Text 1. Understanding physics

Text 1. Understanding physics

(from The Feynman lectures on physics mainly electromagnetism and matter,
by Richard P.Feynman and, Addison-Wesley Publishing company Inc.,
Reading, 1964)

1) The physicist needs a facility in looking at problems from several points of view. The exact analysis of real physical problems is usually quite complicated, and any particular physical situation may be too complicated to analyze directly by solving the differential equation. But one can still get a very good idea of the behavior of a system if one has some feel for the character of the solution in different circumstances. Ideas such as the field lines, capacitance, resistance, and inductance are, for such purposes, very useful. So we will spend much of our time analyzing them. In this way we will get a feel as to what should happen in different electromagnetic situations. On the other hand, none of the heuristic models, such as field lines, is really adequate and accurate for all situations. There is only one precise way ofpresenting the laws, and that is by means of differential equations. They have the advantage of being fundamental and, so far as we know, precise. If you have learned the differential equations you can always go back to them. There is nothing to unlearn.

2) It will take you some time to understand what should happen in different circumstances. You will have to solve the equations. Each time you solve the equations, you will learn something about the character of the solutions. To keep these solutions in mind, it will be useful also to study their meaning in terms of field lines and of other concepts. This is the way you will really "understand" the equations. That is the difference between mathematics and physics. Mathematicians, or people who have very mathematical minds, are often led astray when "studying" physics because they lose sight of the physics. They say: "Look, these differential equations—the Maxwell equations—are all there is to electrodynamics; it is admitted by the physicists that there is nothing which is not contained in the equations. The equations are complicated, but after all they are only mathematical equations and if I understand them mathematically inside out, I will understand the physics inside out." Only it doesn't work that way. Mathematicians who study physics with that point of view —and there have been many of them—usually make little contribution to physics and, in fact, little to mathematics. They fail because the actual physical situations in the real world are so complicated that it is necessary to have a much broader understanding of the equations.

3) What it means really to understand an equation—that is, in more than a strictly mathematical sense—was described by Dirac. He said: "I understand what an equation means if I have a way of figuring out the characteristics of its solution without actually solving it." So if we have a way of knowing what should happen in given circumstances without actually solving the equations, then we "understand" the equations, as applied to these circumstances. A physical understanding is a completely unmathematical, imprecise, and inexact thing, but absolutely necessary for a physicist.

4) Ordinarily, a course like this is given by developing gradually the physical ideas—by starting with simple situations and going on to more and more complicated situations. This requires that you continuously forget things you previously learned—things that are true in certain situations, but which are not true in general. For example, the "law" that the electrical force depends on the square of the distance is not always true. We prefer the opposite approach. We prefer to take first the complete laws, and then to step back and apply them to simple situations, developing the physical ideas as we go along. And that is what we are going to do.

Text 2. Cooling at night by radiation

(from Conceptual Physics by Paul G. Hewitt, City College of San Francisco,
Pearson International Edition, 2006)

1) Bodies that radiate more energy than they receive become cooler. This happens at night when solar radiation is absent. An object left out in the open at night radiates energy into space and, because of the absence of any warmer bodies in the vicinity, receives very little energy from space in return. Thus, it gives out more energy than it receives and becomes cooler. But if the object is a good conductor of heat—such as metal, stone, or concrete —heat conducts to it from the ground, which somewhat stabilizes its temperature. On the other hand, materials such as wood, straw, and grass are poor conductors, and little heat is conducted into them from the ground.

2) These insulating materials are net radiators and get colder than the air. It is common for frost to form on these kinds of materials even when the temperature of the air does not go down to freezing. Have you ever seen a frost-covered lawn or field on a chilly but above-freezing morning before the Sun is up? The next time you see this, notice that the frost forms only on the grass, straw, or other poor conductors, while none forms on the cement, stone, or other good conductors.

3) Hard-core gardeners will cover their favorite plants with a tarp when they expect a frost. The plants radiate just as before, but now they are receiving radiant energy from the tarp rather than from the dark night sky. Because the tarp radiates as an object at the temperature of the surroundings rather than at the temperature of the cold, dark sky, frost doesn't form on the plants' leaves. This is the same reason that plants on a covered porch won't have frost on them, whereas plants exposed to the open sky will.

