low f)

14 SOUND WAVES  Sound is the most common example of a longitudinal wave  In sound waves, the molecules of air transmit the vibrations through compressions and rarefactions  Sounds waves cannot travel in vacuum!

15 SPEED OF SOUND  Speed of sound is affected by:  wind conditions, temperature, humidity and type of medium  NOT affected by its loundness (amplitude)  Sound travels faster in liquids and solids compared to gases  Closely packed molecules in solids and liquids will transmit the vibrations faster than in gases  Sound travels faster at higher T  Faster moving air particles bump into each other more frequently and carry vibrations in shorter time

16 DOPPLER EFFECT  The apparent shift in pitch of sound when the source is moving relative to the observer is called the Doppler Effect  i.e. The sudden change in pitch of an ambulance siren as it goes by an observer is the result of the Doppler effect  Pitch of a sound is the subjective measure of its frequency  High frequency sounds have high pitch, and low frequency sounds have low pitch

17 STANDING WAVES  Waves that are reflected on themselves and appear to “ stand ” are called standing waves  Standing waves produce the variety of sounds in musical instruments! (Animation) violinAnimation

18 RESONANCE  Every object has natural vibrations caused by motion of its molecules  When a forced vibration matches an object ’ s natural vibrations, a dramatic increase in amplitude occurs  This phenomena is called Resonance  Some Examples  Tacoma Narrows Bridge Tacoma Narrows Bridge  Resonant Bridge Resonant Bridge

19 THE END

Some Symbols Used  all  there is or there exists  Change  infinity  multiplication  proportional to  less than or equal to  greater than or equal to  plus or minus  degrees  therefore ≈ approximately equal to  decreases  increases λ – Wavelength E – Energy H – Heat # - Number e - - electron p + - proton