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College Physics 2e | Wave Optics | Young’s Double Slit Experiment | m42508 | |||
College Physics 2e | Wave Optics | Multiple Slit Diffraction | m42512 | |||
College Physics 2e | Wave Optics | Single Slit Diffraction | m42515 | |||
College Physics 2e | Wave Optics | Limits of Resolution: The Rayleigh Criterion | m42517 | |||
College Physics 2e | Wave Optics | Thin Film Interference | m42519 | Problem-Solving Strategies for Wave Optics | **Step 1.** *Examine the situation to determine that interference is involved*. Identify whether slits or thin film interference are considered in the problem. |
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College Physics 2e | Wave Optics | Polarization | m42522 | Polarization by Reflection | By now you can probably guess that Polaroid sunglasses cut the glare in reflected light because that light is polarized. You can check this for yourself by holding Polaroid sunglasses in front of you and rotating them while looking at light reflected from water or glass. As you rotate the sunglasses, you will notice the light gets bright and dim, but not completely black. This implies the reflected light is partially polarized and cannot be completely blocked by a polarizing filter. |
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College Physics 2e | Wave Optics | Polarization | m42522 | Polarization by Scattering | If you hold your Polaroid sunglasses in front of you and rotate them while looking at blue sky, you will see the sky get bright and dim. This is a clear indication that light scattered by air is partially polarized. [link] helps illustrate how this happens. Since light is a transverse EM wave, it vibrates the electrons of air molecules perpendicular to the direction it is traveling. The electrons then radiate like small antennae. Since they are oscillating perpendicular to the direction of the light ray, they produce EM radiation that is polarized perpendicular to the direction of the ray. When viewing the light along a line perpendicular to the original ray, as in [link], there can be no polarization in the scattered light parallel to the original ray, because that would require the original ray to be a longitudinal wave. Along other directions, a component of the other polarization can be projected along the line of sight, and the scattered light will only be partially polarized. Furthermore, multiple scattering can bring light to your eyes from other directions and can contain different polarizations. |
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College Physics 2e | Wave Optics | Polarization | m42522 | Liquid Crystals and Other Polarization Effects in Materials | While you are undoubtedly aware of liquid crystal displays (LCDs) found in watches, calculators, computer screens, cellphones, flat screen televisions, and other myriad places, you may not be aware that they are based on polarization. Liquid crystals are so named because their molecules can be aligned even though they are in a liquid. Liquid crystals have the property that they can rotate the polarization of light passing through them by $\text{90º}$. Furthermore, this property can be turned off by the application of a voltage, as illustrated in [link]. It is possible to manipulate this characteristic quickly and in small well-defined regions to create the contrast patterns we see in so many LCD devices. |
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College Physics 2e | Wave Optics | *Extended Topic* Microscopy Enhanced by the Wave Characteristics of Light | m42290 | |||
College Physics 2e | Special Relativity | Introduction to Special Relativity | m42525 | |||
College Physics 2e | Special Relativity | Einstein’s Postulates | m42528 | Einstein’s First Postulate | The first postulate upon which Einstein based the theory of special relativity relates to reference frames. All velocities are measured relative to some frame of reference. For example, a car’s motion is measured relative to its starting point or the road it is moving over, a projectile’s motion is measured relative to the surface it was launched from, and a planet’s orbit is measured relative to the star it is orbiting around. The simplest frames of reference are those that are not accelerated and are not rotating. Newton’s first law, the law of inertia, holds exactly in such a frame. |
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College Physics 2e | Special Relativity | Einstein’s Postulates | m42528 | Einstein’s Second Postulate | The second postulate upon which Einstein based his theory of special relativity deals with the speed of light. Late in the 19th century, the major tenets of classical physics were well established. Two of the most important were the laws of electricity and magnetism and Newton’s laws. In particular, the laws of electricity and magnetism predict that light travels at $c=3\text{.}\text{00}×{\text{10}}^{8}{\rule{0.25em}{0ex}}\text{m/s}$ in a vacuum, but they do not specify the frame of reference in which light has this speed. |
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College Physics 2e | Special Relativity | Simultaneity And Time Dilation | m42531 | Simultaneity | Consider how we measure elapsed time. If we use a stopwatch, for example, how do we know when to start and stop the watch? One method is to use the arrival of light from the event, such as observing a light turning green to start a drag race. The timing will be more accurate if some sort of electronic detection is used, avoiding human reaction times and other complications. |
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College Physics 2e | Special Relativity | Simultaneity And Time Dilation | m42531 | Time Dilation | The consideration of the measurement of elapsed time and simultaneity leads to an important relativistic effect. |
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College Physics 2e | Special Relativity | Simultaneity And Time Dilation | m42531 | The Twin Paradox | An intriguing consequence of time dilation is that a space traveler moving at a high velocity relative to the Earth would age less than her Earth-bound twin. Imagine the astronaut moving at such a velocity that $\gamma =\text{30}\text{.}0$, as in [link]. A trip that takes 2.00 years in her frame would take 60.0 years in her Earth-bound twin’s frame. Suppose the astronaut traveled 1.00 year to another star system. She briefly explored the area, and then traveled 1.00 year back. If the astronaut was 40 years old when she left, she would be 42 upon her return. Everything on the Earth, however, would have aged 60.0 years. Her twin, if still alive, would be 100 years old. |
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College Physics 2e | Special Relativity | Length Contraction | m42535 | Proper Length | One thing all observers agree upon is relative speed. Even though clocks measure different elapsed times for the same process, they still agree that relative speed, which is distance divided by elapsed time, is the same. This implies that distance, too, depends on the observer’s relative motion. If two observers see different times, then they must also see different distances for relative speed to be the same to each of them. |
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College Physics 2e | Special Relativity | Length Contraction | m42535 | Length Contraction | To develop an equation relating distances measured by different observers, we note that the velocity relative to the Earth-bound observer in our muon example is given by |
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College Physics 2e | Special Relativity | Relativistic Addition of Velocities | m42540 | Classical Velocity Addition | For simplicity, we restrict our consideration of velocity addition to one-dimensional motion. Classically, velocities add like regular numbers in one-dimensional motion. (See [link].) Suppose, for example, a girl is riding in a sled at a speed 1.0 m/s relative to an observer. She throws a snowball first forward, then backward at a speed of 1.5 m/s relative to the sled. We denote direction with plus and minus signs in one dimension; in this example, forward is positive. Let $v$ be the velocity of the sled relative to the Earth, $u$ the velocity of the snowball relative to the Earth-bound observer, and $u\prime$ the velocity of the snowball relative to the sled. |
