Catch the Wave – 8.7 B

TAKS Objective 4 – The student will demonstrate an understanding of motion, forces, and energy.

Learned Science Concepts

  • Unbalanced forces cause changes in the speed or direction of an object’s motion.
  • Waves are generated and can travel through different media.

TEKS 8.7 Science concepts

The student knows that there is a relationship between force and motion. The student is expected to:

(B) recognize that waves are generated and can travel through different media.

Overview

To facilitate the discovery by the student that a relationship exists between force and motion, to help him/her demonstrate the ability to recognize that waves are generated and can travel through different media by

  1. Discovering the existence of various kinds of waves;
  2. Exploring the nature of longitudinal and transverse mechanical waves;
  3. Characterizing waves with regard to period, frequency, amplitude and velocity;
  4. Measuring and developing skill in the measurement of wave speed;
  5. Distinguishing between mechanical waves and Radio or Electromagnetic waves;
  6. Modeling seismic wave action.

Instructional Strategies

These objectives will be met using authentic methods of scientific inquiry; critical thinking skills and scientific problem solving practices.

Objectives

  1. The learner will be able to differentiate between the motion of the wave and the motion of the matter carrying the wave.
  2. The learner will identify waves that require a medium to move through and waves that travel through empty space.
  3. The learner will demonstrate that waves carry energy.
  4. Students will generate waves that pass through different media.
  5. The student will demonstrate that a wave moves forward while the material through which it passes is displace only slightly and momentarily.
  6. Students will measure wave properties.

For Teacher’s Eyes Only

Vocabulary

How Waves Move

Longitudinal- (aka Compression waves) – energy moves parallel to medium (Sound waves, dominos, some earthquake)

Transverse- (aka S waves)- energy moves perpendicular (90º to medium (electromagnetic waves -- radio, infrared, visible light, UV, Xrays . . .)

Two Major Types of Waves

Electromagnetic Waves - produced by the vibration of electrons within atoms on the Sun's surface. Does not require particles to transfer energy; Usually transverse (S) waves

Mechanical Waves - must have a medium to transfer energy. Need particles to interact to transfer energy; Compression (sound), surface (water), both compression & transverse (earthquakes)

Vacuum – a space with no matter- no air, no molecules, no atoms, nothing!

Waves are one of the most important forms of energy transmission that we know. Waves come in many varieties: mechanical waves are actual motions of a material medium while radio, light and X-rays are electromagnetic waves that are fluctuations in the electromagnetic force that travel through empty space. There are other kinds of waves too, such as the people “Wave” that we know from and enjoy at football games. Incidentally, “The Wave” was created in 1981 by Mr. Bill Bissell, the long-time band director of the University of Washington Husky Marching Band, although it is often called the “Mexican Wave” since it was made popular at the 1986 World Cup Soccer tournament in Mexico City. Scientists are also still searching for the faint oscillations in the weak gravitational field of distant Black Holes that would signal the detection of “Gravity Waves.” Even the toppling of dominoes that have been carefully lined up is a wave of sorts.

When mechanical waves travel through a material medium, two possibilities exist for the motion of the medium: along the direction of travel (called longitudinal) and perpendicular to the direction of travel (called transverse). Only longitudinal waves can travel through liquids and gases, but solids allow both. Seismic waves occur as both varieties, longitudinal Pressure or P waves and transverse Shear or S waves. The P waves travel significantly faster than do the S waves, a fact that permits the determination of the distance to the epicenter of an earthquake. The waves you see on the surface of water happens because the water near the surface moves in a circle, as anyone who has watched a fishing cork waiting for a bite can affirm. A water-wave, then, is actually both a transverse and a longitudinal wave because the water moves both up and down and back and forth as a wave passes.

Mechanical waves have many properties in common. The height of the wave or the distance the medium moves from its resting position is called the amplitude of the wave. The time it takes for the wave to repeat the same up-and-down, back-and-forth or side-to-side cycle is called the period of the wave; the period is measured in seconds. The frequency of the wave is the same information as the period only expressed as how many cycles happen in a second; the frequency is the inverse or reciprocal of the period and is measured in the unit of Hertz (Hz). One Hertz is one cycle per second. In one period, the wave will travel a certain distance so that the wave begins to repeat itself. This repeat distance is called the wavelength. Because we have defined the frequency and wavelength as we have, the speed of a wave is equal to its wavelength multiplied by the frequency. In a formula