4) Earth itself exchanges radiation with its surroundings. The Sun is a dominant part of Earth's surroundings during the day. The sun lit half of the Earth absorbs more radiant energy than it emits. At night, if the air is relatively transparent, Earth radiates more energy to deep space than it gets back. As the Bell laboratories researchers Arno Penzias and Robert Wilson learned in 1965, outer space has a temperature—about 2.7 К (2.7 degrees above absolute zero). Space itself emits weak radiation characteristic of that low temperature.

 

Text 4. Solar energy

(from Conceptual Physics by Paul G. Hewitt, City College of San Francisco, Pearson International Edition, 2006)

1) If you step from the shade into the sunshine, you're noticeably warmed. The warmth you feel isn't so much because the Sun is hot, for its surface temperature of 6000 С. is no hotter than the flames of some welding torches. We are warmed principally because the Sun is so big. As a result, it emits enormous amounts of energy, less than one part in a billion of which reaches Earth.

2) Nonetheless, the amount of radiant energy received each second over each square meter at right angles to the Sun's rays at the top of the atmosphere is 1400 joules (1.4 kj). This input of energy is called the solar constant. This is equivalent, in power units, to 1.4 kilowatts per square meter (1.4 kW/m²). The amount of solar power that reaches the ground is attenuated by the atmosphere and reduced by nonperpendicular elevation angles of the Sun. Also, of course, it ceases at night. The solar power received in the United States, averaged over day and night, summer and winter, is about 13% of the solar constant (0.18 kW/ m²).

3) This amount of power, falling on the roof area of a typical American house, is twice the power needed to comfortably heat and cool the house year-round. More and more homes are using solar energy for space heating and water heating. Also gaining in popularity are photovoltaic shingles used in roofing buildings. Solar heating needs a distribution system to move solar energy from the collector to the storage or living space. When the distribution system requires external energy to operate fans or pumps, we have an active system. When the distribution is by natural means (conduction, convection, or radiation), we have a passive system. At the present time, passive systems are essentially problem-free and serve as an economical supplement to conventional heating—even in the northern parts of theUnited States and in Canada.

4) On a larger scale, the problems of utilizing solar power to generate electricity are greater. First, there is the fact that no energy arrives at night. This calls for supplemental sources of energy or efficient energy-storage devices. Variations in weather, particularly in cloud cover, produce a variable energy supply from day to day and from season to season. Even in clear daylight hours, the Sun is high in the sky for only part of the day. At the time of this writing, solar-energy collecting and concentration systems, whether arrays of mirrors or photovoltaic cells, are not yet competitive in cost with electrical power generated by conventional power sources. Projections indicate that the story may be different later in the twenty-first century.

 

Text 6. Condensation

(from Conceptual Physics by Paul G. Hewitt, City College of San Francisco, Pearson International Edition, 2006)

1) The opposite of evaporation is condensation—the changing of a gas to a liquid. When gas molecules near the surface of a liquid are attracted to the liquid, they strike the surface with increased kinetic energy and become part of the liquid. In collisions with low-energy molecules in the liquid, excess kinetic energy is shared with the liquid, increasing the liquid temperature. Condensation is a warming process.

2) A dramatic example of the warming that results from condensation is the energy released by steam when it condenses —a painful experience if it condenses on you. That's why a steam burn is much more damaging than a burn from boiling water of the same temperature; the steam releases considerable energy when it condenses to a liquid and wets the skin. This energy release by condensation is utilized in steam heating systems.

3) Steam is water vapor at a high temperature, usually 100 degrees Centigrade or more. Cooler water vapor also releases energy when it condenses. In taking a shower, for example, you're warmed by condensation of vapor in the shower region—even vapor from a cold shower—if you remain in the moist shower area. You quickly sense the difference if you step outside. Away from the moisture, net evaporation takes place quickly and you feel chilly. But, if you remain in the shower stall, even with the water turned off, the warming effect of condensation counteracts the cooling effect of evaporation. If as much moisture condenses as evaporates, you feel no change in body temperature. If condensation exceeds evaporation, you are warmed. If evaporation exceeds condensation, you are cooled. So now you know why you can dry yourself with a towel much more comfortably if you remain in the shower stall. To dry yourself thoroughly, you can finish the job in a less moist area.