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College Physics 2e | Special Relativity | Relativistic Addition of Velocities | m42540 | Relativistic Velocity Addition | The second postulate of relativity (verified by extensive experimental observation) says that classical velocity addition does not apply to light. Imagine a car traveling at night along a straight road, as in [link]. If classical velocity addition applied to light, then the light from the car’s headlights would approach the observer on the sidewalk at a speed $u=v+c$. But we know that light will move away from the car at speed $c$ relative to the driver of the car, and light will move towards the observer on the sidewalk at speed $c$, too. |
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College Physics 2e | Special Relativity | Relativistic Addition of Velocities | m42540 | Doppler Shift | Although the speed of light does not change with relative velocity, the frequencies and wavelengths of light do. First discussed for sound waves, a Doppler shift occurs in any wave when there is relative motion between source and observer. |
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College Physics 2e | Special Relativity | Relativistic Momentum | m42542 | |||
College Physics 2e | Special Relativity | Relativistic Energy | m42546 | Total Energy and Rest Energy | The first postulate of relativity states that the laws of physics are the same in all inertial frames. Einstein showed that the law of conservation of energy is valid relativistically, if we define energy to include a relativistic factor. |
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College Physics 2e | Special Relativity | Relativistic Energy | m42546 | Stored Energy and Potential Energy | What happens to energy stored in an object at rest, such as the energy put into a battery by charging it, or the energy stored in a toy gun’s compressed spring? The energy input becomes part of the total energy of the object and, thus, increases its rest mass. All stored and potential energy becomes mass in a system. Why is it we don’t ordinarily notice this? In fact, conservation of mass (meaning total mass is constant) was one of the great laws verified by 19th-century science. Why was it not noticed to be incorrect? The following example helps answer these questions. |
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College Physics 2e | Special Relativity | Relativistic Energy | m42546 | Kinetic Energy and the Ultimate Speed Limit | Kinetic energy is energy of motion. Classically, kinetic energy has the familiar expression $\frac{1}{2}{\mathrm{mv}}^{2}$. The relativistic expression for kinetic energy is obtained from the work-energy theorem. This theorem states that the net work on a system goes into kinetic energy. If our system starts from rest, then the work-energy theorem is |
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College Physics 2e | Special Relativity | Relativistic Energy | m42546 | Relativistic Energy and Momentum | We know classically that kinetic energy and momentum are related to each other, since |
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College Physics 2e | Quantum Physics | Introduction to Quantum Physics | m42550 | |||
College Physics 2e | Quantum Physics | Quantization of Energy | m42554 | Planck’s Contribution | Energy is quantized in some systems, meaning that the system can have only certain energies and not a continuum of energies, unlike the classical case. This would be like having only certain speeds at which a car can travel because its kinetic energy can have only certain values. We also find that some forms of energy transfer take place with discrete lumps of energy. While most of us are familiar with the quantization of matter into lumps called atoms, molecules, and the like, we are less aware that energy, too, can be quantized. Some of the earliest clues about the necessity of quantum mechanics over classical physics came from the quantization of energy. |
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College Physics 2e | Quantum Physics | Quantization of Energy | m42554 | Atomic Spectra | Now let us turn our attention to the *emission and absorption of EM radiation by gases*. The Sun is the most common example of a body containing gases emitting an EM spectrum that includes visible light. We also see examples in neon signs and candle flames. Studies of emissions of hot gases began more than two centuries ago, and it was soon recognized that these emission spectra contained huge amounts of information. The type of gas and its temperature, for example, could be determined. We now know that these EM emissions come from electrons transitioning between energy levels in individual atoms and molecules; thus, they are called atomic spectra. Atomic spectra remain an important analytical tool today. [link] shows an example of an emission spectrum obtained by passing an electric discharge through a material. One of the most important characteristics of these spectra is that they are discrete. By this we mean that only certain wavelengths, and hence frequencies, are emitted. This is called a line spectrum. If frequency and energy are associated as $\Delta E=\text{hf}\text{,}$ the energies of the electrons in the emitting atoms and molecules are quantized. This is discussed in more detail later in this chapter. |
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College Physics 2e | Quantum Physics | The Photoelectric Effect | m42558 | |||
College Physics 2e | Quantum Physics | Photon Energies and the Electromagnetic Spectrum | m42563 | Ionizing Radiation | A photon is a quantum of EM radiation. Its energy is given by $E=\text{hf}$ and is related to the frequency $f$ and wavelength $\lambda$ of the radiation by |
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College Physics 2e | Quantum Physics | Photon Energies and the Electromagnetic Spectrum | m42563 | Visible Light | The range of photon energies for visible light from red to violet is 1.63 to 3.26 eV, respectively (left for this chapter’s Problems and Exercises to verify). These energies are on the order of those between outer electron shells in atoms and molecules. This means that these photons can be absorbed by atoms and molecules. A *single* photon can actually stimulate the retina, for example, by altering a receptor molecule that then triggers a nerve impulse. Photons can be absorbed or emitted only by atoms and molecules that have precisely the correct quantized energy step to do so. For example, if a red photon of frequency $f$ encounters a molecule that has an energy step, $\Delta E,$ equal to $\text{hf},$ then the photon can be absorbed. Violet flowers absorb red and reflect violet; this implies there is no energy step between levels in the receptor molecule equal to the violet photon’s energy, but there is an energy step for the red. |
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College Physics 2e | Quantum Physics | Photon Energies and the Electromagnetic Spectrum | m42563 | Lower-Energy Photons | Infrared radiation (IR) has even lower photon energies than visible light and cannot significantly alter atoms and molecules. IR can be absorbed and emitted by atoms and molecules, particularly between closely spaced states. IR is extremely strongly absorbed by water, for example, because water molecules have many states separated by energies on the order of ${\text{10}}^{\text{–5}}{\rule{0.25em}{0ex}}\text{eV}$ to ${\text{10}}^{\text{–2}}{\rule{0.25em}{0ex}}\text{eV,}$ well within the IR and microwave energy ranges. This is why in the IR range, skin is almost jet black, with an emissivity near 1—there are many states in water molecules in the skin that can absorb a large range of IR photon energies. Not all molecules have this property. Air, for example, is nearly transparent to many IR frequencies. |
