This Document Contains Chapters 4 to 7 Chapter 4 Heat and Temperature Contents The Kinetic Molecular Theory Molecules Molecules Interact Phases of Matter Molecules Move Temperature Thermometers Thermometer Scales Heat Heat as Energy Transfer Measures of Heat Specific Heat Heat Flow A Closer Look: Passive Solar Design Energy, Heat, and Molecular Theory Phase Change Evaporation and Condensation Relative Humidity Thermodynamics The First Law of Thermodynamics The Second Law of Thermodynamics The Second Law and Natural Processes Overview This chapter presents a short historical development of ideas about heat, then develops the theoretical basis for the modern understanding of temperature and heat. Concepts of heat and the kinetic molecular theory are closely linked, and observations of heat interactions provide evidence in favor of the kinetic theory. In turn, the particle nature of matter becomes more realistic from the quantitative and qualitative evidence provided by heat. The modern understanding of heat, which is derived from thermodynamics, is that heat is a measure of the amount of internal energy transferred from one object or system to another object or system. Heat corresponds to work at a molecular level and they are not considered separate concepts. Energy, work, and heat are measured in units of joules in science classes for science majors. Calories and Btus are also discussed in this text for nonscience majors because these are the heat measurements in everyday use. A measured amount of heat will affect the temperature of any substance, but the change in temperature will vary with the substance. Each substance has its own specific heat, which is usually smaller than that of water. Thus, less heat (internal energy) is required to raise the temperatures of these substances and, as they cool, they release less heat, or release less internal energy, than water. Conduction, convection, and radiation are important concepts of heat flow, and understanding them will become more important as present-day lifestyles place more and more demands on the energy supply. Suggestions 1. A Brownian motion device and a microscope projector (perhaps borrowed from the zoology department) can be used to show the random kinetic energy involved in Brownian motion. A few crystals of potassium permanganate in a tall graduated cylinder filled with water can be situated so it will remain undisturbed for a week. The resulting diffusion of the purple color can be related to the observed Brownian motion. 2. Linear expansion of heated solids can be demonstrated with a bimetallic strip, which can be compared to the coiled strip in many home thermometers and thermostats. An air thermometer can be demonstrated with an inverted Florence flask with a one-hole stopper and glass tube. The flask is supported by an iron ring on a ring stand with the glass tube extending into a beaker of colored water. When the flask is warmed by your hand air bubbles come from the tube in the water, providing evidence of expansion of the air inside the flask. As the flask cools water is forced into the tube by atmospheric pressure. The water level inside the tube is highly sensitive to temperature variations but is not used as a thermometer because it is also highly sensitive to atmospheric pressure variations. 3. The concept of specific heat can be demonstrated by simultaneously heating a block of iron and an equal mass of water over burner flames. When the water is warm a few drops transferred to the iron will sizzle, indicating the much higher temperature of the iron. The specific heats of water and iron can then be compared and discussed. 4. The concepts of conduction, convection, and radiation may be illustrated with the demonstration conductometer, the convection box, and the radiation outfit, all available from scientific supply houses. Point out that the oven advertised as a “convection oven” means forced convection. All ovens heat food through natural convection. A “convection oven” has a fan, which mechanically mixes the air. 5. The latent heat absorbed or released during a change of state is interesting to students. Ice cubes at 0˚ C placed in a glass of water at 0˚ C absorb latent heat as they melt, and no temperature change is observed during the melting process. Heat obtained from the surroundings is required to melt the ice at 80 cal per gram. When water freezes the same amount of energy (latent heat) is released. Since there is no temperature change, latent heat is not involved in kinetic energy. Since the same amount of heat is released during freezing as is absorbed during melting, the heat must be “hidden” as changes in potential energy. Relate this energy transfer to the energy required to create potential energy in a mechanical system (lift a chair to a tabletop; ice melts). The chair (and the liquid water) has more potential energy as a result of the work done (lifting the chair; separating the molecules from the solid state). When the chair is returned to the floor (the liquid water freezes), the potential energy (latent heat) is released. 6. The concept of relative humidity can be readily demonstrated with a collapsible plastic cup available from most drug stores. Half fill a fully extended cup with water, then ask the class what percent of the cup is filled. The analogy is that air has half the water it can hold at a particular temperature (50 percent relative humidity). Drop the upper ring on the cup, which reduces the capacity about 25 percent (depending on the particular cup used). This is analogous to reducing the air temperature, which reduces the air capacity to hold water. Because of dropping the ring, the cup is now 75 percent full (75 percent relative humidity). Drop the next upper ring and the cup is now 100 percent full, an analogy to saturated air and 100 percent humidity. Drop another ring and water will spill to the floor. Students will probably exclaim “rain!” to which you will ask if rain falls from a clear sky. Point out that the falling water represents condensation, which results in dew or the formation of a cloud. Rain falls from clouds, not clear air. 7. Relationships between heat, boiling, evaporation, condensation, air pressure, and atmospheric pressure can be demonstrated with an ordinary aluminum soda can and a burner. Rinse the can and pour several centimeters of water into the can, asking the class to note how much water you are pouring into the can. Using laboratory tongs, hold the can over a burner flame (with the palm side of your hand up) until the water boils for a minute. Point out that the visible mist coming from the can is not steam (gases are invisible) but is condensed water vapor just like the droplets in a cloud. Quickly invert the can in a large beaker full of cool water. The can will be immediately crushed with a loud pop. As you raise the can from the beaker of water, it will be obvious that more water is now running from the can than you poured in originally. Ask the students to explain what happened. 