Chapter 17 & 18
I. Mechanical Waves
A. Mechanical Wave : A disturbance in matter that carries energy from one place to another
1. Medium: Material through which a wave travels a. Solids, liquids, gases
2. A mechanical wave is created when a source of energy causes a vibration to travel through the medium a. Vibration: A repeating back and forth motion
B. Types of Mechanical Waves (3 types)
1. Transverse Waves: A wave that causes the medium to vibrate at right angles to the direction in which the wave travels a. Crest: The highest point of the wave above the rest position b. Trough: The lowest point below the rest position
2. Longitudinal waves : A wave in which the vibration of the medium is parallel to the direction the wave travels a. Compression: An area where the particles in a medium are spaced close together b. Rarefaction: An area where the particles in a medium are spread out c. P waves produced by earthquakes
3. Surface waves: A wave that travels along a surface separating two media a. Ocean waves
1. Up and down motion is perpendicular to direction in which wave travels
2. Back and forth motion is parallel to direction in which the wave travels a. Do not transport matter from one place to another
II. Properties of Mechanical Waves
A. Frequency and Period
1. Periodic Motion: Any motion that repeats at regular time intervals
2. Period: Time required for one cycle, a complete motion that returns to its starting point
3. Frequency: The number of complete cycles in a given time a. Hertz: Cycles per second
1. Measures frequency
B. Wavelength : Distance between a point on one wave and the same point on another wave
1. Transverse waves a. Between adjacent crests or troughs
2. Longitudinal waves
C. Wave Speed a. Distance between adjacent compressions and rarefactions b. Increased frequency, decrease wavelength
Wave speed = frequency x Wavelength
Wave speed = Wave length/period
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D. Amplitude : The maximum displacement of the medium from its rest position
1. Distance from rest position to crest or trough
2. The more energy a wave has the higher the amplitude
3. Longitudinal wave a. The more energy the more medium will be compressed or displaced
III. Behavior of Waves
A. Reflection: Occurs when a wave bounces off a surface that it cannot pass through
B. Refraction: The bending of a wave as it enters a new medium at an angle
1. Shallow water: New medium
2. Occurs when the two sides of a wave travel at different speeds
C. Diffraction: The bending of a wave as it moves around an obstacle or passes through a narrow opening
D. Interference: Occurs when two or more waves overlap and combine together
Ex: 2 balls collide they can’t continue on their original paths
1. Constructive Interference: Occurs when two or more waves combine to produce a wave with a larger displacement a. Crest meets crest, trough meets trough b. Result; amplitude is increased
2. Destructive Interference: Occurs when two or more waves combine to produce a wave with a smaller displacement a. Crest of one wave meets with the trough of the next wave b. Result; amplitude is reduced
1. Lower crests, more shallow troughs
E. Standing Waves : A wave that appears to stay in one place
1. At certain frequencies interference between a wave and its reflection a. Ex: Pluck a guitar string
2. Node: A point on the standing wave that has no displacement from the rest position
3. Anti-node: A point where a crest or trough occurs midway between two nodes
4. Forms only half a wavelength or a multiple of half a wavelength fits exactly into the vibrating cord
5. Can be adjusted by changing frequency
IV. Sound
A. Sound waves: Longitudinal waves; compressions and rarefactions that travel through a medium
1. Speed a. 342 m/s in dry air at 20 o
C
1. Faster in solids
2. Slower in liquids
3. Slowest in gases b. Depends on density of medium and how elastic it is
-In a solid the particles are close together
2. Intensity and loudness a. Intensity:
The rate at which a wave’s energy flows through a given area
1. Depends on amplitude and distance from source b. Decibel (dB): A unit that compares the density of different sounds c. Loudness: A physical response to the intensity of sound, modified by physical factors
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1. As intensity increases, loudness increases
3. Frequency and pitch a. Frequency of sound wave depends on how fast the source of the sound is vibrating b . Pitch : The frequency of a sound as you perceive it
1. Depends on frequency a. High frequency; high pitch b. Low frequency; low pitch
B. Ultrasound
1. Most people hear sounds between 20 and 20,000 Hz a. Infrasound: Sound at frequencies below what most people can hear b. Ultrasound: Sound at frequencies above what most people can hear
2. Sonar: Technique for determining the distance to an object under water
C. Doppler Effect : A change in sound frequency caused by motion of the sound source, motion of the listener, or both
D. Music
1. Vary pitch by changing the frequency of the standing waves
2. Resonance: Response of a standing wave to another wave of the same frequency
Chapter 18 Electromagnetic Spectrum
I. Electromagnetic Waves
A. Electromagnetic Waves: Transverse waves consisting of changing electric fields and changing magnetic fields
1. How they are produced a. Produced by constantly changing fields b. Electric field: A region of space that exerts electric forces on charged particles
1. Produced by a. Electrically charged particles b. Changing magnetic fields c. Magnetic field: A region of space produced by magnetic forces
1. Produced by a. Magnetic forces b. Magnets c. Changing electric fields d. Vibrating charges
2. How they travel a. Electric fields regenerate each other b. Can travel through a vacuum, empty space or a medium c. Electromagnetic radiation: The transfer of energy by electromagnetic waves traveling through matter or across space
B. The speed of Electromagnetic Waves
1. Michelson’s experiment accurately measured the speed of light
2. The Speed of Light: 3.0 x 10
8
m/s
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C. Wavelength and Frequency
1. Electromagnetic waves vary in wavelength and frequency
D. Wave or Particle? a. As wavelength increases frequency decreases
1. Electromagnetic radiation travels as waves and as a stream of particles a. Wave Model
1. Thomas Young (1773-1829); Wave explanation
2. Fig 5 Light and dark bands show constructive and destructive interference b. Particle Model
1. Photoelectric effect: The emission of electrons from a metal caused by light striking the metal a. Photons: Packets of electromagnetic energy
1. Proposed by Albert Einstein
2. Energy proportional to frequency of the light
E. Intensity
1. The closer you are to the source of light the brighter the light appears a.. Photons travel outward from the light source in all directions
2. Intensity:
The rate at which a wave’s energy flows through a given unit of area a. Brightness b. Decreases as photons travel farther from the source
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