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Waves and Sound

Wave Characteristics

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Transferring Energy Without Transferring Matter

• Vibrating bodies connected to their environment will transfer energy to the environment.

• The vibrations are then transferred though the environment from neighbor to neighbor.

• This energy transfer is called wave motion. • Wave motion moves energy through a medium

without moving the whole medium. • Only the shape or form of wave travels, not the

medium.

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Definition of wave • A periodic disturbance which travels through a

medium from one point in space to the others.

Wave motion means the propagation of waves through a medium.

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TYPES OF WAVESWaves are classified into different types

according to their natures :

Transverse waves Longitudinal waves

Mechanical waves

Transverse waves

Electromagnetic waves

WAVES

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Three types of waves:

• Mechanical waves require a material medium to travel (air, water, ropes).

• Electromagnetic waves do not require a medium to travel (light, radio).

• Matter waves are produced by electrons and particles.

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Electromagnetic Waves

• Material medium is not essential for propagation. e/m waves travel through vacuum.

• Disturbance of electric and magnetic fields traveling through space.

• All electromagnetic waves are transverse waves.

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Electromagnetic Waves

• Light Waves

• Radio Waves

• X-Rays

• No Medium Required

• Travels At The Speed Of Light

• 3 X 108 m/s in a vacuum

• Cannot Be Observed

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Matter Waves

Matter is in the form of particles.

Energy is in the form of waves.

Energy can be quantized and matter can have a wavelength.

Matter displays wavelike properties.

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Mechanical Waves

• Transverse waves cause the medium to move perpendicular to the direction of the wave.

• Longitudinal waves cause the medium to move parallel to the direction of the wave.

• Surface waves are both transverse waves and longitudinal waves mixed in one medium. (Such as water waves)

• Torsional waves produce a twisting motion through the medium – such as the ones which caused the collapse of the Tacoma Narrows Bridge.

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3 Types of Mechanical Waves

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Transverse Wave

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Transverse Waves

These waves cause particles of a medium to vibrate perpendicular to the direction of motion

The waveform appears in the shape of sine curve.

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Longitudinal Waves

These waves cause medium to vibrate in a direction parallel to the direction of motion.

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Longitudinal Waves

Speaker membrane expands, creating a region where the air molecules are packed closely together, a "condensation".  The air pressure in a condensation is higher than normal.

As the membrane moves back, a regionis left behind where few molecules are located, a "rarefaction".  Meanwhile, thecondensation moves forward.

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Surface Waves

Waves out on the ocean's surface are a combination of transverse and longitudinal waves.

The wave height is the distance from a trough to a peak and the wavelength is the peak to peak distance.

When the wave height is 1/7 the wavelength the wave breaks.

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Tacoma Narrows BridgeTorsional Oscillation

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Period: T Frequency: f• The PERIOD is the time

for a particle of the medium to complete one oscillation.

• The SI unit for period is the second

• The FREQUENCY of is the number of cycles per unit time.

The frequency of a wave is equal to the number of waves that pass a particular point in one second.

• The unit is Hertz (Hz) which is a cycle per second.

• FREQUENCY is also the reciprocal of the period.

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Period and Frequency

Tf

1f

T1

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Amplitude: A

• The AMPLITUDE of a wave is the maximum distance of a particle from the equilibrium position.

• The SI unit for amplitude is meter

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Amplitude: A

• The energy content of a wave is indicated by the amplitude.

• Two waves of the same frequency can have different amplitudes.

• The greater the amplitude, the more work a wave can do.

• Amplitude does not affect wavelength, frequency or velocity

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Wavelength:

• The WAVELENGTH of a wave is the length of one complete cycle.

• It is the distance between two consecutive “in phase” points.

• In phase points are those that are moving in step with each other.

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Comparing waves

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Wave Velocity

• The speed of a wave is the product of the wave’s frequency and wavelength.

• The speed does not depend on amplitude or frequency…it depends on the properties of the medium.

• Sound waves depend on temperature of the air and water waves depend on depth of water. fv

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Do you remember……..?

• 4. The periodic wave in the diagram below has a frequency of 40. hertz.

• What is the speed of the wave?

• a. 13 m/s b. 60. m/s c. 27 m/s d. 120 m/s

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More to remember…..

• Austin drops a stone into a dry well 175 m deep. How long, after he drops the stone, does he hear it hit the bottom of the shaft?

