Waves and Energy Transfer
Wave Definition
The presence of waves is evidence of energy moving through some material or area. A wave is created when the energy passing by causes the temporary dislocation of particles. We call the pattern of dislocations a "wave."
Main Wave Classifications
There are three main classifications of waves: mechanical, electromagnetic and matter.
Mechanical Waves
This classification has 3 subdivisions: transverse, longitudinal and surface. All mechanical waves must have a transmitting medium through which to move.
Wave Structure
a) Transverse Waves
In these waves, the dislocated particles vibrate at 900 to the path of the energy. For example, we see ocean waves moving up and down while their energy moves sideways. We can make this type of wave by plucking a stretched elastic band. Here are terms describing the structure of a transverse wave:
crest - the highest point on the waveform
trough - the lowest point on the waveform
principal axis - the rest or equilibrium shape of the waveform
wavelength - the distance between any two corresponding in-phase) points on adjacent waves
amplitude - the displacement between the principal axis and a crest or a trough
height - the displacement between the crest and trough; the sum of the amplitudes
b) Longitudinal Waves
In these waves, the dislocated particles vibrate parallel to the path of the energy. We can make this wave by suspending a slinky between a couple of people, gathering in some of the coils at one end and then letting them go. Here the terms describing the structure of a longitudinal wave:
compression - an area where the vibrating particles are squished together
rarefaction - an area where the vibrating particles are more widely spaced
wavelength - the distance between any two corresponding (in-phase) points on adjacent waves
c) Surface Waves
These are a combo of transverse and longitudinal motions - circular or oval
Electromagnetic Waves
These waves can move through both a vacuum or a transmitting material. Common examples are: light, heat, radio, TV, X-rays. Note: all electromagnetic waves travel at the same enormous speed, 3 * 10 8 m/s.
Mechanical Wave Properties
1. Waves in Water
Wave Production
When the surface of calm water is disturbed, a circular ripple forms around the disturbance point and then spreads outward. As the ripple rings spread and enlarge in circumference, they seem to straighten, just like a section of a huge circle looks straighter than the same length of a small, more tightly curved circle. As the energy spreads out from the center, the speed remains essentially constant but the ripple height decreases as the given amount of storm energy is diffused over a wider and wider area.
A single disturbance produces a single ripple called a pulse. If the disturbance is repeated regularly, the ripple rings are regularly spaced and are called periodic waves. They form a pattern on the water called a wave train. Large ripples are called waves.
Energy Transfer
The energy added to the water at the disturbance center causes the water particles to vibrate. As the energy spreads out through the water particles, they appear as ripples. The height of a ripple diminishes as it moves along because some of the ripple's original disturbance energy is used up in the work of dislocating each new water particle the energy encounters. The wave's speed will be constant until it moves into a patch of water with different properties, e.,g, hotter, deeper, saltier.
Moving Water?
Although ripples may appear to move forward, it is an illusion, just like messages scrolling across a computer screen or waves "blowing" across fields of wheat. The energy moves - the particles stay in the same spot. We know of course that winds, gravity, temperature variations, or tectonic action can set in motion even huge masses of water. These rivers, tides, currents, etc., can carry objects over thousands of kilometres. Do not confuse the concept of energy moving through a mass of water with that of a mass of water in motion.
Observing Waves
Waves in motion can be observed by means of a shallow, transparent bottomed tray called a ripple tank.
Wave Structure
Energy moving through deep ocean water causes particles suspended there to move in the to and fro motion of longitudinal waves. Tiny objects suspended in the water column closer to the surface are caused to move in the up and down motion of transverse waves. Floating objects move in the circular or oval pattern of surface waves.
Wave Behavior Diagrams
Wave crests are irregular but, in diagrams, are often depicted as simple straight or curved lines called wave fronts. To more simply show the motion of wave energy, rays may be drawn. A ray is an arrow, drawn so it always crosses wave fronts at 900. Rays show where each part of a wave front is headed.
1. Rectilinear Propagation: waves move in straight lines until they encounter some change in the transmitting medium, e.g., new temperature, depth, density.