8. Additional demonstrations: (a) Measure the temperature of equal amounts of cold and warm water, then mix them together in an empty glass beaker. Take another temperature reading, showing the final temperature is nearly midway between the two initial temperatures. Discuss Q, m, c, T, and why the temperature was not exactly midway. (b) Commercial demonstration devices are available to compare the rates of heat conduction in metals. One such device, for example, has rods of iron, copper, aluminum, nickel, and brass that are set into a metal ring. A thumbtack is attached to the end of each bar with candle wax and the center ring is heated. (c) Commercial demonstration devices are available to show convection in a smoke box. One such device consists of a box with two openings with chimneys. One side is of a transparent material such as glass or plastic. A small lighted candle (or other heat source) is placed under one chimney and a smoke source is held just over the opening not above the candle. Convection currents can be observed as the smoke moves down one chimney, through the box, and up the other chimney over the heat source. For Class Discussions 1. A metal lid is stuck on a glass jar. What will help you open the jar? a. Heat it under running water. b. Cool it. c. Heat it under running water, then quickly cool it. d. Nothing; you are stuck. 2. When you add heat to a substance the temperature a. always increases. b. sometimes decreases. c. might stay the same. 3. The cheese on a hot pizza takes a long time to cool because it a. is not elastic. b. low specific heat. c. high specific heat. d. has a white color. 4. The great cooling effect produced by water evaporating comes from its high a. conductivity. b. specific heat. c. latent heat. d. transparency. 5. Anytime a temperature difference occurs, you can expect a. cold to move where it is warmer, such as cold moving into a warm house during the winter. b. heat movement from cold to warmer regions. c. heat movement from high temperature regions. d. no energy movement unless it is warm enough. 6. The specific heat of copper is roughly three times as great as the specific heat of gold. Which of the following is true? a. If the same amount of heat is applied to equal masses of gold and copper, the copper will become hotter. b. Copper heats up three times as fast as gold. c. A piece of copper stores three times as much heat as the same mass of gold at the same temperature. d. The melting temperature of copper is roughly three times that of gold. 7. Compared to sea level, water at a high altitude boils at a. a lower temperature. b. a higher temperature. c. 100°C 8. Turning up the heat under a boiling pot of water will a. shorten the cooking time by making it hotter. b. cause no change in the temperature of the water. c. require more time to cook because the water is boiling faster. 9. The heat death of the universe is a time in the future when the universe is supposed to a. have the same temperature. b. have a very high temperature everywhere in the universe. c. freeze Answers: 1a, 2c, 3c, 4c, 5c, 6c, 7a, 8b, 9a Answers to Questions for Thought 1. Temperature is a measure of the average kinetic energy of the molecules of a substance. Heat is the total internal energy of the molecules involved in an energy transfer. 2. As the temperature of a solid increases, the vibrations of the individual molecules become larger. When these vibrations become larger, the average distance between the molecules increases to accommodate these larger oscillations, and the solid expands. In a liquid or a gas, the individual molecules move faster as the temperature increases, and the collisions between individual molecules become more violent. Since the molecules are moving faster, they move farther apart as they travel a larger distance in the time between collisions. 3. Tight packing would tend to decrease the insulation value of glass wool because it would squeeze the wool together and give the heat more paths to travel. It is the presence of many small pockets of air, with unattached molecules, that gives glass wool and other similar insulation materials their insulating properties. 4. The vacuum between the walls prevents heat transfer by means of convection or conduction, while the silvered walls reflect radiated energy back into (or away from) the food, preventing energy transfer by radiation. 5. Cooler air is denser than warmer air. This denser air weighs more per volume than the warmer air and pushes the warmer air out of the way as it sinks down to its lowest level. The warmer, less dense air sits on top of the cooler air because it weighs less per volume. 6. Air is not very dense and conduction is not very efficient at transferring energy because the molecules are much farther apart than they are in solids or liquids. 7. The metal is more efficient at conducting heat away from your hand than wood, so it feels cooler because your hand senses heat leaving your body. 8. Condensation occurs when more vapor molecules are returning to the liquid state than are leaving the liquid state. When a water vapor molecule joins a group of liquid water molecules, it has to give up its latent heat of vaporization. This heat is transferred to the surrounding air molecules such as the air in the bathroom. 9. The 10 pounds of ice provide more cooling because as the ice undergoes the phase change into water, it absorbs heat. Ten pounds of ice water simply absorbs heat according to the value of its specific heat until it reaches room temperature and therefore absorbs less heat. 10. Water condenses out of the air onto the cooler surface of a glass because the air near the glass is cooled, lowering its temperature to the dew point. Since the warmer air can hold more water vapor in the summer, it would have more water vapor to condense. Therefore, you would expect more condensation in the summer. 