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College Physics 2e | Quantum Physics | Photon Momentum | m42568 | Measuring Photon Momentum | The quantum of EM radiation we call a photon has properties analogous to those of particles we can see, such as grains of sand. A photon interacts as a unit in collisions or when absorbed, rather than as an extensive wave. Massive quanta, like electrons, also act like macroscopic particles—something we expect, because they are the smallest units of matter. Particles carry momentum as well as energy. Despite photons having no mass, there has long been evidence that EM radiation carries momentum. (Maxwell and others who studied EM waves predicted that they would carry momentum.) It is now a well-established fact that photons *do* have momentum. In fact, photon momentum is suggested by the photoelectric effect, where photons knock electrons out of a substance. [link] shows macroscopic evidence of photon momentum. |
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College Physics 2e | Quantum Physics | Photon Momentum | m42568 | Relativistic Photon Momentum | There is a relationship between photon momentum $p$ and photon energy $E$ that is consistent with the relation given previously for the relativistic total energy of a particle as ${E}^{2}=\left(\text{pc}{\right)}^{2}+\left(\text{mc}{\right)}^{2}$. We know $m$ is zero for a photon, but $p$ is not, so that ${E}^{2}=\left(\text{pc}{\right)}^{2}+\left(m{c}^{2}{\right)}^{2}$ becomes |
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College Physics 2e | Quantum Physics | The Particle-Wave Duality | m42573 | |||
College Physics 2e | Quantum Physics | The Wave Nature of Matter | m42576 | De Broglie Wavelength | In 1923 a French physics graduate student named Prince Louis-Victor de Broglie (1892–1987) made a radical proposal based on the hope that nature is symmetric. If EM radiation has both particle and wave properties, then nature would be symmetric if matter also had both particle and wave properties. If what we once thought of as an unequivocal wave (EM radiation) is also a particle, then what we think of as an unequivocal particle (matter) may also be a wave. De Broglie’s suggestion, made as part of his doctoral thesis, was so radical that it was greeted with some skepticism. A copy of his thesis was sent to Einstein, who said it was not only probably correct, but that it might be of fundamental importance. With the support of Einstein and a few other prominent physicists, de Broglie was awarded his doctorate. |
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College Physics 2e | Quantum Physics | The Wave Nature of Matter | m42576 | Electron Microscopes | One consequence or use of the wave nature of matter is found in the electron microscope. As we have discussed, there is a limit to the detail observed with any probe having a wavelength. Resolution, or observable detail, is limited to about one wavelength. Since a potential of only 54 V can produce electrons with sub-nanometer wavelengths, it is easy to get electrons with much smaller wavelengths than those of visible light (hundreds of nanometers). Electron microscopes can, thus, be constructed to detect much smaller details than optical microscopes. (See [link].) |
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College Physics 2e | Quantum Physics | Probability: The Heisenberg Uncertainty Principle | m42579 | Probability Distribution | Matter and photons are waves, implying they are spread out over some distance. What is the position of a particle, such as an electron? Is it at the center of the wave? The answer lies in how you measure the position of an electron. Experiments show that you will find the electron at some definite location, unlike a wave. But if you set up exactly the same situation and measure it again, you will find the electron in a different location, often far outside any experimental uncertainty in your measurement. Repeated measurements will display a statistical distribution of locations that appears wavelike. (See [link].) |
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College Physics 2e | Quantum Physics | Probability: The Heisenberg Uncertainty Principle | m42579 | Heisenberg Uncertainty | How does knowing which slit the electron passed through change the pattern? The answer is fundamentally important—*measurement affects the system being observed*. Information can be lost, and in some cases it is impossible to measure two physical quantities simultaneously to exact precision. For example, you can measure the position of a moving electron by scattering light or other electrons from it. Those probes have momentum themselves, and by scattering from the electron, they change its momentum *in a manner that loses information*. There is a limit to absolute knowledge, even in principle. |
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College Physics 2e | Quantum Physics | Probability: The Heisenberg Uncertainty Principle | m42579 | Heisenberg Uncertainty for Energy and Time | There is another form of Heisenberg’s uncertainty principle for *simultaneous measurements of energy and time*. In equation form, |
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College Physics 2e | Quantum Physics | The Particle-Wave Duality Reviewed | m42581 | Integrated Concepts | The problem set for this section involves concepts from this chapter and several others. Physics is most interesting when applied to general situations involving more than a narrow set of physical principles. For example, photons have momentum, hence the relevance of Linear Momentum and Collisions. The following topics are involved in some or all of the problems in this section: |
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College Physics 2e | Atomic Physics | Introduction to Atomic Physics | m42585 | |||
College Physics 2e | Atomic Physics | Discovery of the Atom | m42589 | |||
College Physics 2e | Atomic Physics | Discovery of the Parts of the Atom: Electrons and Nuclei | m42592 | The Electron | Gas discharge tubes, such as that shown in [link], consist of an evacuated glass tube containing two metal electrodes and a rarefied gas. When a high voltage is applied to the electrodes, the gas glows. These tubes were the precursors to today’s neon lights. They were first studied seriously by Heinrich Geissler, a German inventor and glassblower, starting in the 1860s. The English scientist William Crookes, among others, continued to study what for some time were called Crookes tubes, wherein electrons are freed from atoms and molecules in the rarefied gas inside the tube and are accelerated from the cathode (negative) to the anode (positive) by the high potential. These “*cathode rays*” collide with the gas atoms and molecules and excite them, resulting in the emission of electromagnetic (EM) radiation that makes the electrons’ path visible as a ray that spreads and fades as it moves away from the cathode. |
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College Physics 2e | Atomic Physics | Discovery of the Parts of the Atom: Electrons and Nuclei | m42592 | The Nucleus | Here, we examine the first direct evidence of the size and mass of the nucleus. In later chapters, we will examine many other aspects of nuclear physics, but the basic information on nuclear size and mass is so important to understanding the atom that we consider it here. |
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College Physics 2e | Atomic Physics | Bohr’s Theory of the Hydrogen Atom | m42596 | Mysteries of Atomic Spectra | As noted in Quantization of Energy , the energies of some small systems are quantized. Atomic and molecular emission and absorption spectra have been known for over a century to be discrete (or quantized). (See [link].) Maxwell and others had realized that there must be a connection between the spectrum of an atom and its structure, something like the resonant frequencies of musical instruments. But, in spite of years of efforts by many great minds, no one had a workable theory. (It was a running joke that any theory of atomic and molecular spectra could be destroyed by throwing a book of data at it, so complex were the spectra.) Following Einstein’s proposal of photons with quantized energies directly proportional to their wavelengths, it became even more evident that electrons in atoms can exist only in discrete orbits. |