2. Reflection: when waves reach a large straight barrier, their angles of impact (angle of incidence) and of reflection are equal. Note: when a wave strikes a material with very different properties, much of the incident wave's energy is reflected back and in opposite shape. For example, most of the energy in a wave crest slapping into a rock face returns to the ocean as a trough. Here are terms to describe the behavior of the wave at the barrier:
barrier - a surface into which the energy is unable to move
reference line - an imaginary line at 900 to the barrier
incident ray - the ray approaching the barrier
angle of incidence - the angle between the incident ray and the reference line
reflected ray - the ray leaving the barrier after being reflected from there
angle of reflection - the angle between the reflected ray and the reference line
3. Refraction: wave speed and water depth are directly related. As a wave moves into shallower water, friction with the bottom increases, slowing down its speed. The wave fronts bend, or refract, as they slow down. Note: when a wave runs into a material with only slightly different properties, only a bit of the original energy reflects and in the same shape. For example, if a cold, salt water wave crest runs into warm, fresh water, just a bit of the wave's energy is reflected back as a crest. Most of the energy moves ahead and is refracted. As the wave crosses the boundary, its speed and wavelength change but not its frequency. Here are terms to describe the behavior of the wave at the boundary:
boundary - a zone through which the energy passes as it moves into a new transmitting material
reference line - an imaginary line at 900 to the boundary
incident ray - the ray approaching the boundary
angle of incidence - the angle between the incident ray and the reference line
refracted ray - the ray moving away from the boundary after passing across it
angle of refraction - the angle between the refracted ray and the reference line
4. Diffraction: waves can diffract, i.e., bend around edges. If a wave moves through an aperture, that section passing through its center is unaffected. Where the wavefront ends touch the aperture edges, diffraction reshapes them into arcs. The amount of diffraction is affected by aperture size and wavelength. The "zone of no diffraction" in front of a narrow aperture may be so narrow as to be unnoticed between the diffraction arcs. A wide aperture will produce a more obvious zone within which the incident waves are unchanged; on either side are areas of diffraction.
5. Superposition: pictures of large bodies of water reveal that usually, many sets of wave trains are passing simultaneously through each other.
6. Interference
As the wave trains pass through each other, they superimpose on each other and their energies will combine in both positive and negative fashions.
a) Constructive Interference occurs when the superimposed waves are in phase, i.e., with the crests lined up with each other and the troughs lined up with each other. The wave energies add to produce a new wave of increased height.
b) Destructive Interference occurs when the superimposed waves are out of phase, i.e., with the crests lined up with the troughs. The wave energies subtract to create a new wave of reduced height.
7. Interference Pattern: if two disturbance centers are adjacent, their circular wave trains will intersect and create a complex wave pattern. Radiating from the disturbance area like the spokes of a bicycle wheel are zones of constructive and destructive interference. The water within the zones of constructive interference will appear choppy. Water within the zones of destructive interference will appear calm.
2. Waves in Air
Wave Production
A body vibrating in air sends out waves of energy which cause the air particles to vibrate. As the air particles vibrate to and fro, they create areas of compression and expansion. A single vibration produces one pulse or beat of sound.
Energy Transfer
The energy added to the air by the string causes it to bunch together. It is this energy that spreads out through the air, causing it to compress. The intensity of the sound diminishes as it moves because the energy is being used up in the work of compressing successive air masses.
Moving Air?
Air is elastic. Once compressed, it tries to expand and release the energy gained during compression. Looking at the motion of a vertical, vibrating string shows how a sound wave is created. As the string arcs to the right, it compresses the pocket of air just in front of it. These air particles then run into those in the next pocket out, compressing them, and so on. In this way, the compression energy moves away from the string and out through the successive air masses. As the string rebounds to the left, it removes pressure on the air in the first pocket, allowing it to expand. This allows the air in the next pocket out to expand, and so on. In this way the expansion energy moves away from the string and out through the successive air masses. Although the energy moves out through the air pockets, the particles in each pocket show just a simple to and fro motion.
We know of course that the earth's rotation and Tair variations can set in motion huge masses of air. These breezes, winds, gales, hurricanes, etc., can carry objects for thousands of kilometers. Do not confuse the concept of energy moving through air with that of a moving air mass.
Wave Structure
Sound waves are longitudinal. A wave is made of a sequence of compressions and expansions (rarefactions.) The loudness of a sound depends upon how much air undergoes compression-expansion and the degree to which it occurs.
Wave Behavior
The behaviors of water waves hold true for sound waves. The pitch of a sound is proportional to its frequency. A reflected sound wave is an echo. Multiple echos are called reverberation. Soft and/or irregular surfaces trap more sound than they reflect. The absorbed sound causes the absorbant to vibrate and/or increase in temperature.
1. Resonance (Sympathetic Vibration): sound waves passing around a body will cause it to vibrate. If the pitch of the incident sound waves is at the natural vibration frequency of the object, they will cause it to vibrate strongly and loudly. Many musical instruments produce their characteristic rich sound because of resonance. The vibration of a violin's strings causes both the case and the air inside to vibrate. The vibration imparted by a flute player's breath to the air column inside the flute causes it to resonate.
2. Interference and Beats: sound waves can interfere with each other as can water waves. The result is the formation of beats which are heard as a wavering quality to the sound. Beats are the audio equivalent of an interference pattern on water.
Wave Math
Quantities important in describing wave behavior are frequency, period and speed. Wave frequency tells us how many waves pass by a certain spot, e.g, 10.3 waves per second. It tells us how often something happens. Wave period tells us how long it takes a single wave to pass by a certain spot, e.g., 5.7 seconds per wave. The speed of waves is proportional to certain characteristics of the transmitting medium, e.g., sound wave speed is controlled by the density and temperature of the transmitting medium.