11. One hundred degree Celsius steam contains more energy (540 cal/g) than 100°C water, so the steam burn would be more severe. 12. Cooling of air reduces the capacity of air to hold water vapor. Relative humidity is a ratio of water in the air to how much water it can hold. Thus a decrease of capacity increases the relative humidity, even when the amount of water vapor in the air is constant. Group B Solutions 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. For Further Analysis 1. Table salt takes the shape of a container and has a definite volume. These are the properties of a liquid. 2. Similarities – both heat and temperature involve internal energy, the energy of molecules. Difference – temperature involves an average of the random kinetic energy of the molecules in a body; heat involves the total internal energy, both kinetic and potential. Temperature of a body can be measured anytime; heat can be measured only by adding it or removing it from a body. 3. Clarifying the meaning of these words will show that answers will vary. 4. Analysis will show that copper is more practical and more energy efficient. 5. Developing criteria for evaluation will depend on the local climate. 6. Analysis will show that moving heat without the latent heat of vaporization would restrict the movement to that would take place by conduction, convection, or radiation, which would not be practical. 7. Evaluating and pursuing an alternative concept of the future will result in answers that vary, but should be logical. Chapter 5 Wave Motion and Sound Contents Forces and Elastic Materials Forces and Vibrations Describing Vibrations Waves Kinds of Waves Waves in Air Hearing Waves in Air A Closer Look: Ultrasonics Describing Waves Sound Waves A Closer Look: Hearing Problems Velocity in Air Refraction and Reflection Interference Energy and Sound Loudness Resonance Sources of Sounds Vibrating Strings Sounds from Moving Sources Overview Sound is an ideal way to present velocity, frequency, and wavelength as well as other measurable characteristics of all types of waves. Some attributes of sound are subjective. The same holds true of electromagnetic waves and others but to a lesser extent. Experience and training provide recognition of the variance present in sound waves, such as pitch, loudness, and quality. With proper demonstrations, you can determine the subjectivity of these measures and ascertain which students are musically inclined or trained. The chapter begins with vibrations and the characteristics of vibrations, then logically proceeds to the waves, or disturbances, that are produced by these vibrations. Characteristic wave terms are introduced, then used to explore some of the more interesting sound phenomena that students can relate to. Students are familiar with echoes but many have no conception of their cause. It is subjective if a listener hears a reverberation or an echo. There are many methods by which various musical sounds are produced. All sources of musical sounds are represented in the chapter, along with some theory behind some musical notes. The Doppler effect applies to all waves, but it is easier to discuss and to demonstrate with sound. Any constant sound that is in motion relative to the listener will produce the effect. Light also undergoes the Doppler effect, and this application is discussed in other chapters. Suggestions 1. To illustrate how vibrating matter creates sound, place one end of a ruler, metal strip, or meter stick on the edge of a desk. Hold the end down securely and pluck the opposite end. Practice will enable you to determine proper adjustments. Students should also relate the frequency of the sound produced to the length and rate of the vibrating object. The vibrations of a struck tuning fork are difficult to observe. They can be illustrated by holding a vibrating tuning fork in one hand and a sheet of paper in the other, lightly touching the vibrating end against the paper. A dramatic (and somewhat humorous) illustration is obtained by plunging a vibrating tuning fork into a beaker of water. 2. Vibrate your arm up and down as you hold a piece of chalk against the chalkboard, walking along the length of the board. Point out that the wave trace obtained is a graph of the vibration and not in itself a wave. Compare the wave traces obtained with more rapid vibration (greater frequency) and more energy (greater amplitude). Ask if anyone hears your vibrating arm, which leads to a discussion of audible waves. 3. A twenty foot long or so one-inch diameter helical spring is ideal to demonstrate both longitudinal and transverse waves. Shaking the coil from side to side produces a transverse wave, while holding the spring taut and releasing a group of bunched-up coils produces longitudinal waves. The motion of a string tied to the center part clearly shows the characteristic disturbances produced by these two kinds of waves. The coil can also be used later to illustrate standing waves at several different frequencies. 4. A “trick” demonstration involving forced vibrations maintains interest and helps teach the basic wave terms. State that you will illustrate what happens to a sound wave when it is inside a container. Strike a tuning fork and hold it in one hand with the rod end near the tabletop but covered by your hand. Tell the class that you are going to grasp a wave by its crest, then drop it into a beaker (or some other container). Act as if you are grasping a crest with your thumb and finger near the end of the vibrating tuning fork, then act as if you drop the wave into the container. The trick here is to touch the base of the rod firmly (but quietly) to the table top as you “release the wave” over the beaker with your other hand. With a little practice, it will seem as if there is an increase in loudness as you “drop” the wave. You may need to do this several times before someone catches on. You now have the class's undivided attention as you explain the concept of forced vibrations—that the tuning fork touched to the table starts the whole table vibrating. This is an ideal place to reinforce the conservation of energy concept. Compare the time that the whole table vibrates (with a louder sound) with the time that the tuning fork only vibrates (with a softer sound). 5. A signal generator can be used to illustrate the frequency ranges that students of various ages are able to hear. Two signal generators can be used to illustrate the beat concept. Hooking the generators to an oscilloscope provides a visual representation of what they are hearing. 