4) Spend a July afternoon in dry Tucson or Phoenix where evaporation is appreciably greater than condensation. The result of this pronounced evaporation is a much cooler feeling than you would experience in a same-temperature July afternoon in New York City or New Orleans. In these humid locations, condensation noticeably counteracts evaporation, and you feel the warming effect as vapor in the air condenses on your skin. You are literally being bombarded by the impact of H2O molecules in the air slamming into you. Put more mildly, you are warmed by the condensation of vapor in the air upon your skin.

 

Condensation is the phase change of water __________ into a _________. During the condensation on process, water __________ lose the 600 cal/gm of latent heat that were added during the evaporation process. When latent heat is ____(called the @latent heat of fusion@), it is converted into sensible heat which warms the surrounding air. Condensation takes place in the presence of condensation nuclei and when the air is nearly saturated.

Water vapor is darting around so fast in the air that the molecules tend to bounce off one another without bonding. Even if a few pure water molecules were to _____ and bind together, the -------- TENSION CREATED BY SUCH A TINY SPHERE IS SO GREAT THAT IT IS EXTREMELY DIFFICULT FOR ADDITIONAL WATER MOLECULES TO BECOME INCORPORATED INTO THE MASS.

 

Text 7. Wireline channels

(from Fundamentals of Electrical Engineering I by Don Johnson, the Connexions Project, Rice University, Houston TX, 2002)

1) Wireline channels were the first used for electrical communications in the mid-nineteenth century for the telegraph. Here, the channel is one of several wires connecting transmitter to receiver. The transmitter simply creates a voltage related to the message signal and applies it to the wire(s). We must have a circuit -a closed path -that supports current flow. In the case of single-wire communications, the earth is used as the current's return path. In fact, the term ‘ ground’ for the reference node in circuits originated in single-wire telegraphs. You can imagine that the earth's electrical characteristics are highly variable, and they are. Single-wire metallic channels cannot support high-quality signal transmission having a bandwidth beyond a few hundred Hertz over any appreciable distance.

2) Consequently, most wireline channels today essentially consist of pairs of conducting wires Figure 6.1, and the transmitter applies a message-related voltage across the pair. How these pairs of wires are physically configured greatly affects their transmission characteristics.One example is ‘ twisted pair’, wherein the wires are wrapped about each other. Telephone cables are one example of a twisted pair channel. Another is ‘ coaxial cable’, where a concentric conductor surrounds a central wire with a dielectric material in between. Coaxial cable, fondly called "co-ax" by engineers, is what Ethernet uses as its channel. In either case, wireline channels form a dedicated circuit between transmitter and receiver.

3) As we shall find subsequently, several transmissions can share the circuit by amplitude modulation techniques; commercial cable TV is an example. These information-carrying circuits are designed so that interference from nearby electromagnetic sources is minimized. Thus, by the time signals arrive at the receiver, they are relatively interference- and noise-free.

4) Both twisted pair and co-ax are examples of ‘ transmission lines’, which all have the circuit model shown in Figure 6.2 for an infinitesimally small length. This circuit model arises from solving Maxwell's equations for the particular transmission line geometry.

5) The so-called distributed parameter model for two-wire cables has the depicted circuit model structure. Element values depend on geometry and the properties of materials used to construct the transmission line. The series resistance comes from the conductor used in the wires and from the conductor's geometry. The inductance and the capacitance derive from transmission line geometry, and the parallel conductance from the medium between the wire pair. Note that all the circuit elements have values expressed by the product of a constant times a length.

 

Text 9. Lightning

(from The Feynman lectures on physics mainly electromagnetism and matter, by Richard P.Feynman, Addison-Wesley Publishing company Inc., Reading, 1964)

1) The first evidence of what happens in a lightning stroke was obtained in photographs taken with a camera held by hand and moved back and forth with the shutter open—while pointed toward a place where lightning was expected. The first photographs obtained this way showed clearly that lightning strokes are usually multiple discharges along the same path.

