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Physical Science. Waves Slides subject to change. Sinking rock does work pushing surface water aside. Energy transferred to water. Water surface molecules push nearby molecules ... Wave “propagates” through the water. Energy Transfer.

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Physical science

Physical Science

Waves

Slides subject to change


Energy transfer

Sinking rock does work pushing surface water aside. Energy transferred to water.

Water surface molecules push nearby molecules ...

Wave “propagates” through the water.

Energy Transfer

  • A wave is a disturbance that propagates through space and time.

  • Waves transfer energy.


Longitudinal wave
Longitudinal Wave transferred to water.

  • Longitudinal wave: Particles move in same direction as the wave.

  • Examples: Sound waves, “Slinky” spring.

Animation courtesy of Dr. Dan Russell, Kettering University


Transverse wave
Transverse Wave transferred to water.

  • Transverse wave: Particles move perpendicular to wave direction.

  • Examples: Electromagnetic waves (radio, television, optical), waves on a string.

Animation courtesy of Dr. Dan Russell, Kettering University


Wave properties
Wave Properties transferred to water.

  • A medium propagates waves.

  • Amplitude is the maximum displacement from equilibrium.


Period
Period transferred to water.

  • Period (T), is the time between successive peaks.

Period

time →

Time t


“Period” is the transferred to water.time between successive peaks (or wave crests).


Frequency
Frequency transferred to water.

  • Frequency is the rate at which peaks are arriving.

  • Units are generally events per time, such as revolutions per minute. Cycles per second units are called Hertz (Hz).

  • Examples

    • Surfer waves: 4 crests per minute.

    • Electrical Outlet: 60 Hz


Music
Music transferred to water.

  • Music “middle C” 262 Hz (cycles per second).


Relate period and frequency
Relate Period and Frequency transferred to water.

  • If middle C is 262 Hz, what is the time between crests?

  • Given Formula

  • f = 262 Hz f = 1/T

  • therefore, 262 = 1/T, T = 0.0038 seconds

f = 1/T


Wavelength
Wavelength transferred to water.

  • Associated with a traveling wave.

  • Wavelength (λ) is the distance between successive crests.

Traveling Wave



Sound
Sound transferred to water.

  • The speed of sound is 344 m/s under standard conditions in air (sea level, 20 °C).

  • That’s about 768 mph.

  • Wave speed = wavelength times frequency

    v = λ f


Wavelength1
Wavelength transferred to water.

If the frequency of a concert tone is 262 Hz, what is the wavelength λ?

Given Formula

v = 344 m/s v = λ f

f = 262 Hz

v = λ f

344 = λ(262)

λ = 1.31 m


Electromagnetic waves
Electromagnetic Waves transferred to water.

The wave, or "disturbance," is a transverse electric field, which is invisible. Causes charged particles to move.

Light, microwaves, x-rays, TV, and radio transmissions are various kinds of electromagnetic waves.

  • The electric field interacts with electrons and protons.


Electromagnetic waves1
Electromagnetic Waves transferred to water.

Speed of light c = 3.0x108 m/s

KFI AM radio broadcasts at f = 640 kHz. What is the wavelength?

Given Formula

v = 3.0x108 m/s v = λ f

f = 640 kHz = 640x103 Hz

3x108 = λ (640x103)

λ = 470 m Radio wavelength is important in antenna design.


Radar
Radar transferred to water.

Icebergs on ship radar

Aircraft search radar

Wikipedia


Sound wave characteristics
Sound Wave Characteristics transferred to water.

  • Sound is propagation of compression waves through matter (solid, liquid, or gas).

  • Three regions.

    • Ultrasonic > 20,000 Hz (medicine, some animals – dogs, bats – can hear)

    • Audible 20 Hz – 20,000 Hz, (human hearing) Age Test

    • Infrasonic < 20 Hz (earthquakes, some animals – cattle, elephants - can hear or feel?)


Intensity
Intensity transferred to water.

  • Measure intensity = rate of energy transfer through a given area (power/area = W/m2).

  • Sound Intensity

    • Minimum intensity human can hear is about 10-12 W/m2, the threshold of hearing ... a mosquito at 10 feet!

    • Intensity decreases the farther you are from the source. Goes as 1/r2.


Sound loudness
Sound Loudness transferred to water.

  • Loudness, in bels (after Alexander Grahm Bell), of a sound of intensity I is defined to be

  • I0 is the minimum intensity detectable by the human ear.


Sound loudness1
Sound Loudness transferred to water.

  • Logarithms are powers of 10.

  • If a sound is 100 times more intense than another, its loudness is 2 bels more (factor of 100 or 102)

  • If one sound is 6 bel, and another is 9 bel, it is 1000 times more intense (103).


Sound loudness2
Sound Loudness transferred to water.

  • The bel is a large unit, so a sub-unit, the decibel, is generally used.