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College Physics 2e | Atomic Physics | Bohr’s Theory of the Hydrogen Atom | m42596 | Bohr’s Solution for Hydrogen | Bohr was able to derive the formula for the hydrogen spectrum using basic physics, the planetary model of the atom, and some very important new proposals. His first proposal is that only certain orbits are allowed: we say that *the orbits of electrons in atoms are quantized*. Each orbit has a different energy, and electrons can move to a higher orbit by absorbing energy and drop to a lower orbit by emitting energy. If the orbits are quantized, the amount of energy absorbed or emitted is also quantized, producing discrete spectra. Photon absorption and emission are among the primary methods of transferring energy into and out of atoms. The energies of the photons are quantized, and their energy is explained as being equal to the change in energy of the electron when it moves from one orbit to another. In equation form, this is |
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College Physics 2e | Atomic Physics | Bohr’s Theory of the Hydrogen Atom | m42596 | Triumphs and Limits of the Bohr Theory | Bohr did what no one had been able to do before. Not only did he explain the spectrum of hydrogen, he correctly calculated the size of the atom from basic physics. Some of his ideas are broadly applicable. Electron orbital energies are quantized in all atoms and molecules. Angular momentum is quantized. The electrons do not spiral into the nucleus, as expected classically (accelerated charges radiate, so that the electron orbits classically would decay quickly, and the electrons would sit on the nucleus—matter would collapse). These are major triumphs. |
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College Physics 2e | Atomic Physics | X Rays: Atomic Origins and Applications | m42599 | Medical and Other Diagnostic Uses of X-rays | All of us can identify diagnostic uses of x-ray photons. Among these are the universal dental and medical x rays that have become an essential part of medical diagnostics. (See [link] and [link].) X rays are also used to inspect our luggage at airports, as shown in [link], and for early detection of cracks in crucial aircraft components. An x ray is not only a noun meaning high-energy photon, it also is an image produced by x rays, and it has been made into a familiar verb—to be x-rayed. |
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College Physics 2e | Atomic Physics | X Rays: Atomic Origins and Applications | m42599 | X-Ray Diffraction and Crystallography | Since x-ray photons are very energetic, they have relatively short wavelengths. For example, the 54.4-keV ${K}_{\alpha }$ x ray of [link] has a wavelength $\lambda =\text{hc}/E=0\text{.}\text{0228 nm}$. Thus, typical x-ray photons act like rays when they encounter macroscopic objects, like teeth, and produce sharp shadows; however, since atoms are on the order of 0.1 nm in size, x rays can be used to detect the location, shape, and size of atoms and molecules. The process is called x-ray diffraction, because it involves the diffraction and interference of x rays to produce patterns that can be analyzed for information about the structures that scattered the x rays. Perhaps the most famous example of x-ray diffraction is the discovery of the double-helix structure of DNA in 1953 by an international team of scientists working at the Cavendish Laboratory—American James Watson, Englishman Francis Crick, and New Zealand–born Maurice Wilkins. Using x-ray diffraction data produced by Rosalind Franklin, they were the first to discern the structure of DNA that is so crucial to life. For this, Watson, Crick, and Wilkins were awarded the 1962 Nobel Prize in Physiology or Medicine. There is much debate and controversy over the issue that Rosalind Franklin was not included in the prize. |
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College Physics 2e | Atomic Physics | Applications of Atomic Excitations and De-Excitations | m42602 | Fluorescence and Phosphorescence | The ability of a material to emit various wavelengths of light is similarly related to its atomic energy levels. [link] shows a scorpion illuminated by a UV lamp, sometimes called a black light. Some rocks also glow in black light, the particular colors being a function of the rock’s mineral composition. Black lights are also used to make certain posters glow. |
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College Physics 2e | Atomic Physics | Applications of Atomic Excitations and De-Excitations | m42602 | Lasers | Lasers today are commonplace. Lasers are used to read bar codes at stores and in libraries, laser shows are staged for entertainment, laser printers produce high-quality images at relatively low cost, and lasers send prodigious numbers of telephone messages through optical fibers. Among other things, lasers are also employed in surveying, weapons guidance, tumor eradication, retinal welding, and for reading DVDs, Blu-rays, and computer or game console CD-ROMs. |
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College Physics 2e | Atomic Physics | The Wave Nature of Matter Causes Quantization | m42606 | |||
College Physics 2e | Atomic Physics | Patterns in Spectra Reveal More Quantization | m42609 | |||
College Physics 2e | Atomic Physics | Quantum Numbers and Rules | m42614 | Intrinsic Spin Angular Momentum Is Quantized in Magnitude and Direction | There are two more quantum numbers of immediate concern. Both were first discovered for electrons in conjunction with fine structure in atomic spectra. It is now well established that electrons and other fundamental particles have *intrinsic spin*, roughly analogous to a planet spinning on its axis. This spin is a fundamental characteristic of particles, and only one magnitude of intrinsic spin is allowed for a given type of particle. Intrinsic angular momentum is quantized independently of orbital angular momentum. Additionally, the direction of the spin is also quantized. It has been found that the magnitude of the intrinsic (internal) spin angular momentum, $s$, of an electron is given by |
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College Physics 2e | Atomic Physics | The Pauli Exclusion Principle | m42618 | Multiple-Electron Atoms | All atoms except hydrogen are multiple-electron atoms. The physical and chemical properties of elements are directly related to the number of electrons a neutral atom has. The periodic table of the elements groups elements with similar properties into columns. This systematic organization is related to the number of electrons in a neutral atom, called the atomic number, $Z$. We shall see in this section that the exclusion principle is key to the underlying explanations, and that it applies far beyond the realm of atomic physics. |
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College Physics 2e | Atomic Physics | The Pauli Exclusion Principle | m42618 | Shells and Subshells | Because of the Pauli exclusion principle, only hydrogen and helium can have all of their electrons in the $n=1$ state. Lithium (see the periodic table) has three electrons, and so one must be in the $n=2$ level. This leads to the concept of shells and shell filling. As we progress up in the number of electrons, we go from hydrogen to helium, lithium, beryllium, boron, and so on, and we see that there are limits to the number of electrons for each value of $n$. Higher values of the shell $n$ correspond to higher energies, and they can allow more electrons because of the various combinations of $l,{\rule{0.25em}{0ex}}{m}_{l}$, and ${m}_{s}$ that are possible. Each value of the principal quantum number $n$ thus corresponds to an atomic shell into which a limited number of electrons can go. Shells and the number of electrons in them determine the physical and chemical properties of atoms, since it is the outermost electrons that interact most with anything outside the atom. |
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College Physics 2e | Atomic Physics | The Pauli Exclusion Principle | m42618 | Shell Filling and the Periodic Table | [link] shows electron configurations for the first 20 elements in the periodic table, starting with hydrogen and its single electron and ending with calcium. The Pauli exclusion principle determines the maximum number of electrons allowed in each shell and subshell. But the order in which the shells and subshells are filled is complicated because of the large numbers of interactions between electrons. |
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College Physics 2e | Radioactivity and Nuclear Physics | Introduction to Radioactivity and Nuclear Physics | m42620 | |||