2) Later, the "Boys" camera, which has two lenses mounted 180° apart on a rapidly rotating disc, was d eveloped. The image made by each lens moves across the film —the picture is spread out in time. If, for instance, the stroke repeats, there will be two images side by side. By comparing the images of the two lenses, it is possible to work out the details of the time sequence of the flashes. Figure 9-14 shows a photograph taken with a "Boys" camera.

3) We will now describe the lightning. Again, we don't understand exactly how it works. We will give a qualitative description of what it looks like, but we won't go into any details of why it does what it appears to do. We will describe only the ordinary case of the cloud with a negative bottom over flat country. Its potential is much more negative than the earth underneath, so negative electrons will be accelerated toward the earth. What happens is the following. It all starts with a thing called a "step leader," which is not as bright as the stroke of lightning.

4) On the photographs one can see a little bright spot at the beginning that starts from the cloud and moves downward very rapidly—at a sixth of the speed of light! It goes only about 50 meters and stops. It pauses for about 50 microseconds, and then takes another step. It pauses again and then goes another step, and so on. It moves in a series of steps toward the ground, along a path like that shown in Fig. 9-15. In the leader there are negative charges from the cloud; the whole column is full of negative charge. Also, the air is becoming ionized by the rapidly moving charges that produce the leader, so the air becomes a conductor along the path traced out.

5) The moment the leader touches the ground, we have a conducting "wire" that runs all the way up to the cloud and is full of negative charge. Now, at last, the negative charge of the cloud can simply escape and run out. The electrons at the bottom of the leader are the first ones to realize this; they dump out, leaving positive charge behind that attracts more negative charge from higher up in the leader, which in its turn pours out, etc. So finally all the negative charge in a part of the cloud runs out along the column in a rapid and energetic way. So the lightning stroke you see runs upwards from the ground, as indicated in Fig. 9-16. In fact, this main stroke—by far the brightest Dart—is called the return…

 

Text 10. Light quanta

(from Conceptual Physics by Paul G. Hewitt, City College of San Francisco, Pearson International Edition, 2006)

1) The classical physics that we have so far studied deals with two categories of the phenomena: particles and waves. According to our everyday experience, "particles" are tiny objects tike bullets. They have mass and they obey Newton's laws—they travel through space in straight lines unless a force acts upon them. Likewise, according to our everyday experience, "waves," like waves in the ocean, are phenomena that extend in space. When a wave travels through an opening or around a barrier, the wave diffracts and different parts of the wave interfere. Therefore, particles and waves are easy to distinguish from each other. In fact, they have properties that are mutually exclusive. Nonetheless, the question of how to classify light was a mystery for centuries.

2) One of the early theories about the nature of light is that of Plato, who lived in the fifth and fourth centuries BC. Plato thought that light consisted of streamers emitted by the eye. Euclid, who lived roughly a century later, also held this view. On the other hand, the Pythagoreans believed that light emanated from luminous bodies in the form of very fine particles, while Empedocles, a predecessor of Plato, taught that light is composed of high-speed waves of some sort. For more than 2000 years, the questions remained unanswered. Does tight consist of waves or particles?

3) In 1704, Isaac Newton described light as a stream of particles or corpuscles. He held this view despite his knowledge of what we now call polarization and despite his experiment with tight reflecting from glass plates, in which he noticed fringes of brightness and darkness (Newton's rings). He knew that his particles of light had to have certain wave properties too. Christian Huygens, a contemporary of Newton, advocated a wave theory of light.

4) With all this history as background, Thomas Young, in 1801, performed the "double-slit experiment," which seemed to prove, finally, that light is a wave phenomenon. This view was reinforced in 1862 by Maxwell's prediction that light carries energy in oscillating electric and magnetic fields. Twenty-five years later, Heinrich Hertz used sparking electric circuits to demonstrate the reality of electromagnetic waves (of radio frequency). In 1905, however, Albert Einstein published a Nobel Рrizе-winning paper that challenged the wave theory of light by arguing that light interacts with matter, not in continuous waves, as Maxwell envisioned, but in tiny packets of energy that we now call photons. This discovery didn't wipe out light waves. It revealed, instead, that light is both wave and particle.

