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)