Showing papers in "Physics Today in 1976"
TL;DR: In this article, the authors define the concept of debye shielding and define a set of criteria for plasmas in terms of temperature, debye Shielding, and debye shielding.
Abstract: 1. Introduction.- Occurrence of Plasmas in Nature.- Definition of Plasma.- Concept of Temperature.- Debye Shielding.- The Plasma Parameter.- Criteria for Plasmas.- Applications of Plasma Physics.- 2. Single-Particle Motions.- Uniform E and B Field.- Nonuniform B Field.- Nonuniform E Field.- TimeVarying E Field.- Time-Varying B Field.- Summary of Guiding Center Drifts.- Adiabatic Invariants.- 3. Plasmas as Fluids.- Relation of Plasma Physics to Ordinary Electromagnetics.- The Fluid Equation of Motion.- Fluid Drifts Perpendicular to B.- Fluid Drifts Parallel to B.- The Plasma Approximation.- 4. Waves in Plasmas.- Representation of Waves.- Group Velocity.- Plasma Oscillations.- Electron Plasma Waves.- Sound Waves.- Ion Waves.- Validity of the Plasma Approximation.- Comparison of Ion and Electron Waves.- Electrostatic Electron Oscillations Perpendicular to B.- Electrostatic Ion Waves Perpendicular to B.- The Lower Hybrid Frequency.- Electromagnetic Waves with B0 =.- Experimental Applications.- Electromagnetic Waves Perpendicular to B0.- Cutoffs and Resonances.- Electromagnetic Waves Parallel to B0.- Experimental Consequences.- Hydromagnetic Wave.- Magnetosonic Waves.- Summary of Elementary Plasma Waves.- The CMA Diagram.- 5. Diffusion and Resistivity.- Diffusion and Mobility in Weakly Ionized Gases.- Decay of a Plasma by Diffusion.- Steady State Solutions.- Recombination.- Diffusion Across a Magnetic Field.- Collisions in Fully Ionized Plasmas.- The Single-Fluid MHD Equations.- Diffusion in Fully Ionized Plasmas.- Solutions of the Diffusion Equation.- Bohm Diffusion and Neoclassical Diffusion.- 6. Equilibrium and Stability.- Introductio.- Hydromagnetic Equilibrium.- The Concept of ss.- Diffusion of Magnetic Field into a Plasma.- Classification of Instabilities.- Two-Stream Instability.- The \"Gravitational\" Instability.- Resistive Drift Waves.- 7. Introduction to Kinetic Theory.- The Meaning off(v).- Equations of Kinetic Theory.- Derivation of the Fluid Equations.- Plasma Oscillations and Landau Damping.- The Meaning of Landau Damping.- A Physical Derivation of Landau Damping.- BGK and Van Kampen Modes.- Experimental Verification.- Ion Landau Damping.- 8. Nonlinear Effects.- Sheaths.- Ion Acoustic Shock Waves.- The Ponderomotive Force.- Parametric Instabilities.- Plasma Echoes.- Nonlinear Landau Damping.- 9. Introduction to Controlled Fusion.- The Problem of Controlled Fusion.- Magnetic Confinement: Toruses.- Mirrors.- Pinches.- Laser-Fusion.- Plasma Heating.- Fusion Technology.- Summary.- Units.- Useful Constants and Formulas.- Useful Vector Relations.
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TL;DR: This article proposed powder metallurgy, which allows the homogeneous melt to be cooled in tiny droplets, making it possible at least to limit segregation to the scale of the resulting powder particles.
Abstract: How can we make an alloy to fit a specific materials requirement? The oldest method of alloy fabrication, casting, has two inherent limitations: Phases with high melting points are difficult to melt; and the cooling of the alloys from the melt is slow, so that alloy segregation and phase separation have time to occur. The other traditional method, powder metallurgy, has helped with the second of these problems: Allowing the homogeneous melt to be cooled in tiny droplets makes it possible at least to limit segregation to the scale of the resulting powder particles.