College Physics 2e | Radioactivity and Nuclear Physics | Nuclear Radioactivity | m42623 | Discovery of Nuclear Radioactivity | In 1896, the French physicist Antoine Henri Becquerel (1852–1908) accidentally found that a uranium-rich mineral called pitchblende emits invisible, penetrating rays that can darken a photographic plate enclosed in an opaque envelope. The rays therefore carry energy; but amazingly, the pitchblende emits them continuously without any energy input. This is an apparent violation of the law of conservation of energy, one that we now understand is due to the conversion of a small amount of mass into energy, as related in Einstein’s famous equation $\mathit{E}={\mathit{mc}}^{2}$. It was soon evident that Becquerel’s rays originate in the nuclei of the atoms and have other unique characteristics. The emission of these rays is called **nuclear radioactivity** or simply radioactivity. The rays themselves are called nuclear radiation. A nucleus that spontaneously destroys part of its mass to emit radiation is said to decay (a term also used to describe the emission of radiation by atoms in excited states). A substance or object that emits nuclear radiation is said to be radioactive. |
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College Physics 2e | Radioactivity and Nuclear Physics | Nuclear Radioactivity | m42623 | Alpha, Beta, and Gamma | Research begun by people such as New Zealander Ernest Rutherford soon after the discovery of nuclear radiation indicated that different types of rays are emitted. Eventually, three types were distinguished and named alpha$\left(\alpha \right)$, beta$\left(\beta \right)$, and gamma$\left(\gamma \right)$, because, like x-rays, their identities were initially unknown. [link] shows what happens if the rays are passed through a magnetic field. The $\gamma$s are unaffected, while the $\alpha$ s and $\beta$ s are deflected in opposite directions, indicating the $\alpha$ s are positive, the $\beta$ s negative, and the $\gamma$ s uncharged. Rutherford used both magnetic and electric fields to show that $\alpha$ s have a positive charge twice the magnitude of an electron, or $+2\mid {q}_{e}\mid$. In the process, he found the $\alpha$ s charge to mass ratio to be several thousand times smaller than the electron’s. Later on, Rutherford collected $\alpha$ s from a radioactive source and passed an electric discharge through them, obtaining the spectrum of recently discovered helium gas. Among many important discoveries made by Rutherford and his collaborators was the proof that $\alpha$ *radiation is the emission of a helium nucleus*. Rutherford won the Nobel Prize in chemistry in 1908 for his early work. He continued to make important contributions until his death in 1934. |
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College Physics 2e | Radioactivity and Nuclear Physics | Nuclear Radioactivity | m42623 | Ionization and Range | Two of the most important characteristics of $\alpha$, $\beta$, and $\gamma$ rays were recognized very early. All three types of nuclear radiation produce *ionization* in materials, but they penetrate different distances in materials—that is, they have different *ranges*. Let us examine why they have these characteristics and what are some of the consequences. |
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College Physics 2e | Radioactivity and Nuclear Physics | Radiation Detection and Detectors | m42627 | Human Application | The first direct detection of radiation was Becquerel’s fogged photographic plate. Photographic film is still the most common detector of ionizing radiation, being used routinely in medical and dental x rays. Nuclear radiation is also captured on film, such as seen in [link]. The mechanism for film exposure by ionizing radiation is similar to that by photons. A quantum of energy interacts with the emulsion and alters it chemically, thus exposing the film. The quantum come from an $\alpha$-particle, $\beta$-particle, or photon, provided it has more than the few eV of energy needed to induce the chemical change (as does all ionizing radiation). The process is not 100% efficient, since not all incident radiation interacts and not all interactions produce the chemical change. The amount of film darkening is related to exposure, but the darkening also depends on the type of radiation, so that absorbers and other devices must be used to obtain energy, charge, and particle-identification information. |
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College Physics 2e | Radioactivity and Nuclear Physics | Substructure of the Nucleus | m42631 | Nuclear Forces and Stability | What forces hold a nucleus together? The nucleus is very small and its protons, being positive, exert tremendous repulsive forces on one another. (The Coulomb force increases as charges get closer, since it is proportional to $1/{r}^{2}$, even at the tiny distances found in nuclei.) The answer is that two previously unknown forces hold the nucleus together and make it into a tightly packed ball of nucleons. These forces are called the *weak and strong nuclear forces*. Nuclear forces are so short ranged that they fall to zero strength when nucleons are separated by only a few fm. However, like glue, they are strongly attracted when the nucleons get close to one another. The strong nuclear force is about 100 times more attractive than the repulsive EM force, easily holding the nucleons together. Nuclear forces become extremely repulsive if the nucleons get too close, making nucleons strongly resist being pushed inside one another, something like ball bearings. |
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College Physics 2e | Radioactivity and Nuclear Physics | Nuclear Decay and Conservation Laws | m42633 | Alpha Decay | In alpha decay, a ${}^{4}\text{He}$ nucleus simply breaks away from the parent nucleus, leaving a daughter with two fewer protons and two fewer neutrons than the parent (see [link]). One example of $\alpha$ decay is shown in [link] for ${}^{\text{238}}\text{U}$. Another nuclide that undergoes $\alpha$ decay is ${}^{\text{239}}\text{Pu}$. The decay equations for these two nuclides are |
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College Physics 2e | Radioactivity and Nuclear Physics | Nuclear Decay and Conservation Laws | m42633 | Beta Decay | There are actually *three* types of beta decay. The first discovered was “ordinary” beta decay and is called ${\beta }^{-}$ decay or electron emission. The symbol ${\beta }^{-}$ represents *an electron emitted in nuclear beta decay*. Cobalt-60 is a nuclide that ${\beta }^{-}$ decays in the following manner: |
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College Physics 2e | Radioactivity and Nuclear Physics | Nuclear Decay and Conservation Laws | m42633 | Gamma Decay | Gamma decay is the simplest form of nuclear decay—it is the emission of energetic photons by nuclei left in an excited state by some earlier process. Protons and neutrons in an excited nucleus are in higher orbitals, and they fall to lower levels by photon emission (analogous to electrons in excited atoms). Nuclear excited states have lifetimes typically of only about ${\text{10}}^{-\text{14}}$ s, an indication of the great strength of the forces pulling the nucleons to lower states. The $\gamma$ decay equation is simply |
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College Physics 2e | Radioactivity and Nuclear Physics | Half-Life and Activity | m42636 | Half-Life | Why use a term like half-life rather than lifetime? The answer can be found by examining [link], which shows how the number of radioactive nuclei in a sample decreases with time. The *time in which half of the original number of nuclei decay* is defined as the half-life, ${t}_{1/2}$. Half of the remaining nuclei decay in the next half-life. Further, half of that amount decays in the following half-life. Therefore, the number of radioactive nuclei decreases from $N$ to $N/2$ in one half-life, then to $N/4$ in the next, and to $N/8$ in the next, and so on. If $N$ is a large number, then *many* half-lives (not just two) pass before all of the nuclei decay. Nuclear decay is an example of a purely statistical process. A more precise definition of half-life is that *each nucleus has a 50% chance of living for a time equal to one half-life ${t}_{1/2}$*. Thus, if $N$ is reasonably large, half of the original nuclei decay in a time of one half-life. If an individual nucleus makes it through that time, it still has a 50% chance of surviving through another half-life. Even if it happens to make it through hundreds of half-lives, it still has a 50% chance of surviving through one more. The probability of decay is the same no matter when you start counting. This is like random coin flipping. The chance of heads is 50%, no matter what has happened before. |