6. Additional demonstrations: (a) Do the classic demonstration showing that the transmission of sound requires a medium. Place an electric bell inside a vacuum pump bell jar, evacuating the jar as the bell rings. Note the bell hammer continues vibrating as the sound level dies out. Stop the pump and restore air to the jar to hear the bell again. (b) Use citizen band radios or mobile telephones to measure the speed of sound over a distance. One student signals the beginning of a sound over the radio or telephone and a student at the other end starts a stopwatch at the signal, stopping it when the sound is heard. The signal used will depend on what is available locally. Measure the distance and make the correction for temperature. (c) Use a commercial demonstration device that varies the height of a flame with the frequencies of sounds. (d) Demonstrate resonance with commercial resonator boxes fitted with identical frequency sounding bars or tuning forks. (e) Demonstrate beats with commercial resonator boxes fitted with tuning forks that are 1 Hz apart in frequency. (f) Ask the class to close their eyes, then drop a previously hidden metal drill bit. Ask the class to open their eyes as you hold up the bit and a board, asking which you dropped. Explain the concept of natural frequency. For Class Discussions 1. How can astronauts talk to one another on the Moon, where there is no air? a. Air is not needed if one is standing with heavy boots on something solid. b. By radio transmissions. c. By staying inside a pressurized space suit. d. This is impossible; they could not talk. 2. An airplane moves over your location at a speed greater than the speed of sound, so you should hear a. nothing since the plane was moving faster than sound. b. nothing, then a sonic boom, followed by the distant sound of engines. c. a sonic boom only at the instant the plane breaks the sound barrier, then silence. d. two sharp sonic booms, followed by silence. 3. Does a high frequency sound travel faster than a low frequency sound? a. Yes. b. No. c. Sometimes it does, depending on the conditions. 4. What happens if a source is moving toward you at a high rate of speed? a. The sound will be traveling faster than from a stationary source. b. The sound will be moving faster only in the direction of travel. c. A higher frequency will be heard by you only. d. A higher frequency will be heard by all observers in all directions. 5. What happens if you are moving at a high rate of speed with a sound source? You will hear a. a higher frequency than people you are approaching. b. the same frequency as people you are approaching. c. a frequency that is the same as from the source when it is not moving. 6. During a track and field meet, the time difference between seeing the smoke from a starter's gun and hearing the “bang” would be less a. on a warmer day. b. on a cooler day. c. if a more powerful shell is used. d. if a less powerful shell is used. 7. At the same temperature, the gas that would carry a sound with the greatest velocity would be a. oxygen. b. hydrogen. c. carbon dioxide. d. The velocity would be the same in each gas. 8. As you go up a mountain the air becomes less dense, and with everything else constant, the overall velocity of sound a. increases steadily with increasing altitude. b. decreases steadily with increasing altitude. c. increases slowly at first, then rapidly with increasing altitude. d. does not change. Answers: 1b, 2b, 3b, 4c, 5c, 6a, 7b, 8d. Answers to Questions for Thought 1. A wave is a disturbance that moves through a medium such as a solid or the air. 2. No, because there is no force acting on the air to return it to its original position after the wave has dislocated it. 3. Loosen. Since the beat frequency depends upon the difference between the two frequencies, you wish to go in the direction of fewer beats per second. 4. There is no medium such as air to transmit sound on the moon. 5. The condition where the frequency of an external force matches the frequency of an object is resonance. 6. Gas molecules have a greater kinetic energy and move faster in warm air than in cold air. These molecules are able to transfer an impulse from one molecule to the next faster. 7. Longer wavelengths have lower frequencies. Since the velocity of sound is equal to the product of the frequency times the wavelength, the velocity is a constant. 8. The energy of the sound wave is eventually dissipated into heat. 9. The presence and strength of various overtones determine the characteristic sound of a musical note. 10. The sonic boom is from the building up of a pressure wave in front of the moving aircraft. Since this pressure wave is present as long as the plane is moving faster than the speed of sound, the aircraft continually makes a sonic boom. 11. An echo is the return of a sound wave to its source after the wave has been reflected. 12. They all produce standing waves or resonance in whatever is oscillating. Group B Solutions 1. 2. 3. 4. 5. 6. 7. 8. Air: Water: For Further Analysis 1. If the speed of sound decreased with frequency, all the musical notes would become separated with higher frequencies arriving first. 2. Similarities – both longitudinal and transverse waves are mechanical disturbances that move through matter. Differences – In a longitudinal wave the disturbance acts in a direction parallel to the direction the wave is moving; In a transverse wave the disturbance acts in a direction perpendicular to the direction the wave is moving. Examples will vary. 3. Possible explanation: The empty room has more echoes that bounce around and garble the sound. 4. Starting with two distinct tones, a fast beat develops, which slows as the frequencies approach each other while tuning the instrument. It slows to a single pitch when tuned. 5. Two identical sounds moving through the air that are 180 degrees out of phase will cancel each other. 6. Vibrations are the sources of all sounds, which is supported by observation as well generalizations. 7. Unlike objects, sounds waves have the ability to pass through one another. 