TL;DR: The diamond anvil pressure cell was developed in stepwise fashion from a rather qualitative instrument to the sophisticated quantitative research tool it is today, capable of reaching static pressures in the megabar range.
Abstract: Scientific advances can be made either in a dramatic and revolutionary manner or in a slow evolutionary way, sometimes covering a span of several years. An example of the latter process is the diamond‐anvil pressure cell, which in about a decade and a half has developed in stepwise fashion from a rather qualitative instrument to the sophisticated quantitative research tool it is today, capable of reaching static pressures in the megabar range.
TL;DR: In this paper, the authors present the PERFECT SOLID, the perfect solids, and the structure of isolaed defects in the context of simulated reality and experience.
Abstract: PART I: THE PERFECT SOLID PART II: ELECTRONIC STRUCTURE OF ISOLATED DEFECTS PART III: CALCULATION OF OBSERVABLE PROPERTIES OF DEFECTS PART IV: COMPARISON OF THEORY AND EXPERIMENT APPENDIX I: SUM RULES APPENDIX II: THE FACTORIZATION OF SECULAR EQUATIONS
TL;DR: In this paper, a short survey of the important experimental results of the last fifty years is given, showing that a critical unbiased study of these results already gives an answer to the question; theory, as we shall see, cannot add much to this answer.
Abstract: The question, “What is an elementary particle?” must find its answer primarily in experiment, although it must also be confronted with philosphical considerations. I will therefore begin by giving a short survey of the important experimental results of the last fifty years. This survey will show that a critical unbiased study of these results already gives an answer to the question; theory, as we shall see, cannot add much to this answer.
TL;DR: The expanding universe is 1010 years old and has a radius of 1010 light years or 1028 cm as mentioned in this paper, and most galaxies contain diffuse, low density interstellar matter.
Abstract: The expanding universe is 1010 years old and has a radius of 1010 light years or 1028 cm. Matter in the universe, distributed in a highly non‐uniform manner, is concentrated largely in galaxies, which occupy only a minute fraction (10−7) of the volume of the universe. In addition to stars, most galaxies contain diffuse, low density interstellar matter. Because interstellar matter is most easily studied in our own galaxy, the discussion here will focus on it, although this discussion should be pertinent to other galaxies as well.
TL;DR: It was from Heisenberg, as his first doctorate student, that I caught the spirit of research, and that I received the encouragement to make my own contributions.
Abstract: It is appropriate in this year, when we celebrate the 50th anniversary of quantum mechanics, and during which we have been saddened by the death of one of its leading founders, Werner Heisenberg, to reminisce about the formative years of the new mechanics. At the time when the foundations of physics were being replaced with totally new concepts I was a student of physics. I sat in the colloquium audience when Peter Debye made the suggestions to Erwin Schrodinger that started him on the study of de Broglie waves and the search for their wave equation. It was from Heisenberg, as his first doctorate student, that I caught the spirit of research, and that I received the encouragement to make my own contributions.
TL;DR: Bellman, Casto, Goldberg, Kalaba, Nelson, Scott, Wing, and others have made major contributions to the literatule on invatiant imbedding as discussed by the authors.
Abstract: two or three decades. Numerous articles and several books have appeared during this time on the subject matter. Bellman, Casto, Goldberg, Kalaba, Nelson, Scott, Wing, and others have made major contributions to the literatule on invat-iant imbedding. The book by Bellman and Wing is a broad covecrage of the methodology. Other books such as those by Wing [1], Casti and Kalaba [2], and Scott [3] are in some ways directed toward a specific application of the method. The fundamental concepts of invariant imbedding are covered in Chapter 1. The reflection and transmission of particles by a medium are discussed, as well as the equations which characterize the process. Chapter 2 discusses fulrther refinements and generalization of the method. The perturbation approach as well as the Riccati transformation are examined for consistency with the \"particle counting\" approach. Some aspects of the nonlinear problem are also discussed. Functional equations are presented in Chapter 3 and questions on existence, uniqueness, and conservation telations are answered in Chapter 4. The first of these two chapters limits the discussion to the scalar case; whereas the material in Chapter 4 covers the general case of processes defined by matrix equations. The remainder of the book is devoted to the application of the method of invariance, or invariant imbedding, to specific areas. Chapter 5 concentrates on the randoni walk probleni, with the classical approach also given. Thematerial in Chapter 6 is limited to the wave propagation problem. Time-dependent processes are introduced in Chapter 7 with material on transport, wave, and diffusion equations presented. Calculation of eigenvalues for Storm-Liouville-type systems are
TL;DR: In this paper, a new representation of equilibrium thermodynamics, one that is couched in a mathematical language, is presented, with an intrinsically geometrical structure, quite different from that generally employed.