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College Physics 2e | Radioactivity and Nuclear Physics | Half-Life and Activity | m42636 | Activity, the Rate of Decay | What do we mean when we say a source is highly radioactive? Generally, this means the number of decays per unit time is very high. We define activity $R$ to be the rate of decay expressed in decays per unit time. In equation form, this is |
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College Physics 2e | Radioactivity and Nuclear Physics | Half-Life and Activity | m42636 | Human and Medical Applications | Activity $R$ decreases in time, going to half its original value in one half-life, then to one-fourth its original value in the next half-life, and so on. Since $R=\frac{0\text{.}\text{693}N}{{t}_{1/2}}$, the activity decreases as the number of radioactive nuclei decreases. The equation for $R$ as a function of time is found by combining the equations $N={N}_{0}{e}^{-\mathrm{\lambda t}}$ and $R=\frac{0\text{.}\text{693}N}{{t}_{1/2}}$, yielding |
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College Physics 2e | Radioactivity and Nuclear Physics | Binding Energy | m42640 | Problem-Solving Strategies | null |
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College Physics 2e | Radioactivity and Nuclear Physics | Tunneling | m42644 | |||
College Physics 2e | Medical Applications of Nuclear Physics | Introduction to Applications of Nuclear Physics | m42646 | |||
College Physics 2e | Medical Applications of Nuclear Physics | Diagnostics and Medical Imaging | m42649 | Medical Application | [link] lists certain medical diagnostic uses of radiopharmaceuticals, including isotopes and activities that are typically administered. Many organs can be imaged with a variety of nuclear isotopes replacing a stable element by a radioactive isotope. One common diagnostic employs iodine to image the thyroid, since iodine is concentrated in that organ. The most active thyroid cells, including cancerous cells, concentrate the most iodine and, therefore, emit the most radiation. Conversely, hypothyroidism is indicated by lack of iodine uptake. Note that there is more than one isotope that can be used for several types of scans. Another common nuclear diagnostic is the thallium scan for the cardiovascular system, particularly used to evaluate blockages in the coronary arteries and examine heart activity. The salt TlCl can be used, because it acts like NaCl and follows the blood. Gallium-67 accumulates where there is rapid cell growth, such as in tumors and sites of infection. Hence, it is useful in cancer imaging. Usually, the patient receives the injection one day and has a whole body scan 3 or 4 days later because it can take several days for the gallium to build up. |
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College Physics 2e | Medical Applications of Nuclear Physics | Biological Effects of Ionizing Radiation | m42652 | Radiation Protection | Laws regulate radiation doses to which people can be exposed. The greatest occupational whole-body dose that is allowed depends upon the country and is about 20 to 50 mSv/y and is rarely reached by medical and nuclear power workers. Higher doses are allowed for the hands. Much lower doses are permitted for the reproductive organs and the fetuses of pregnant women. Inadvertent doses to the public are limited to $1/\text{10}$ of occupational doses, except for those caused by nuclear power, which cannot legally expose the public to more than $1/\text{1000}$ of the occupational limit or 0.05 mSv/y (5 mrem/y). This has been exceeded in the United States only at the time of the Three Mile Island (TMI) accident in 1979. Chernobyl is another story. Extensive monitoring with a variety of radiation detectors is performed to assure radiation safety. Increased ventilation in uranium mines has lowered the dose there to about 1 mSv/y. |
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College Physics 2e | Medical Applications of Nuclear Physics | Biological Effects of Ionizing Radiation | m42652 | Problem-Solving Strategy | You need to follow certain steps for dose calculations, which are |
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College Physics 2e | Medical Applications of Nuclear Physics | Biological Effects of Ionizing Radiation | m42652 | Risk versus Benefit | Medical doses of radiation are also limited. Diagnostic doses are generally low and have further lowered with improved techniques and faster films. With the possible exception of routine dental x-rays, radiation is used diagnostically only when needed so that the low risk is justified by the benefit of the diagnosis. Chest x-rays give the lowest doses—about 0.1 mSv to the tissue affected, with less than 5 percent scattering into tissues that are not directly imaged. Other x-ray procedures range upward to about 10 mSv in a CT scan, and about 5 mSv (0.5 rem) per dental x-ray, again both only affecting the tissue imaged. Medical images with radiopharmaceuticals give doses ranging from 1 to 5 mSv, usually localized. One exception is the thyroid scan using ${}^{\text{131}}\text{I}$. Because of its relatively long half-life, it exposes the thyroid to about 0.75 Sv. The isotope ${}^{\text{123}}\text{I}$ is more difficult to produce, but its short half-life limits thyroid exposure to about 15 mSv. |
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College Physics 2e | Medical Applications of Nuclear Physics | Therapeutic Uses of Ionizing Radiation | m42654 | Medical Application | The earliest uses of ionizing radiation on humans were mostly harmful, with many at the level of snake oil as seen in [link]. Radium-doped cosmetics that glowed in the dark were used around the time of World War I. As recently as the 1950s, radon mine tours were promoted as healthful and rejuvenating—those who toured were exposed but gained no benefits. Radium salts were sold as health elixirs for many years. The gruesome death of a wealthy industrialist, who became psychologically addicted to the brew, alerted the unsuspecting to the dangers of radium salt elixirs. Most abuses finally ended after the legislation in the 1950s. |
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College Physics 2e | Medical Applications of Nuclear Physics | Food Irradiation | m42656 | |||
College Physics 2e | Medical Applications of Nuclear Physics | Fusion | m42659 | |||
College Physics 2e | Medical Applications of Nuclear Physics | Fission | m42662 | |||
College Physics 2e | Medical Applications of Nuclear Physics | Nuclear Weapons | m42665 | |||
College Physics 2e | Particle Physics | Introduction to Particle Physics | m42667 | |||
College Physics 2e | Particle Physics | The Yukawa Particle and the Heisenberg Uncertainty Principle Revisited | m42669 | |||
College Physics 2e | Particle Physics | The Four Basic Forces | m42671 | |||
College Physics 2e | Particle Physics | Accelerators Create Matter from Energy | m42718 | Early Accelerators | An early accelerator is a relatively simple, large-scale version of the electron gun. The Van de Graaff (named after the Dutch physicist), which you have likely seen in physics demonstrations, is a small version of the ones used for nuclear research since their invention for that purpose in 1932. For more, see [link]. These machines are electrostatic, creating potentials as great as 50 MV, and are used to accelerate a variety of nuclei for a range of experiments. Energies produced by Van de Graaffs are insufficient to produce new particles, but they have been instrumental in exploring several aspects of the nucleus. Another, equally famous, early accelerator is the cyclotron, invented in 1930 by the American physicist, E. O. Lawrence (1901–1958). For a visual representation with more detail, see [link]. Cyclotrons use fixed-frequency alternating electric fields to accelerate particles. The particles spiral outward in a magnetic field, making increasingly larger radius orbits during acceleration. This clever arrangement allows the successive addition of electric potential energy and so greater particle energies are possible than in a Van de Graaff. Lawrence was involved in many early discoveries and in the promotion of physics programs in American universities. He was awarded the 1939 Nobel Prize in Physics for the cyclotron and nuclear activations, and he has an element and two major laboratories named for him. |
