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. OR 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. http://www.phys.unsw.edu.au/jw/dB.html 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.

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)