Speed of wave = wavelength x frequency

The speed of a mechanical wave is determined by only two properties of the material: (1) the stiffness or springiness of the medium and (2) its inertia or the density. What determines how fast a wave moves, then, is (1) how hard one piece of the medium pushes or pulls on the other and (2) how much inertia that neighboring piece of material has. The velocity of sound increases with temperature, since air—when it is heated—is less dense than when it is colder, while on the other hand the springiness of the air is practically unaffected by the higher temperature. So the lower density allows the disturbance to travel faster from one part of the air to the next. At room temperature (20 C) the speed of sound is about 343. meters per second (around 768 miles per hour). For every degree C higher, the speed goes up by 0.6 m/s (1.3 mph). It goes down at the same rate as well. Thinking along those same lines, because helium gas is very much less dense than air, owing to the light mass of the helium atoms that make up the gas, and because the springiness of helium is very nearly the same as air, helium has a much higher speed of sound than does air. So, in general—all other things being equal—the less dense a material is the faster is the speed of the wave.

But the speed of sound is much faster in water than it is in air even though water is about eight hundred (800) times more massive than is air! Why is that? The difference is in the stiffness of water relative to air. A bottle of air can easily be compressed; it makes a “soft” spring. A bottle completely filled with water is very difficult to compress, on the other hand. Water makes a very stiff spring if confined. Here the stiffness wins out over the density to make the speed of sound about 1000 m/second, about three times the speed of sound in air.

The concept of stiffness versus density is a very powerful concept for thinking about mechanical waves. For example, it is easier to bend a piece of material than it is to crush it. Thus rock is “stiffer” for longitudinal waves than it is for transverse waves. Transverse seismic waves (S waves), that require the medium to bend, travel slower than compression waves (P waves). This fact permits geophysicists to estimate the distance to the epicenter of an earthquake by measuring the difference in the time of arrival of the P and S waves. The time difference multiplied by the speed difference is equal to the distance to the epicenter from the seismograph.

The difference in wave speed is important in other ways, too. Whenever there is an abrupt change in the medium through which the wave is traveling, there is a reflection. Seismic exploration takes advantage of this property of waves to reveal hidden geological formations inside the earth’s crust that might have trapped petroleum or natural gas. As a matter of fact, this is the only way that geophysicists and geologists have been able to infer the internal structure of the earth with its crust, lithosphere and dense core. The velocity of the seismic waves differs in each of the materials and echoes occur when ever the sound wave crosses the boundary from one material to another.

The same approach has been used to get information about the interior of the sun: small sun-quakes happen that are associated with solar flares. Then by a careful examination of how the surface of the sun shakes, solar astrophysicists can figure out how the solar-wave traveled inside the sun out of our sight. The same properties of reflection from materials with different wave speeds even is used in ultrasound imaging to take a look at unborn babies in very much the same way.

There is, however, another important class of waves that are very different in nature from mechanical waves: the electromagnetic wave. EM waves, as they are sometimes called, are “a disturbance in the force,” the electromagnetic force, that is. The EM force is the force that makes your socks cling together when they come out of the drier; its what makes plastic kitchen wrap cling to a bowl. Whenever lighting strikes, for example, a huge discharge of electricity happens and the electric and magnetic fields in the space for miles around are disturbed. The disturbance does not travel instantly out from the spark but still it travels at a very high speed : 300 million meters per second or 186,000 miles per second, about 1 million times faster than sound does. If you are listening to an AM radio, the lightning strikes can be heard as static bursts of sound. EM waves happen whenever a charge accelerates or an electrical current changes rapidly. Radio is an example of just such an EM wave. But radio is not sound. Radio can travel through empty space; sound, which requires a material medium, cannot. The information about the sound is coded either as changes in the amplitude or frequency of the radio wave. The radio receiver decodes the information and produces a sound that is like the one coded at the radio studio. Television is also a radio wave in which not only the sound information, but also the visual information (three colors and the brightness) is coded in a set of companion radio waves.

Charges can be accelerated in other ways. When a material is heated the atoms begin to move to and fro. This causes the charges of the nuclei to be accelerated. EM waves are produced—radiant heat, infrared radiation. If the material is hotter it becomes incandescent, it glows. The atoms are moving fast enough to produce EM waves whose frequency is the range of visible light. Even hotter and one detects ultraviolet light.