  • If one sound is 100 times louder than another, it is 2 bels or 20 dB louder.


Intensity levels
Intensity Levels transferred to water.

  • Think POWERS OF TEN

  • Threshold of hearing 0 dB, a mosquito 10 feet away.

  • Humming of a refrigerator 40 db (104· I0)

  • Conversation 60 dB (106· I0).

  • Leaf blower user 90 dB (109· I0).

  • Rock band 110 dB (1011· I0).


Typical problem
Typical Problem transferred to water.

  • A subway train has loudness 90 dB.

  • Rock band loudness of 110 dB.

  • How many times greater is the sound intensity of the band than that of the train?

  • The rock band is 20 dB louder

  • Divide dB by 10

  • 20/10 = 2

  • Intensity is 102 or 100 times greater.


Harmful impact of sound
Harmful Impact of Sound transferred to water.

  • Sounds of less than 75 decibels, even after long exposure, are unlikely to cause hearing loss.

  • Exposure to harmful sounds causes damage to the sensitive hair cells of the cochlea – the inner ear.

  • Hearing injured by noise

    • From an intense brief impulse, such as an explosion.

    • From continuous exposure to noise, such as in a woodworking shop.


More on hearing loss
More on Hearing Loss transferred to water.

  • The decibel level and time of exposure are the most important considerations.

  • Some sounds – artillery, explosions – are so loud (+140 db), ANY brief exposure to them at close range can cause permanent damage and hearing loss.


More on hearing loss1
More on Hearing Loss transferred to water.

  • Sounds at 100 decibels (such as loud music through stereo headphones) will take a while longer (1-2 hours of exposure) to cause permanent damage.

  • Ipods are tested by Apple up to 103 db.


Standing waves
Standing Waves transferred to water.

  • Mode of vibration in a string or column of air with unique pattern.

  • Traveling wave that reflects off an end in such a way that the medium appears to vibrate in segments or regions.

  • Standing wave animation.


Fundamental frequency
Fundamental Frequency transferred to water.

  • The frequency when this pattern appears is the fundamental frequency, or “first harmonic.”

  • This is the primary frequency you hear when you pluck a guitar string.

λ/2


Second harmonic
Second Harmonic transferred to water.

  • Double the frequency and the “second harmonic” appears.

λ/4

λ/4


Higher harmonics
Higher Harmonics transferred to water.

  • Many oscillators, including the human voice or a bowed violin string are composed of harmonics.

  • The quality, or timbre of that sound is a result of the relative strengths of the individual harmonic frequencies.


Resonance
Resonance transferred to water.

  • All oscillators have a natural frequency.

  • Add energy in synch with that natural frequency results in resonance.

  • Example: A swing.

  • Resonance

  • Tacoma Narrows Bridge, WA (1940)


Doppler frequency shift
Doppler Frequency Shift transferred to water.

  • Source moves towards you, waves are bunched up, you hear higher pitch.

  • Source moves away from you, waves are stretched out, you hear lower pitch.

  • Fire engine

  • Train

Higher pitch here

Lower pitch here


Beat frequencies
Beat Frequencies transferred to water.

  • If two sound waves arrive at our ears simultaneously.

    • Our ears hear the average frequency of the two waves.

    • Also hear the intensity increase and decrease – wavering beats.

      fbeat = f1 – f2

  • Musicians use beat phenomena to tune their instruments. Standard for musical pitch A = 440 Hz.


Speed of sound
Speed of Sound transferred to water.

  • vsound = 344 m/s or 1,126 ft/s (770 mi/h) at sea level and 20 °C.

  • Time for sound to go 1.0 mile = 4.7 s.

    • How far is a thunderstorm? Count out seconds between lightning and thunder.

    • One mile approximately every 5 seconds.

  • vsound varies with temperature: if air warmer, sound goes faster.


Move faster than the wave
Move Faster Than the Wave transferred to water.

  • Boat moving in water faster than waves can propagate.

  • Forms a V−shaped wake, sometimes even from the stern of the boat.

Pressure wave build-up.


Sound barrier
Sound Barrier transferred to water.

Pressure wave build-up

Wikipedia

High−performance aircraft speeds measured in Mach numbers.

Mach 1.0 = speed of sound.


Bell x 1
Bell X-1 transferred to water.

Chuck Yeager breaks sound barrier, Oct 14, 1947. Mojave Desert. Mach 1.06.


Supersonic
Supersonic transferred to water.

  • Concorde supersonic transport (SST).

  • Mach 2.04 (1,350 mph) cruising speed.

  • You can't hear the sonic "boom" if you are inside. Why?

  • First flown 1969.

  • Crash in Paris July 25, 2000.

  • Last flight October 2003.

  • Design characteristics inside.

  • What it felt like inside.

  • Concord take off

  • Sonic Boom


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