waves - northern highlands regional hs / overview · the superposition principle •many waves can...
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Waves Chapter 20
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20.1 Waves
• A wave is an oscillation that travels from one place to another.
• Because waves can change motion, they are a travelling form on energy.
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Recognizing Waves
Waves are present:
• When you see a vibration that moves.
Example: A guitar string after it is plucked
• When something makes or responds to sound. Example: A drum or your ears
• When something makes or responds to light. Example: A light bulb or your eyes
• When technology allows us to “see through” objects.
Examples: ultrasound, CAT scans, MRI scans, and X rays
• When information travels through the air (or space) without wires.
Example: A satellite dish for receiving television signals. 3
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Properties of Waves
• Like oscillations, waves have the properties of frequency, period, and amplitude.
• Waves also have two new properties: speed and wavelength.
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Wave Travel
• For a wave to travel, molecules need to be connected or in contact with each other.
• Because are molecules collide, waves can travel in the air.
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Wave Types
• A transverse wave has oscillations that move perpendicular to the direction the wave moves.
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• A longitudinal wave has oscillations that move in the same direction the wave moves.
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Frequency
• The frequency of a wave is measured how often it goes up and down.
• Frequency is measured in hertz (Hz)
One hertz is one cycle per second.
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Amplitude
• The amplitude of a wave is the maximum amount the wave causes anything to move away from equilibrium.
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Wavelength
• A wave is a series of high and low points
• A wavelength is the distance from any point on a wave to the same point on the next cycle of the wave
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Wave Speed
• When a wave moves through water, the water itself stays in the same place.
• Light = 300,000,000 m/s
• Sound = 660 mph
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Standing Waves
• A wave that is confined in a space.
• Can be sound, water, a vibrating string, even light.
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Harmonics
• Like all oscillators, a string has a natural frequency.
• The lowest is called the fundamental. (#1)
• As frequency increases, wavelength decreases. (They are inversely proportional.)
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THE MOTION OF WAVES
Section 20.2
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Wave Patterns and Direction
• Waves spread out from where they begin, or propagate.
• Plane waves form straight lines, circular waves form a pattern of circular wave fronts.
• The direction a wave moves depends on the shape of the wave fronts.
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Boundaries
• Waves are affected by boundaries, which are edges or surfaces where conditions or materials suddenly change.
• For example: the surface of glass or water.
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At Boundaries
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• Reflection – wavelength and frequency DO NOT change.
• Refraction – wavelength and frequency usually DO change.
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Diffraction
• Bending around corners or passing through openings.
• When a wave is diffracted, it is changed by the hole or edge, usually in direction or shape.
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Absorption
• Means the amplitude of a wave gets smaller and smaller.
• The wave energy is transferred to the absorbing material.
• Theater curtains absorb sound energy
• Tinted glass absorbs light energy
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WAVE INTERFERENCE AND ENERGY
Section 20.3
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Wave Interference
• You almost never hear a single wave with only one frequency. You really hear a complex mixture of waves all mixed together.
• Interference happens when two or more waves mix together.
• Radio and TV use the interference of two waves to carry music and video.
• Water waves add up to make a gigantic wave, called a rogue wave.
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The Superposition Principle
• Many waves can be in the same system at the same time.
• The superposition principle states that the total vibration at any point is the sum of the vibrations from each individual wave.
• The sound and light waves you experience are the superposition of thousands of waves with different frequencies and amplitudes.
• Your eyes, ears, and brain separate the waves in order to recognize individual sounds and colors.
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Constructive Interference
• When waves meet, they combine to make a single large pulse.
• The waves add up to make a larger amplitude.
• If two sounds waves constructively interfere, loudness increases.
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Destructive Interference
• When the two pulses meet, they cancel each other out.
• The waves add up to make a wave with smaller or zero amplitude
• After interfering, both waves separate and travel on their own and still store energy.
• Noise cancelling headphones work this way. 23
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Natural Frequencies and Resonance • Waves can have natural frequency and resonance just like
oscillators, but they must be caught in a system with boundaries.
• Light keeps going in a straight line – there is no resonance. But, catch light between two perfect mirrors and you can get resonance of light waves, which is how a laser works.
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Resonance and Reflections
• Resonance in waves comes from the interference of a wave with its own reflections.
• One end of the string is tied to the wall making a boundary
• A pulse launched on the top reflects off the wall and comes back on the bottom of the string
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Resonance and Constructive Interference • To build up a large wave, you wait until a reflected pulse has
returned to your hand before launching a new pulse.
• Resonance is created by adding new pulses so that each adds to the reflected pulse in constructive interference.
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Why Resonance is Important
• The concepts of resonance and natural frequency apply to a huge range of natural and human-made systems.
• The tides of the oceans, musical instruments, the laser, the way our ears separate sound, and even a microwave oven are all examples of waves and resonance.
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Waves and Energy
• A wave is a form of moving energy.
• When you drop a stone into a pool, most of the stone’s kinetic energy is converted into water waves.
• The waves spread out carrying the energy far from the place where the stone fell.
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Frequency and Energy
• The energy of a wave is proportional to its frequency.
• Higher frequency means higher energy.
• This is obvious for a jump rope. You have to move the rope up and down twice, doing twice as much work, to make the rope swing at twice the frequency.
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Frequency and Energy (cont.)
• The wave with the higher frequency has more energy.
• The energy of a wave is proportional to its frequency.
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Amplitude and Energy
• The energy of a wave is also proportional to amplitude.
• Given two standing waves of the same frequency, the wave with the larger amplitude has more energy.
• With a vibrating string, the potential energy of the wave comes from the stretching of the string.
• Larger amplitude means the string has to stretch more and therefore stores more energy.
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Why are Standing Waves Useful? • Standing waves are used to store energy at specific
frequencies.
• With the wave on the string you observed how a small input of energy at the natural frequency accumulated over time to build a wave with much more energy.
• Musical instruments use standing waves to create sound energy of exactly the right frequency.
• Radio transmitters and cell phones also use standing waves to create power at specific frequencies. 32