8. If the marching band happens to match the natural frequency of the bridge, resonance could start the bridge swaying and possibly cause damage Chapter 6 Electricity Contents Electric Charge Measuring Electrical Charge Measuring Electrical Force Electric Current Resistance AC and DC A Closer Look: Hydrogen and Fuel Cells The Electric Circuit Electrical Power and Work A Closer Look: Household Circuits and Safety Magnetism Moving Charges and Magnetic Fields Magnetic Fields Interact A Moving Magnet Produces an Electric Field A Closer Look: Solar Cells Overview This chapter presents the basic concepts of charge and current, and students develop the understanding that electricity is another form of energy perfectly analogous to other forms of energy. It begins by discussing static electricity, which is basic to any study of electrical phenomena, and some very basic atomic theory. Static electric charges are “at rest” but are produced by an excess or deficiency of electrons brought about by friction, generally between unlike insulators. Electric current is defined as the flow of charge rather than the flow of electrons. Current in a conductor approaches the velocity of light, a rate of movement not possible for an electron moving from atom to atom under ordinary circumstances. To move an electric charge against the electric field of other electrons requires work, creating electric potential energy that is in every way analogous to gravitational potential energy. In either case work is done against a field to develop the potential energy. The same quantity of work is realized (disregarding losses) when the potential energy is utilized at some place in a circuit. When a joule of work is done on a coulomb of charge to move it against an electric field, the difference in potential between the points is a joule per coulomb. This is called a volt. Students seem to have two difficulties with this concept, (1) that a coulomb is a quantity of charge and (2) that volt means work per charge. They must be helped over this difficulty and understand that a current of one coulomb per second is called an ampere. Understanding voltage and amperage is necessary before they can be combined to discuss resistance (Ohm's law), work, and power meaningfully. Avoid the temptation to discuss the differences in resistors, voltage, and current in series and parallel circuits unless students are majoring in programs of a technical nature. Magnetism and its properties are of interest to students, and this chapter presents the major characteristics of magnets and magnetic fields. The fundamental relationship between electric and magnetic fields is discussed and then applied to many common phenomena in the environment and to technological devices. The generation and conditioning of alternating current between its source and its consumption is considered a primary way of transferring energy long distances. The active cutting of the imaginary lines of force that surround a magnet by a conducting wire causes current to flow in the conductor. If the direction of the moving magnet is reversed, the current flow also reverses. This constant reversal of the moving magnet produces alternating current. Home appliances operate on 60 hertz alternating current. A coil of wire rotating about the poles of a magnet also would produce alternating current. Devices that convert mechanical energy into electrical energy are called generators. Motors convert electrical energy into mechanical energy. Except for efficiency loss due to resistance and other uncontrollable factors, a combination of the two devices could produce a seemingly endless supply of electricity. To reduce energy loss from resistance through heating, alternating current is stepped up by transformers, since the current is inversely proportional to voltage if power is held constant. Alternating current is stepped back down before it enters the home. Direct current cannot be stepped up efficiently without complex circuitry, it cannot be stepped down as easily as alternating current, and it is more costly to generate than alternating current. Suggestions 1. Demonstrate static electric phenomena with hard rubber rods, cat fur, glass rods, and silk cloth. Use electroscopes and pith balls to show accumulated charges (see the Laboratory Manual). If you are in an area of high humidity, use an electrophorus in place of the rods, fur, and silk cloth. Demonstrate how to generate static electric charges, the identity of the charge that accumulates on an electroscope, how like and opposite charges affect each other, and how static electricity accumulates on the surface of a conductor. 2. Bring a negatively charged rod near a small but steady stream of running water and note how the water is attracted. Ask the class what the charge of the water appears to be. Bring a positively charged rod near the stream of water and note the effect. These observations point out how both electric charges affect the neutral charge of the water. 3. Demonstrate Ohm’s law with resistors of known values connected to a large demonstration ammeter and voltmeter. Take readings quickly to avoid heating (and changing the value of the resistors), and let the class do the calculations. 4. The relationships between amperage, voltage, work, and power are popular among students when applied to the cost of using certain electrical appliances. For example, how much does it cost to use a 1,300 watt hair dryer for 15 minutes? 5. The magnetic field of a magnet can be demonstrated to a large class with an overhead projector. Place a clear plastic sheet over a bar magnet. Sprinkle iron filings on the sheet and tap the sheet with your finger. Arrangements that show attraction and repulsion between two magnets can also be dramatically illustrated in this manner. 6. Use a ring stand and clamps to support a length of stiff copper wire vertically through the center of a flat piece of cardboard. Sprinkle iron filings around the wire, then connect the ends to a dry cell or power supply. Tap the cardboard, then disconnect the power supply immediately when the concentric circles form. Use the left-hand rule to determine polarity of the magnetic field. 7. Magnetize a large paper clip (or soft iron nail) by placing it on one end of a strong bar magnet, then tapping it. Remove the clip from the magnet and show that it is magnetic by bringing it near a second, unmagnetized paper clip. Throw the magnetic paper clip (nail) hard against the floor, then check it again with a different paper clip to show that it has lost most of its magnetism. Explain the entire procedure with the magnetic domain concept. 