 

Text 16. Material particles

(from Quantum Physics Berkeley physics course-v.4 by Eyvind H. Wichmann, Mcgraw-hill book Co. MW, 1971)

1) Heisenberg's Matrix Mechanics is a particular formulation of quantum mechanics in which the vector space aspect of the theory is emphasized, whereas the wave equations play a secondary role. At first Heisenberg's theory appears to be very different from the wave theories, such as Schrodinger's wave mechanics, but the different kinds of theories are in fact completely equivalent, and lead to the same physical predictions. They have a common abstract skeleton, and this skeleton is the abstract vector space theory. Since we cannot assume that the reader has already learned about matrices in his mathematics courses we will have to omit the discussion of Heisenberg's theory in this book. The theory is not particularly difficult, but since there are so many other things for the reader to learn we do not want to load the discussion with a presentation of matrix theory.

2) Werner Heisenberg's first paper on the subject appeared in 1925. In this paper matrix theory is not mentioned explicitly because Heisenberg had not realized that his mathematical operations had a matrix theory interpretation. The connection with matrix theory was soon thereafter clarified in an important paper by Max Born and Pascual Jordan.

3) The reader should note that historically matrix mechanics was invented and developed before Schrodinger had invented his wave mechanics. We have said that it is a natural idea to regard the set of all solutions of a linear differential equation as a vector space, and thereby be led to consider algebraic aspects of the equation. There is no doubt that had Schrodinger's wave mechanics been invented first, matrix mechanics would soon have been discovered as a reformulation of the wave theory. This was, however, not the way things actually happened. The historical sequence of events is almost incredible to this author, and he regardsthe invention of matrix mechanics as one of the most astounding accomplishments in physical theory.

4) That matrix mechanics and wave mechanics are physically equivalent was proved by Schrodinger in 1926.

 

Text 18. Plasma

(from Conceptual Physics by Paul G. Hewitt, City College of San Francisco, Pearson International Edition, 2006)

1) In addition to solids, liquids, and gases, there is a fourth phase of matter — plasma (not to be confused with the clear liquid part of blood, also called plasma). It is the least common phase in our everyday environment, but it is the most prevalent phase of matter in the universe as a whole. The Sun and other stars are largely plasma.

2) A plasma is an electrified gas. The atoms that make it up are ionized, stripped of one or more electrons, with a corresponding number of free electrons. Recall that a neutral atom has as many positive protons inside the nucleus as it has negative electrons outside the nucleus. When one or more of these electrons is stripped from the atom, the atom has more positive charge than negative charge and becomes a positive ion. (Under some conditions, it may have extra electrons, in which case it is a negative ion.) Although the electrons and ions are themselves electrically charged, the plasma as a whole is electrically neutral because there are still equal numbers of positive and negative charges, just as there are in an ordinary gas. Nevertheless, a plasma and a gas have very different properties. The plasma readily conducts electric current, it absorbs certain kinds of radiation that pass unhindered through a gas, and it can be shaped, molded, and moved by electric and magnetic fields.

3) Our Sun is a ball of hot plasma. Plasmas on Earth are created in laboratories by heating gases to very high temperatures, making them so hot that electrons are "boiled" off the atoms. Plasmas may also be created at lower temperatures by bombarding atoms with high-energy particles or radiation.

4) If you're reading this by light emitted by a fluorescent lamp, you don't have to look far to see plasma in action. Within the glowing tube of the lamp is plasma that contains argon and mercury ions (as well as many neutral atoms of these elements). When you turn the lamp on, a high voltage between electrodes at each end of the tube causes electrons to flow. These electrons ionize some atoms, forming plasma, which provides a conducting path that keeps the current flowing. The current activates some mercury atoms, causing them to emit radiation, mostly in the invisible ultraviolet region. This radiation causes the phosphor coating on the tube's inner surface to glow with visible light.

