Physical Science

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# Physical Science - PowerPoint PPT Presentation

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

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
• A wave is a disturbance that propagates through space and time.
• Waves transfer energy.
Longitudinal Wave
• 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: 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
• A medium propagates waves.
• Amplitude is the maximum displacement from equilibrium.
Period
• Period (T), is the time between successive peaks.

Period

time →

Time t

Frequency
• 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 “middle C” 262 Hz (cycles per second).
Relate Period and Frequency
• 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
• Associated with a traveling wave.
• Wavelength (λ) is the distance between successive crests.

Traveling Wave

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

v = λ f

Wavelength

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

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 Waves

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.

Wikipedia

Sound Wave Characteristics
• 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
• 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
• 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 Loudness
• 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 Loudness
• 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
• 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
• 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
• 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
• 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 Loss
• 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
• 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
• 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
• Double the frequency and the “second harmonic” appears.

λ/4

λ/4

Higher Harmonics
• 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
• 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
• 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
• 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
• 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
• 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

Pressure wave build-up

Wikipedia

High−performance aircraft speeds measured in Mach numbers.

Mach 1.0 = speed of sound.

Bell X-1

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

Supersonic
• 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