8. Connect a coil of wire to a large demonstration galvanometer and move the coil toward and away from the magnetic field of a strong magnet (e.g., from a speaker). Show that the needle of the galvanometer moves in one direction when the wire coil moves toward the magnet and in the other direction when the wire coil moves away from the magnet. Illustrate that the movement is relative by now moving the magnet toward and away from the stationary coil of wire. 9. The operation of an electric motor or generator can be demonstrated with the use of a St. Louis motor, a variable direct current power supply, and a small light bulb. The adjustments of the magnets are critical but easily accomplished. Check to be sure that the bar magnets are installed with the poles of each magnet turned opposite the poles of the other. 10. Additional demonstrations: (a) Demonstrate the nature of static electricity with suspended pith balls, electroscopes, a rubber rod rubbed with cat’s fur, and a glass rod rubbed with silk. Suspend inflated rubber balloons with thread, then give them a negative charge by rubbing with cat’s fur. (b) Use a small hand-turned generator to demonstrate that work is done to generate electric current. Vary the speed to show that the current generated is directly proportional to the work done. (c) Demonstrate Ohm’s law using a large ammeter and voltmeter with resistors of known values. (d) Show the relationship between a magnetic field and polarity by sprinkling iron filings on a transparent sheet of plastic over a magnet. Ask students to predict shape of fields around horseshoe magnets, bar magnets, and others, then check their predictions. (e) Display rheostats, relays, meters, telephone parts, speakers, and other devices that work with electromagnets. (f) Display a simple St. Louis type motor connected to a dry cell to show how current moving through the motor can do work. Replace the dry cell with a galvanometer, then give the armature several whirls to show that current can be generated by doing work. For Class Discussions 1. Three pieces of amber are hung from threads, then each is charged by rubbing piece A with fur, piece B with silk, and piece C with nylon. It is now observed that piece A and B repel each other and that Piece B and C repel each other. This means a. pieces A and C have opposite sign charges. b. pieces A and C have the same sign charge. c. piece B does not have a charge. d. all three pieces have the same sign charge. 2. If the pieces of amber in the previous question acquired a positive charge, the each of the rubbing materials a. acquired an equal negative charge. b. acquired a positive charge, too. c. acquired a charge, but not equal and opposite to the amber. 3. When a substance acquires a charge from an excess of electrons, would the mutual repulsion repel them uniformly throughout the substance? a. Yes. b. No. 4. Considering the separation of charges, is it possible to have a high voltage without any current? a. Yes. b. No. 5. Which of the following is a measure of electrical work? a. kilowatt b. C c. kWh d. C/s 6. A bird lands on a non-insulated, 80,000 volt high line. Will it receive a shock? a. Yes. b. No. 7. When an object acquires a negative charge it actually a. gains mass. b. loses mass. c. has a constant mass. 8. A positive and a negative charge are initially 2 cm apart. What happens to the force on each as they are moved closer and closer together? It a. increases while moving. b. decreases while moving. c. remains constant. 9. The current in the secondary coil of a transformer is produced by a a. varying magnetic field. b. varying electric field. c. constant magnetic field. d. constant electric field. 10. The nature of the force that enables an electric motor to do its job is a. electric. b. magnetic. c. electromagnetic Answers: 1d, 2a, 3b (just on the surface), 4a, 5c, 6b (no voltage difference on bird), 7a, 8a, 9a, 10b. Answers to Questions for Thought 1. The balloon has a net charge as a result of being rubbed. When the balloon is brought near a wall, the net charge on the balloon moves electrons around in the wall. As a result, a small region near the balloon has a net charge of opposite sign than the balloon. The overall wall is still electrically neutral; there are now small regions that have net charges. The force from the opposite signed charges in the balloon and the wall causes the balloon to stick to the wall. There it will stay until enough charge has leaked away to cancel the charge on the balloon. 2. Excess charge is building up on your body from the carpet as you walk across it. When a metal object is touched, the charge flows out of your body, through the lower resistance of the metal. It finds a path into the ground, which supplied the charge to make up for what you removed from the carpet. 3. An electron carries a negative charge and can be moved to and from objects relatively easy. Since electrons cannot be divided into parts that can move separately, the smallest charge it is possible to have or to move is the charge of one electron. The charge of one electron is referred to as the fundamental charge. 4. The electrons move rapidly inside a wire bouncing against each other like molecules in a gas. Since so many collisions occur, an individual electron cannot move from one end of a wire to another rapidly. The electric field inside the wire, which exerts a force on the electrons, can move rapidly though the wire because it does not require something to carry it. The force from the electric field gives the electrons a drift velocity, which constitutes a current. 5. A kWh is work multiplied by time. Since a watt is energy per time, a kWh is a unit of energy or work. 6. In direct current (dc), the current always flows in a single direction. In alternating current (ac), the flow of current changes direction with a regular frequency. 7. A magnetic pole is a region where the force of magnetic attraction seems to be concentrated. The pole that seeks, or points to a generally north direction, is called a north pole, and the other pole is called a south pole. 8. In an unmagnetized piece of iron, the magnetic domains are pointing in random directions such that the net field is zero. In a magnetized piece of iron, most of the domains are aligned so that their fields add to make a larger field. 