5) Similarly, the neon gas in an advertising sign becomes a plasma when its atoms are ionized by electron bombardment. Neon atoms, after being activated by electric current, emit predominantly red light. The different colors seen in these signs correspond to plasmas made up of different kinds of atoms. Argon, for example, glows blue, while helium glows pink. Sodium vapor lamps used in street lighting emit yellow light stimulated by glowing plasmas (Figure 14.27). A recent plasma innovation is the flat plasma TV screen. The screen is made up of many thousands of pixels, each of which is composed of three separate subpixel cells. One cell has a phosphor that fluoresces red, another has a phosphor that fluoresces green, and the other blue. The pixels are sandwiched between a network of electrodes that are charged thousands of times in a small fraction of a second, producing electric currents that flow through gases in the cells. As in a fluorescent lamp, the gases convert to glowing plasmas that release ultraviolet light that stimulates the phosphors. The combination of cell colors makes up the pixel color. The image on the screen is the blend of pixel colors activated by the TV control signal.

6) The aurora borealis and the aurora australis (called the northern and southern lights, respectively) are glowing plasmas in the upper atmosphere. Layers of low-temperature plasma encircle the whole Earth. Occasionally, showers of electrons from outer space and radiation belts enter "magnetic windows" near Earth's poles, crashing into the layers of plasma and producing light.

7) These layers of plasma, which extend upward some 80 kilometers, make up the ionosphere, and they act as mirrors to low-frequency radio waves. Higher-frequency radio and TV waves pass through the ionosphere. This is why you can pick up radio stations from long distances on your lower- frequency AM radio, but you have to be in the "line of sight" of broadcasting or relay antennas to pick up higher-frequency FM and TV signals. Have you ever noticed that, at night, you can sometimes receive very distant stations on your AM radio? This is because plasma layers settle closer together in the absence of the energizing sunlight and consequently are better reflectors of radio waves.

 

Text 1. Understanding physics

(from The Feynman lectures on physics mainly electromagnetism and matter,
by Richard P.Feynman and, Addison-Wesley Publishing company Inc.,
Reading, 1964)

1) The physicist needs a facility in looking at problems from several points of view. The exact analysis of real physical problems is usually quite complicated, and any particular physical situation may be too complicated to analyze directly by solving the differential equation. But one can still get a very good idea of the behavior of a system if one has some feel for the character of the solution in different circumstances. Ideas such as the field lines, capacitance, resistance, and inductance are, for such purposes, very useful. So we will spend much of our time analyzing them. In this way we will get a feel as to what should happen in different electromagnetic situations. On the other hand, none of the heuristic models, such as field lines, is really adequate and accurate for all situations. There is only one precise way ofpresenting the laws, and that is by means of differential equations. They have the advantage of being fundamental and, so far as we know, precise. If you have learned the differential equations you can always go back to them. There is nothing to unlearn.

2) It will take you some time to understand what should happen in different circumstances. You will have to solve the equations. Each time you solve the equations, you will learn something about the character of the solutions. To keep these solutions in mind, it will be useful also to study their meaning in terms of field lines and of other concepts. This is the way you will really "understand" the equations. That is the difference between mathematics and physics. Mathematicians, or people who have very mathematical minds, are often led astray when "studying" physics because they lose sight of the physics. They say: "Look, these differential equations—the Maxwell equations—are all there is to electrodynamics; it is admitted by the physicists that there is nothing which is not contained in the equations. The equations are complicated, but after all they are only mathematical equations and if I understand them mathematically inside out, I will understand the physics inside out." Only it doesn't work that way. Mathematicians who study physics with that point of view —and there have been many of them—usually make little contribution to physics and, in fact, little to mathematics. They fail because the actual physical situations in the real world are so complicated that it is necessary to have a much broader understanding of the equations.

3) What it means really to understand an equation—that is, in more than a strictly mathematical sense—was described by Dirac. He said: "I understand what an equation means if I have a way of figuring out the characteristics of its solution without actually solving it." So if we have a way of knowing what should happen in given circumstances without actually solving the equations, then we "understand" the equations, as applied to these circumstances. A physical understanding is a completely unmathematical, imprecise, and inexact thing, but absolutely necessary for a physicist.

4) Ordinarily, a course like this is given by developing gradually the physical ideas—by starting with simple situations and going on to more and more complicated situations. This requires that you continuously forget things you previously learned—things that are true in certain situations, but which are not true in general. For example, the "law" that the electrical force depends on the square of the distance is not always true. We prefer the opposite approach. We prefer to take first the complete laws, and then to step back and apply them to simple situations, developing the physical ideas as we go along. And that is what we are going to do.