Waves
What is a Wave?
Wave- a periodic disturbance in a solid, liquid or gas as energy is transmitted through the medium.
Medium- The matter through which a wave travels.
Waves that require a medium are called mechanical waves.
Electromagnetic waves (light waves) do not require a medium.
What is a Wave? Cont.
Waves carry energy and can do work.
What is a Wave? Cont.
The energy in a wave may spread out as the wave travels.
Vibrations and Waves
Most waves are created by vibrating charged particles
In a mechanical wave, particles in the medium also vibrate.
Vibrations involve transformations of energy, from Kinetic to Potential.
Simple harmonic motion- a motion that is neither driven nor damped.
Vibrations and Waves cont.
Waves can pass through a series of vibrating objects.
Damped harmonic motion- the vibration fades out as energy is transferred from one object to another.
The motion of particles in a medium is like the motion of masses on springs.
Transverse & Longitudinal Waves
Transverse- particles of the medium move perpendicular to the direction of the wave.
Longitudinal- the particles of the medium vibrate parallel to the direction of the wave. Mechanical longitudinal waves -compressional waves or pressure waves.
Surface Waves
In surface waves, particles move in circles
http://www.classzone.com/books/earth_science/terc/content/visualizations/es1604/es1604page01.cfm?chapter_no=visualization
http://en.wikipedia.org/wiki/Image:Wave_motion-i18n.svg
Characteristics of Waves
Wave Properties:– Amplitude- measures the amount of particle vibration.
Crests- highest point of a transverse wave Troughs- lowest part of a transverse wave
Characteristics of Waves cont.
Wavelength (λ)- measures the distance between 2 equivalent (same) parts of a wave. Measured in meters.
Characteristics of Waves cont.
Period (T)- measures how long it takes for waves to make a complete cycle. Measured in seconds
Period
Characteristics of Waves cont.
Frequency (f)- the number of cycles or vibrations per unit of time. Measure in hertz (Hz). 1 vibration / 1s = 1Hz
Frequency- period equation- Frequency = ___1___
period
Light comes in a wide range of frequencies and wavelengths.
– Our eyes can detect light with frequencies ranging from 4.3X1014 Hz to 7.5X1014 Hz. This light range is called VISIBLE LIGHT.
Wave Speed
If speed is calculated by the distance ÷ time. Then the wave speed is calculated by the wavelength ÷ period. OROR
Speed = Wavelength • Frequency (v = λ • f) The wave speed depends on the medium.
– Kinetic theory explains difference in wave speed. (the arrangement of the molecules)
Practice Problems
The average wavelength in a series of ocean waves is 15.0m. A wave crest arrives at the shore on the average every 10.0s, so the frequency is 0.100 Hz. WHAT IS THE AVERAGE SPEED of the wave?
Green light has a wavelength of 5.20 X 10-7m. The speed of light is 3.00 X 108 m/s. Calculate the frequency of green light wave.
The Doppler Effect
-An observed change in the frequency of a wave when the source or observer is moving.
– Pitch is determined by the frequency – Frequency changes when the source of waves is moving.
http://www.colorado.edu/physics/2000/applets/doppler2.html
Sonic Boom- breaking the sound barrier
U.S. Navy F/A-18 at transonic speed. The origin of the cloud is debated; the leading theory claims: "a drop in air pressure at the plane described by the Prandtl-Glauert Singularity occurs so that moist air condenses there to form water droplets."
SOUND
Properties of sound:– Sound waves- are longitudinal waves which
cause the air molecules to vibrate in the same direction.
– Speed of sound depends on medium & temp.
Speed of sound- in air at room temperature is about 346 m/s (760 mi/h)
SOUND cont.
– Loudness is determined by intensity. Measured in decibels dB
– Pitch is a measure of how high or low a sound is perceived to be depending on the frequency of the sound wave.
http://www.phys.unsw.edu.au/jw/dB.html
SOUND cont.
The human ear can hear sounds from sources that vibrate as slowly as 20 Hz and as fast as 20,000Hz (acoustic sound).
Infrasound- frequencies lower than 20Hz (can’t hear)
Ultrasound- frequencies higher than 20,000 Hz (can’t hear)
Musical Instruments
All musical instruments use standing waves to produce sound.
Harmonics give every instrument a unique sound (gives several pitches)
Instruments use resonance to amplify sound.
Resonance- when 2 objects naturally vibrate at the same frequencies.
Hearing and The Ear
Sound waves come in the ear- vibrate the eardrum (tympanic membrane)- vibrate the ossicles (3 bones)- send waves through the cochlea (resonance occurs)- vibrates hairs which stimulate the nerve.
Brain interprets vibrations
Ultrasound and Sonar
SONAR- sound navigation and ranging- system that uses acoustic signals, echo returns to determine the location of objects or to communicate
Ultrasound imaging- very high frequencies (1 million-
1.5 million Hz) can be used to create computer images (sonograms)
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