9. If the voltage is small, the current is large for a particular amount of power. Increasing the voltage decreases the current. Large currents promote many collisions of electrons inside the wire with other electrons and positive ions. Each collision takes energy from the electric field, diverting it into kinetic energy of the positive ions and heating the wire, so there are fewer power losses with lower currents. Thus a higher voltage means less power loss since the current is lower. 10. The electromagnetic generator uses induction to generate a current in loops of wire moving in a magnetic field. Electrons in the loops of wire are forced toward one end by the magnetic field, which sets up a potential difference. 11. The earth's north magnetic pole is actually a magnetic south pole located near the geographic North Pole. 12. The electron is moving, creating its own magnetic field. The interaction between the magnetic field of the electron and the external magnetic field creates a force on the electron, causing it to move. Group B Solutions 1. 2. 3. 4. 5. (a) (b) 6. 7. 8. (a) (b) 9. $2.69/day 30 day/month = $80.70/month 10. Since 2 times 10.83 A is 21.66 A, two devices will clearly overload a 15 A circuit. 11. (a) (b) 12. (a) (b) (c) For Further Analysis 1. The generation of electricity involve much more than the movement of electrons, including the establishment of electrical potential between two points in a circuit, the creation of an electric field as a result, and the movement of electrons as caused by the field. 2. Similarities – both are electric fields moving through a conductor, which carries energy. Differences – In alternating current the field moves electrons back and forth; in direct current the field moves electrons in one direction. 3. Answers will vary, but it does appear that people should be warned about amps, not volts. 4. Answers will vary, but should be logical and supported by identified insights, beliefs, and arguments. 5. Answers will vary, but should show the understanding that solar energy is free, but the bucket to catch it is very expensive. 6. Similarities – both are fields, which is a change of the condition of space around an electrical charge or magnetic field. Differences – an electrical field is found around a charge; a magnetic field is found around a magnet or current carrying wire. Chapter 7 Light Contents Sources of Light Properties of Light Light Interacts with Matter Reflection Refraction A Closer Look: Optics Dispersion and Color A Closer Look: The Rainbow Evidence for Waves Interference Polarization Evidence for Particles Photoelectric Effect Quantization of Energy The Present Theory A Closer Look: Compact Discs Relativity Special Relativity General Relativity Overview Electromagnetic waves have many characteristics of interest, including their methods of production, their properties, and their uses. A review of wavelength, frequency, and wave velocity as they apply to sound relates these concepts to electromagnetic waves. Sounds are produced by vibrating matter and electromagnetic waves are produced by an accelerated charge. The energy involved in this acceleration results in the various parts of the electromagnetic spectrum, with visible light making up only a small part of the total spectrum. After a consideration of the sources of light, the properties of light are considered from a light ray treatment of the laws of reflection and refraction. The early controversy concerning the nature of light is discussed next, leading to evidence for both the particle theory and the wave theory of light. The properties of light that serve as evidence that light is a wave (reflection, refraction, diffraction, interference, and polarization) are considered, followed by the evidence that light is a stream of particles (photoelectric effect and quantization of energy). This leads to the present theory, that light has both wave and particle properties at the same time. Students enjoy a controversy, and this one is based mostly on their everyday notion that light must be either one or the other since everything else in their everyday world appears to be one or the other. However, there is no reason to identify light as being either waves or particles. The modern model of light (and subatomic particles to be discussed in a future chapter) considers light to have both properties at the same time. The model is just that, a model, and it is used because it seems to explain what is observed. Suggestions 1. Discussion and demonstrations of light and other electromagnetic radiations that are not visible are suitable to introduce this chapter. A demonstration of infrared can be accomplished by heating a large piece of iron in a burner flame for several minutes, then holding it close to a radiometer. The radiometer vanes spin more rapidly from the infrared radiation emitted by the hot iron. Explain that the radiometer contains some air. The black sides of the vanes absorb infrared radiation, which shows up as a slight temperature increase on the blackened surface. The silvered sides of the vanes, on the other hand, reflect the radiation without absorbing it. Air particles next to the warm, black surface collide with the surface and rebound with more energy than other particles on the silvered side of the vanes. As a result of the many impulses received from many, many particles on the blackened side the vanes spin in a direction consistent with this explanation. 2. Reflection and refraction can be effectively demonstrated at the same time by filling a small aquarium with water that has been made murky by mixing in some chalk dust. A strong beam of light is aimed downward at a mirror, which reflects the light upward through the aquarium water. A darkened room and chalk dust (or smoke from a smoke source) makes the light beam visible in the air. The same setup can be used to show total internal reflection by directing the beam at increasing angles until it is totally reflected from the inner surface. 3. A prism can be used to cast the spectrum of sunlight on a wall, leading to a discussion of refraction, dispersion, and color. 4. Diffraction can be illustrated with a classroom spectroscope or small squares of plastic replica diffraction gratings. Students should look toward a light source with the grating, either to the right or left of center as they rotate the grating until they see a spectrum. A cloth handkerchief that is stretched tightly in front of a laser beam will show a good diffraction pattern. 5. The mnemonic of “ROY G. BIV” helps students remember the spectrum of major colors, although indigo is not totally accepted as a color (has any one seen indigo in a rainbow?). 