Abstract: It is perhaps appropriate that, in a year marking the 100th anniversary of his landmark paper in thermodynamics, new developments should call fresh attention to the special beauty and profundity of the work of J Willard Gibbs Recent work has proved the possibility of constructing a new representation of equilibrium thermodynamics, one that is couched in a mathematical language—an intrinsically geometrical structure—quite different from that generally employed
TL;DR: A complete description of the evolution of states in the course of such transitions remains a distant goal as discussed by the authors, in spite of our rather extensive knowledge of chemical reactions, and a start on this problem has been made in recent years.
Abstract: A key step in any chemical reaction is the transition of bonding electrons from one stable wave‐mechanical state to another. A complete description of the evolution of states in the course of such transitions remains a distant goal—in spite of our rather extensive knowledge of chemical reactions. This article reviews a start on this problem that has been made in recent years.
TL;DR: In a one-page Letter to the Editor of Naturwissenschaften dated 17 October 1925, Goudsmit et al. as mentioned in this paper proposed the idea that each electron rotates with an angular momentum ℏ/2 and carries, besides its charge e, a magnetic moment equal to one Bohr magneton, i.e.
Abstract: In a one‐page Letter to the Editor of Naturwissenschaften dated 17 October 1925, Samuel A. Goudsmit and I proposed the idea that each electron rotates with an angular momentum ℏ/2 and carries, besides its charge e, a magnetic moment equal to one Bohr magneton, eℏ/2mc. (Here, as usual, ℏ is the modified Planck constant, m the mass of the electron and c the speed of light.) Sam, in his accompanying article, tells something of those times, fifty years ago. We have often talked about the circumstances that led to our idea, but it was mainly Goudsmit's recollections that have appeared in print before now—they are, however, not readily accessible in English. Although I gave a short account of the discovery of the spin as a part of my inaugural address for the Lorentz professorship in Leiden in 1955, it therefore appears to be my turn to reminisce.
TL;DR: The main difference between an industrial and a post-industrial society is that the sources of innovation in a postindustrial society are derived increasingly from the codification of theoretical knowledge, rather than from "random" inventions as mentioned in this paper.
Abstract: A number of countries in the West, the United States among them, are now passing from an industrial into a post‐industrial phase of society. The change primarily affects the socio‐technical dimensions of society and is generally independent of the nature of political change or political structure. The main difference between an industrial and a post‐industrial society is that the sources of innovation in a post‐industrial society are derived increasingly from the codification of theoretical knowledge, rather than from “random” inventions. Every society in human history has been dependent upon knowledge, but it is only in recent years that the accumulation and distribution of theoretical knowledge has come to the fore as a directive force of innovation and change.
TL;DR: For the case of slow positrons, the position appears very well established, and the prospect is now opened for interpreting, in more detail, positron collisions with more complex systems as discussed by the authors.
Abstract: The study of the behavior of slow positrons in gases has recently become a very lively field. New fast counting techniques are making it possible to enlarge much further the base already established in earlier work, as well as bringing into practical possibility measurements that previously seemed unattainable. (Typical of the recent experimental systems is the apparatus shown in the photograph, figure 1, and diagrammatically in figure 2.) For collisions with helium atoms at energies below the positronium formation threshold the position appears very well established, and the prospect is now opened for interpreting, in more detail, positron collisions with more complex systems.