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College Physics 2e | Particle Physics | Accelerators Create Matter from Energy | m42718 | Modern Behemoths and Colliding Beams | Physicists have built ever-larger machines, first to reduce the wavelength of the probe and obtain greater detail, then to put greater energy into collisions to create new particles. Each major energy increase brought new information, sometimes producing spectacular progress, motivating the next step. One major innovation was driven by the desire to create more massive particles. Since momentum needs to be conserved in a collision, the particles created by a beam hitting a stationary target should recoil. This means that part of the energy input goes into recoil kinetic energy, significantly limiting the fraction of the beam energy that can be converted into new particles. One solution to this problem is to have head-on collisions between particles moving in opposite directions. Colliding beams are made to meet head-on at points where massive detectors are located. Since the total incoming momentum is zero, it is possible to create particles with momenta and kinetic energies near zero. Particles with masses equivalent to twice the beam energy can thus be created. Another innovation is to create the antimatter counterpart of the beam particle, which thus has the opposite charge and circulates in the opposite direction in the same beam pipe. For a schematic representation, see [link]. |
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College Physics 2e | Particle Physics | Particles, Patterns, and Conservation Laws | m42674 | Matter and Antimatter | The positron was only the first example of antimatter. Every particle in nature has an antimatter counterpart, although some particles, like the photon, are their own antiparticles. Antimatter has charge opposite to that of matter (for example, the positron is positive while the electron is negative) but is nearly identical otherwise, having the same mass, intrinsic spin, half-life, and so on. When a particle and its antimatter counterpart interact, they annihilate one another, usually totally converting their masses to pure energy in the form of photons as seen in [link]. Neutral particles, such as neutrons, have neutral antimatter counterparts, which also annihilate when they interact. Certain neutral particles are their own antiparticle and live correspondingly short lives. For example, the neutral pion ${\pi }^{0}$ is its own antiparticle and has a half-life about ${\text{10}}^{-8}$ shorter than ${\pi }^{+}$ and ${\pi }^{-}$, which are each other’s antiparticles. Without exception, nature is symmetric—all particles have antimatter counterparts. For example, antiprotons and antineutrons were first created in accelerator experiments in 1956 and the antiproton is negative. Antihydrogen atoms, consisting of an antiproton and antielectron, were observed in 1995 at CERN, too. It is possible to contain large-scale antimatter particles such as antiprotons by using electromagnetic traps that confine the particles within a magnetic field so that they don't annihilate with other particles. However, particles of the same charge repel each other, so the more particles that are contained in a trap, the more energy is needed to power the magnetic field that contains them. It is not currently possible to store a significant quantity of antiprotons. At any rate, we now see that negative charge is associated with both low-mass (electrons) and high-mass particles (antiprotons) and the apparent asymmetry is not there. But this knowledge does raise another question—why is there such a predominance of matter and so little antimatter? Possible explanations emerge later in this and the next chapter. |
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College Physics 2e | Particle Physics | Particles, Patterns, and Conservation Laws | m42674 | Hadrons and Leptons | Particles can also be revealingly grouped according to what forces they feel between them. All particles (even those that are massless) are affected by gravity, since gravity affects the space and time in which particles exist. All charged particles are affected by the electromagnetic force, as are neutral particles that have an internal distribution of charge (such as the neutron with its magnetic moment). Special names are given to particles that feel the strong and weak nuclear forces. Hadrons are particles that feel the strong nuclear force, whereas leptons are particles that do not. The proton, neutron, and the pions are examples of hadrons. The electron, positron, muons, and neutrinos are examples of leptons, the name meaning low mass. Leptons feel the weak nuclear force. In fact, all particles feel the weak nuclear force. This means that hadrons are distinguished by being able to feel both the strong and weak nuclear forces. |
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College Physics 2e | Particle Physics | Particles, Patterns, and Conservation Laws | m42674 | Mesons and Baryons | Now, note that the hadrons in the table given above are divided into two subgroups, called mesons (originally for medium mass) and baryons (the name originally meaning large mass). The division between mesons and baryons is actually based on their observed decay modes and is not strictly associated with their masses. Mesons are hadrons that can decay to leptons and leave no hadrons, which implies that mesons are not conserved in number. Baryons are hadrons that always decay to another baryon. A new physical quantity called baryon number $B$ seems to always be conserved in nature and is listed for the various particles in the table given above. Mesons and leptons have $B=0$ so that they can decay to other particles with $B=0$. But baryons have $B\text{=+}1$ if they are matter, and $B=-1$ if they are antimatter. The conservation of total baryon number is a more general rule than first noted in nuclear physics, where it was observed that the total number of nucleons was always conserved in nuclear reactions and decays. That rule in nuclear physics is just one consequence of the conservation of the total baryon number. |
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College Physics 2e | Particle Physics | Particles, Patterns, and Conservation Laws | m42674 | Forces, Reactions, and Reaction Rates | The forces that act between particles regulate how they interact with other particles. For example, pions feel the strong force and do not penetrate as far in matter as do muons, which do not feel the strong force. (This was the way those who discovered the muon knew it could not be the particle that carries the strong force—its penetration or range was too great for it to be feeling the strong force.) Similarly, reactions that create other particles, like cosmic rays interacting with nuclei in the atmosphere, have greater probability if they are caused by the strong force than if they are caused by the weak force. Such knowledge has been useful to physicists while analyzing the particles produced by various accelerators. |
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College Physics 2e | Particle Physics | Quarks: Is That All There Is? | m42678 | Conception of Quarks | Quarks were first proposed independently by American physicists Murray Gell-Mann and George Zweig in 1963. Their quaint name was taken by Gell-Mann from a James Joyce novel—Gell-Mann was also largely responsible for the concept and name of strangeness. (Whimsical names are common in particle physics, reflecting the personalities of modern physicists.) Originally, three quark types—or flavors—were proposed to account for the then-known mesons and baryons. These quark flavors are named up (*u*), down (*d*), and strange (*s*). All quarks have half-integral spin and are thus fermions. All mesons have integral spin while all baryons have half-integral spin. Therefore, mesons should be made up of an even number of quarks while baryons need to be made up of an odd number of quarks. [link] shows the quark substructure of the proton, neutron, and two pions. The most radical proposal by Gell-Mann and Zweig is the fractional charges of quarks, which are $±\left(\frac{2}{3}\right){q}_{e}$ and $\left(\frac{1}{3}\right){q}_{e}$, whereas all directly observed particles have charges that are integral multiples of ${q}_{e}$. Note that the fractional value of the quark does not violate the fact that the *e* is the smallest unit of charge that is observed, because a free quark cannot exist. [link] lists characteristics of the six quark flavors that are now thought to exist. Discoveries made since 1963 have required extra quark flavors, which are divided into three families quite analogous to leptons. |