6. Additional demonstrations: (a) Hold a radiometer near a heated iron block supported on a ring stand to confirm the existence of electromagnetic radiations that are not visible. Discuss how infrared warms the black sides of the vanes, causing air molecules to rebound with more of a “kick” than they do from the silvered side. This “kick” gives the vanes their motion. (b) Show a spectrum produced by a prism. Mount the prism so the spectrum is cast on a wall. (c) Fill a large beaker or aquarium with water and mix in some chalk dust. Aim a narrow beam of light from a slide projector (or laser) at some angle to demonstrate refraction. (d) Cut inexpensive plastic replica grating into 5 cm squares. Instruct students to handle the squares only by the edges to avoid “oiling the gratings.” Tell them to hold the grating close to one eye, viewing a light source through the grating. They should rotate the grating in 90 degree intervals until a spectrum or spectrum lines are seen. These are usually to the left or right of center. Neon, mercury, and sodium lights are good sources for a line spectrum. For Class Discussions 1. Light from an incandescent source comes from a. glowing atoms with high kinetic energy. b. fast moving electrons, which tend to glow. c. accelerating electrons. d. oxidation at the atomic level. 2. One of the following behaviors of light means it must consist of waves. a. interference. b. photoelectric effect. c. refraction. d. reflection. 3. From a space station on the Moon you would be able to tell planets from the stars because stars twinkle and planets do not. a. Yes. b. No. c. Who knows? 4. If you look straight down and see a fish under the water, it appears to be a. deeper in the water than it really is. b. closer to the surface than it really is. c. where it actually is. 5. If you look at a fish that is some distance from you, it appears to be a. deeper in the water than it really is. b. closer to the surface than it really is. c. where it actually is. 6. A fish under the water always appears to be a. larger than it really is. b. smaller than it really is. c. the size it actually is. 7. Can you can distinguish a mirage of water from real water by using Polaroid sunglasses? a. Yes. b. No. Answers: 1c, 2a, 3b (no atmosphere; no twinkling), 4b, 5b, 6a, 7a (light from water is reflected and thus polarized; a mirage is refracted light and is not polarized). Answers to Questions for Thought 1. The frequency of the wave. 2. Diffraction supports a wave theory of light. A wave front diffracts only if the opening is about the same size as the wavelength. A wave front passing through a large opening will generate wavelets that retain the shape of the wave. A small opening will let the wave generate only one wavelet, which moves out in all directions from the opening. 3. Blue light carries more energy. No, because the energy difference between these two colors is very small. The number of photons determines the intensity of the light. 4. The photoelectric effect supports the particle model, because the effect depended on the frequency and not the intensity of the light. This means that particles of certain energy were creating the effect and not the absorption of a wave. 5. The energy in the light is transferred to the absorbing material. 6. The bluish star is at a higher temperature because higher temperature objects emit more photons of shorter wavelengths (and higher energies) than objects with lower temperatures. 7. Internal reflection occurs when the angle of refraction is equal to or greater than 90°. This occurs more in the diamond because the critical angle depends upon the ratio of the indices of refraction of the air to the stone. The diamond has a very high index of refraction. 8. The hot air above the surface of the highway has a lower index of refraction than the air above it, so light striking the warmer air is refracted upward. This light is interpreted by your brain to be reflected light. 9. Look at the clear sky at an angle of about 90° from the sun. The scattered light from this direction is partially polarized, so if the sky appears to darken as the glasses are turned, the glasses are polarized. If you have a pair of polarizing sunglasses, turn a lens of the unknown pair over a stationary lens of the known, polarizing pair. If the unknown pair is polarizing, light coming through the lens will appear to darken then brighten. 10. Reflected light is slightly polarized. When the polarization of the reflected light is parallel with the polarizing sunglasses, it appears brighter. When the polarization of the light is perpendicular with the sunglasses it appears darker. 11. Planck's findings were revolutionary because they meant that vibrating molecules could only have a fixed amount of energy that could only be multiples of a certain amount called the quanta of energy. All previous experience led everyone to believe that energy could exist in a continuous range of amounts. 12. Neither model totally explains all behavior of light, while a combination of these two models, using each when it is useful, explains all the behaviors of light. There is nothing in conventional experience that behaves as a particle in some situations and a wave in different situations, so the concept is hard to visualize. Group B Solutions 1. (a) (b) (c) 2. 3. 4. 5. This is the index of refraction for glass. 6. 7. This is less than the required photon energy, so no chemical reaction occurs in the paper. 8. (a) (b) This is in the frequency range of red light. 9. 10. For Further Analysis 1. Answers will vary, but should describe reflection as light bouncing from a surface and refraction as the bending of light when passing between two different optical media. 2. Answers will vary in this Socratic discussion attempt to clarify the concept of convection. 3. Answers will vary in this attempt to reason dialogically about the nature of light. 4. Answers will vary in the attempt to compare and contrast the used of convex and concave lenses. 5. E is a quantity of energy, which is a particle property, and f is frequency, a wave property. The equation states they are equal, so it equates particle and wave properties. 6. Same – both are electromagnetic radiation that travels at the speed of light. Different – radio waves are longer wavelength. Instructor Manual for Integrated Science Bill W. Tillery, Eldon D. Enger , Frederick C. Ross 9780073512259
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