If one takes an electron, accelerates it with a high voltage and allows it to run into a molybdenum slug, it will be stopped very suddenly. This abrupt deceleration will cause a burst of EM radiation of very high frequency and short wavelength. Originally these rays were not identified and were called X-rays, for the unknown ray. Later scientists learned that X-rays are just penetrating cousins of more familiar EM waves in the electromagnetic spectrum. That’s what that device is doing in the dentist’s office when you get dental X-rays; it is making electromagnetic waves by slamming electrons into a target to get X-rays when they stop suddenly.

All waves carry energy from one place to another. In the case of EM waves we feel the heat energy from a campfire; microwaves heat our muffins; sunlight warms us on a winter morning. The electric fields of EM waves make the electrons in materials move. The electrons bump into the atoms, making them move. When the atoms move, heat happens, and the energy is absorbed by the object. No electricity flows from the radiator, only energy in the form of a disturbance in the electric and magnetic field. The source of the energy may be trillions of miles away, as is the case with distant galaxies whose light we see after billions of years of travel.

Mechanical waves carry energy, too, even if the medium does not flow from the source; rather, one part of the medium “hand off” the energy to the next and then to the next and so on until it reaches you. Our ears are incredibly sensitive. The average human can detect a sound whose sound energy intensity is equivalent to the light intensity of a 100 Watt reading lamp being viewed at a distance of about 2000 miles! Clearly earthquakes carry an immense amount of energy in the seismic waves. The Richter scale is a measure of the amount of energy released by the earthquake at its source. A helpful (but somewhat advanced) source of information is the University of Nevada, Reno, Seismological Laboratory,

Even dominoes falling down, one after the other, send energy down the chain. And let’s not forget the nimble surfers who slide down the face of rapidly advancing ocean waves. The energy that propels them forward is the energy of the water wave that lifts them up. Instead of being lifted first up, then down at the wave passes, the surfboard allows them to stay continually on the lifting side of the wave and to exploit the energy of the wave, and that’s not to speak of the exhilarating ride that they enjoy.

Waves are everywhere and share many interesting features. We hope this little tutorial will help you enjoy Catching the Wave!

Summary:

  • There are many kinds of waves.
  • Mechanical waves can be transverse or longitudinal.
  • Mechanical waves are a form of motion in which one part of the object moves relative to another, rather than an overall motion of the object from one place to another.
  • Amplitude is the size of a wave.
  • Period is the time it takes to make one oscillation
  • Frequency is how many times in a second a wave wiggles.
  • Wavelength is the distance from one point a wave to a corresponding point on the next cycle.
  • Radio, light, heat radiation, microwaves and X-rays are all electromagnetic waves that do not require any medium other than space.
  • All mechanical waves need a material medium to shake.
  • The speed of a mechanical wave is determined by how stiff and how dense is the medium.
  • Seismic waves are of two types: longitudinal P-waves and transverse S-waves.
  • Waves carry energy from one place to another without the medium moving nearly as far.

Student Misconceptions

 Misconception

When waves—particularly sound waves—move, the media flows from the source to the observer, like a river.

 Science Concept

In fact, the medium moves very little. In the case of sound waves the motion of the air is only about the thickness of a human hair.

Rebuild Concept

Use physical examples in the cheerleader wave or the domino fall to show the wave moves but the medium does not.

 Misconception

Radio waves are sound waves

 Science Concept

The radio is not sound. Radio waves are “a disturbance in the force,” the electromagnetic force, that is. The information about the sound is coded either in the strength (amplitude) of the disturbance to produce Amplitude Modulated (AM) radio or in the frequency to make Frequency Modulated (FM) radio. ( XM radio is just FM radio broadcast from satellites.)

Rebuild Concept

Point out that radio waves move through space where there is no medium.

 Misconception

There is sound in the vacuum of space.

 Science Concept

Sound is a mechanical wave that requires a material medium to shake. There is no air in Outer Space; therefore sound cannot travel through the emptiness of space.

Rebuild Concept

Use a bell jar vacuum system and ringing bell. As the bell jar is evacuated the ringing gets dimmer until it can no longer be heard.

 Misconception

Sound moves up and down and left to right, that is, it is a transverse wave.

 Science Concept

The molecules of air move forward and backward along the direction the wave is traveling, a longitudinal wave. Fluids like air and water have no way to allow a transverse wave to travel through them.

Rebuild Concept

Use a slinky to demonstrate longitudinal and transverse waves. Show a simulation of compression waves hitting the ear drum to show how sound is transmitted to the ear.

Student Prior Knowledge

Waves are not introduced in the sixth and seventh grade TEKS. In order to build upon learning experiences, students must understand the motion of matter, understand the concert of energy, and know what is meant by a medium.