• The speed of sound waves is 340 m/s.

• Assume g = 10 m/s/s

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Warm up

• Sitting on the beach I notice the waves motion . I see a crest pass me every 2 seconds. The crests are

3 meters apart. What is the waves’ velocity?

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Waves and SoundWave Actions

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Waves and Boundaries

When a wave encounters an interface (boundary), it may be reflected and/or transmitted!

An incident wave goes into a new medium as a transmitted wave and bounces back as a reflected wave.

Density difference determines how much of the wave is transmitted and how much is reflected.

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Waves and Boundaries

Small Difference Between the Densities of the two Media

Most of wave is transmitted.

Amplitude of transmitted wave will be greater than amplitude of reflected wave.

Large Difference Between the Densities of the two MediaMost of incident wave is reflected.

Amplitude of reflected wave is greater than amplitude of transmitted wave.

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Less Dense to More Dense Medium

• When a wave hits the boundary of a more dense medium, the reflected wave is inverted.

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More dense to a Less Dense Medium

• When a wave hits the boundary of a less dense medium, the reflected wave is erect.

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Reflection of WavesReflection of Waves

• A traveling wave is reflectedreflected when it hits a barrier.

• This phenomenon can easily be observed when a traveling water wave hits a reflector in the ripple tank.

Reflectedwaves

Reflectedwaves

Reflector

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Law of Reflection

When an incident wave encounters a boundary at an angle, , from the normal, its reflected wave will be reflected at the same angle from the normal.

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Do you remember???

• What is a boundary?• What determines the speed of a wave?• What happens to a wave hitting a boundary?• How do you know if a wave will reflect

erect or inverted?• What does not change at a boundary?• What must change at a boundary?

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When a wave passes into a new medium…

• Its wavelength changes…• Its speed changes with density…• Its frequency remains the same…

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Refraction

Refraction is the bending of a wave as it passes from one medium to another medium of different propagation speed.

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The Densities of the Media

• If the light passes from a less dense to a more dense medium, it bends toward the normal.

• If the light passes from a more dense to a less dense medium, it bends away from the normal.

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Refraction of WavesRefraction of Waves

The speed of a water wave increases with depth.

This change in speed is accompanied by refractionrefraction.

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DiffractionDiffraction is the spreading of a wave disturbance

around and beyond the edge of a barrier. When a traveling water wave hits an obstacle, the

wave fronts spreads out round the edge and becomes curved.

The wavelength of the wave is not changed in diffraction.

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Superposition Principle

Wave interference occurs when two or more waves act simultaneously on a medium.

Whenever two or more waves pass through each other, the resulting disturbance at a given point in the medium may usually be found by adding the individual displacements that each wave would have caused. (Principle of Superposition)

= +

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Principle of Superposition

The displacement of a medium caused by two or more waves is the algebraic sum of the displacements caused by the individual waves.

When colliding waves combine (add) the result can either be bigger or smaller than the original waves.

The waves add “constructively” or “destructively” depending on the relative sign of each wave.

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Superposition & Interference

• Consider two harmonic waves A and B meeting at x = 0.

• The displacement versus time for each is shown below:

CONSTRUCTIVEINTERFERENCE

DESTRUCTIVEINTERFERENCE

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Constructive Interference

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Constructive Interference• Occurs when wave displacements are in phase.• Two waves combine to create a larger wave.

Constructive interference occurs when the waves are trying to displace the medium in the same direction.

More than one wave can occupy the same space at the same time.

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Destructive Interference

• Occurs when wave displacements are opposite in phase.

• One wave cancels the other out.

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Destructive Interference

• When these two waves are completely overlapping, there will be complete destructive interference.

• Destructive interference occurs when the waves are trying to displace the medium in opposite directions.

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Before and AfterWaves have the ability to pass through one another

and not change.

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Waves Moving in and Out of Phase

• When the 2 waves are in phase, the resulting disturbance has a maximum amplitude.

• When the 2 waves are out of phase, the resulting disturbance has a minimum amplitude.

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Interference in Water Waves

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Standing Wave in Action

Nodes are places on a standing wave that do not appear to move.

Antinodes are places on a standing wave that are the greatest displacement from equilibrium.

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Standing Waves

• Waves traveling in opposite directions, with the same frequency, wavelength, and amplitude, produce a standing wave.