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College Physics 2e | Particle Physics | Quarks: Is That All There Is? | m42678 | How Does it Work? | To understand how these quark substructures work, let us specifically examine the proton, neutron, and the two pions pictured in [link] before moving on to more general considerations. First, the proton *p* is composed of the three quarks *uud*, so that its total charge is $+\left(\frac{2}{3}\right){q}_{e}+\left(\frac{2}{3}\right){q}_{e}-\left(\frac{1}{3}\right){q}_{e}={q}_{e}$, as expected. With the spins aligned as in the figure, the proton’s intrinsic spin is $+\left(\frac{1}{2}\right)+\left(\frac{1}{2}\right)-\left(\frac{1}{2}\right)=\left(\frac{1}{2}\right)$, also as expected. Note that the spins of the up quarks are aligned, so that they would be in the same state except that they have different colors (another quantum number to be elaborated upon a little later). Quarks obey the Pauli exclusion principle. Similar comments apply to the neutron *n*, which is composed of the three quarks *udd*. Note also that the neutron is made of charges that add to zero but move internally, producing its well-known magnetic moment. When the neutron ${\beta }^{-}$ decays, it does so by changing the flavor of one of its quarks. Writing neutron ${\beta }^{-}$ decay in terms of quarks, |
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College Physics 2e | Particle Physics | Quarks: Is That All There Is? | m42678 | All Combinations are Possible | All quark combinations are possible. [link] lists some of these combinations. When Gell-Mann and Zweig proposed the original three quark flavors, particles corresponding to all combinations of those three had not been observed. The pattern was there, but it was incomplete—much as had been the case in the periodic table of the elements and the chart of nuclides. The ${\Omega }^{-}$ particle, in particular, had not been discovered but was predicted by quark theory. Its combination of three strange quarks, $\text{sss}$, gives it a strangeness of $-3$ (see [link]) and other predictable characteristics, such as spin, charge, approximate mass, and lifetime. If the quark picture is complete, the ${\Omega }^{-}$ should exist. It was first observed in 1964 at Brookhaven National Laboratory and had the predicted characteristics as seen in [link]. The discovery of the ${\Omega }^{-}$ was convincing indirect evidence for the existence of the three original quark flavors and boosted theoretical and experimental efforts to further explore particle physics in terms of quarks. |
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College Physics 2e | Particle Physics | Quarks: Is That All There Is? | m42678 | Now, Let Us Talk About Direct Evidence | At first, physicists expected that, with sufficient energy, we should be able to free quarks and observe them directly. This has not proved possible. There is still no direct observation of a fractional charge or any isolated quark. When large energies are put into collisions, other particles are created—but no quarks emerge. There is nearly direct evidence for quarks that is quite compelling. By 1967, experiments at SLAC scattering 20-GeV electrons from protons had produced results like Rutherford had obtained for the nucleus nearly 60 years earlier. The SLAC scattering experiments showed unambiguously that there were three pointlike (meaning they had sizes considerably smaller than the probe’s wavelength) charges inside the proton as seen in [link]. This evidence made all but the most skeptical admit that there was validity to the quark substructure of hadrons. |
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College Physics 2e | Particle Physics | Quarks: Is That All There Is? | m42678 | Quarks Have Their Ups and Downs | The quark model actually lost some of its early popularity because the original model with three quarks had to be modified. The up and down quarks seemed to compose normal matter as seen in [link], while the single strange quark explained strangeness. Why didn’t it have a counterpart? A fourth quark flavor called charm (*c*) was proposed as the counterpart of the strange quark to make things symmetric—there would be two normal quarks (*u* and *d*) and two exotic quarks (*s* and *c*). Furthermore, at that time only four leptons were known, two normal and two exotic. It was attractive that there would be four quarks and four leptons. The problem was that no known particles contained a charmed quark. Suddenly, in November of 1974, two groups (one headed by C. C. Ting at Brookhaven National Laboratory and the other by Burton Richter at SLAC) independently and nearly simultaneously discovered a new meson with characteristics that made it clear that its substructure is $c\stackrel{-}{c}$. It was called *J* by one group and psi ($\psi$) by the other and now is known as the $J/\psi$ meson. Since then, numerous particles have been discovered containing the charmed quark, consistent in every way with the quark model. The discovery of the $J/\psi$ meson had such a rejuvenating effect on quark theory that it is now called the November Revolution. Ting and Richter shared the 1976 Nobel Prize. |
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College Physics 2e | Particle Physics | Quarks: Is That All There Is? | m42678 | What’s Color got to do with it?—A Whiter Shade of Pale | As mentioned and shown in [link], quarks carry another quantum number, which we call color. Of course, it is not the color we sense with visible light, but its properties are analogous to those of three primary and three secondary colors. Specifically, a quark can have one of three color values we call **red** ($R$), **green** ($G$), and **blue** ($B$) in analogy to those primary visible colors. Antiquarks have three values we call **antired or cyan**$\left(\stackrel{-}{R}\right)$, **antigreen or magenta**$\left(\stackrel{-}{G}\right)$, and **antiblue or yellow**$\left(\stackrel{-}{B}\right)$ in analogy to those secondary visible colors. The reason for these names is that when certain visual colors are combined, the eye sees white. The analogy of the colors combining to white is used to explain why baryons are made of three quarks, why mesons are a quark and an antiquark, and why we cannot isolate a single quark. The force between the quarks is such that their combined colors produce white. This is illustrated in [link]. A baryon must have one of each primary color or RGB, which produces white. A meson must have a primary color and its anticolor, also producing white. |
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College Physics 2e | Particle Physics | Quarks: Is That All There Is? | m42678 | The Three Families | Fundamental particles are thought to be one of three types—leptons, quarks, or carrier particles. Each of those three types is further divided into three analogous families as illustrated in [link]. We have examined leptons and quarks in some detail. Each has six members (and their six antiparticles) divided into three analogous families. The first family is normal matter, of which most things are composed. The second is exotic, and the third more exotic and more massive than the second. The only stable particles are in the first family, which also has unstable members. |
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College Physics 2e | Particle Physics | GUTs: The Unification of Forces | m42680 | |||
College Physics 2e | Frontiers of Physics | Introduction to Frontiers of Physics | m42683 |
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