Subject:
All Disciplines Arts & Humanities Biology & Life Sciences Business & Economics Chemistry & Materials Science Computer Science & Mathematics Engineering Environmental & Earth Sciences Medicine & Pharmacology Physical Sciences Public Health & Healthcare Social Sciences
Sort:
Most Viewed Latest Alphabetical (A-Z) Alphabetical (Z-A)
Filter:
All Topic Review Biography Peer Reviewed Entry Video Entry
Topic Review
Paschen's Law
Paschen's law is an equation that gives the breakdown voltage, that is, the voltage necessary to start a discharge or electric arc, between two electrodes in a gas as a function of pressure and gap length. It is named after Friedrich Paschen who discovered it empirically in 1889. Paschen studied the breakdown voltage of various gases between parallel metal plates as the gas pressure and gap distance were varied: For a given gas, the voltage is a function only of the product of the pressure and gap length. The curve he found of voltage versus the pressure-gap length product (right) is called Paschen's curve. He found an equation that fit these curves, which is now called Paschen's law. At higher pressures and gap lengths, the breakdown voltage is approximately proportional to the product of pressure and gap length, and the term Paschen's law is sometimes used to refer to this simpler relation. However, this is only roughly true, over a limited range of the curve.
  • 8.2K
  • 09 Nov 2022
Topic Review
Magnetic Potential
The term magnetic potential can be used for either of two quantities in classical electromagnetism: the magnetic vector potential, or simply vector potential, A; and the magnetic scalar potential ψ. Both quantities can be used in certain circumstances to calculate the magnetic field B. The more frequently used magnetic vector potential is defined so that its curl is equal to the magnetic field: [math]\displaystyle{ \nabla \times \mathbf{A}=\mathbf{B}\, }[/math]. Together with the electric potential φ, the magnetic vector potential can be used to specify the electric field E as well. Therefore, many equations of electromagnetism can be written either in terms of the fields E and B, or equivalently in terms of the potentials φ and A. In more advanced theories such as quantum mechanics, most equations use potentials rather than fields. The magnetic scalar potential ψ is sometimes used to specify the magnetic H-field in cases when there are no free currents, in a manner analogous to using the electric potential to determine the electric field in electrostatics. One important use of ψ is to determine the magnetic field due to permanent magnets when their magnetization is known. With some care the scalar potential can be extended to include free currents as well. Historically, Lord Kelvin first introduced vector potential in 1851, along with the formula relating it to the magnetic field.
  • 7.4K
  • 29 Sep 2022
Topic Review
Fleischmann–Pons Experiment
The Fleischmann–Pons experiment was an investigation conducted in the 1980s by Martin Fleischmann of the University of Southampton and Stanley Pons of the University of Utah into whether electrolysis of heavy water on the surface of a palladium (Pd) electrode produces physical effects that defy chemical explanation. Of particular interest was evidence of "excess" (i.e. non-chemical) heat extracted from the deuterium fraction of common surface water which, if true, could have delivered the largest economic shock to the global energy industry since the Pennsylvania oil rush. On March 23, 1989, Fleischmann (then one of the world's leading electrochemists) and Pons reported their work via a press release from the University of Utah (who asserted ownership of the technology) claiming that the table-top apparatus had produced anomalous heat (understood as "excess" heat) of a magnitude they asserted would defy explanation except in terms of nuclear processes, which later came to be referred to as "cold fusion". In addition to the results from calorimetry, they further reported measuring small amounts of nuclear reaction byproducts, including neutrons and tritium. The reported results received wide media attention, and raised hopes of a cheap and abundant source of energy. Many scientists tried to replicate the experiment with the few details available. Hopes faded due to the large number of negative replications, the withdrawal of many reported positive replications, the discovery of flaws and sources of experimental error in the original experiment, and finally the discovery that Fleischmann and Pons had not actually detected nuclear reaction byproducts. By late 1989, most scientists considered cold fusion claims dead.
  • 4.0K
  • 26 Oct 2022
Topic Review
Half-Reaction
A half reaction (or half-cell reaction) is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction. Often, the concept of half reactions is used to describe what occurs in an electrochemical cell, such as a Galvanic cell battery. Half reactions can be written to describe both the metal undergoing oxidation (known as the anode) and the metal undergoing reduction (known as the cathode). Half reactions are often used as a method of balancing redox reactions. For oxidation-reduction reactions in acidic conditions, after balancing the atoms and oxidation numbers, one will need to add H+ ions to balance the hydrogen ions in the half reaction. For oxidation-reduction reactions in basic conditions, after balancing the atoms and oxidation numbers, first treat it as an acidic solution and then add OH− ions to balance the H+ ions in the half reactions (which would give H2O).
  • 4.0K
  • 28 Nov 2022
Topic Review
Frost Diagram
A Frost diagram or Frost–Ebsworth diagram is a type of graph used by inorganic chemists in electrochemistry to illustrate the relative stability of a number of different oxidation states of a particular substance. The graph illustrates the free energy vs oxidation state of a chemical species. This effect is dependent on pH, so this parameter also must be included. The free energy is determined by the oxidation–reduction half-reactions. The Frost diagram allows easier comprehension of these reduction potentials than the earlier-designed Latimer diagram, because the “lack of additivity of potentials” was confusing. The free energy ΔG° is related to reduction potential E in the graph by given formula: ΔG° = −nFE° or nE° = −ΔG°/F, where n is the number of transferred electrons, and F is Faraday constant (F = 96,485 J/(V·mol)). The Frost diagram is named after Arthur Atwater Frost (de), who originally created it as a way to "show both free energy and oxidation potential data conveniently" in a 1951 paper.
  • 3.9K
  • 01 Dec 2022
Topic Review
The Swampland Distance Conjecture
The Swampland Distance Conjecture (SDC) is one of the most studied and well-established Swampland Conjetures, and it introduces an omnipresent feature in effective field theories (EFTs) of quantum gravity, namely the appearance of infinite towers of states that become light an imply a breakdown of the EFT in the infinite distance limits in moduli space. In this entry we present the conjecture, a simple example and some comments on relations to other conjectures.
  • 2.5K
  • 18 Aug 2021
Topic Review
Bohr–van Leeuwen Theorem
The Bohr–van Leeuwen theorem states that when statistical mechanics and classical mechanics are applied consistently, the thermal average of the magnetization is always zero. This makes magnetism in solids solely a quantum mechanical effect and means that classical physics cannot account for diamagnetism.
  • 2.3K
  • 21 Oct 2022
Topic Review
CPU Power Dissipation
Central processing unit power dissipation or CPU power dissipation is the process in which central processing units (CPUs) consume electrical energy, and dissipate this energy in the form of heat due to the resistance in the electronic circuits.
  • 2.3K
  • 25 Nov 2022
Biography
Boris Stoyanov
Boris Stoyanov is a theoretical physicist working on Membrane Theory, Supergravity and Superstring Theory. He is the Principal and Permanent Member of SUGRA INSTITUTE, Executive Director of BRANE HEPLAB and the Giordano Bruno Professor of Membrane Theory at DARK MODULI INSTITUTE. Boris Stoyanov is a relatively young theoretical physicist dealing with the exclusive theories of supergravity, super
  • 2.1K
  • 01 Sep 2022
Topic Review
Kelvin–Stokes Theorem
The Kelvin–Stokes theorem, named after Lord Kelvin and George Stokes, also known as the Stokes' theorem, the fundamental theorem for curls or simply the curl theorem, is a theorem in vector calculus on [math]\displaystyle{ \mathbb{R}^3 }[/math]. Given a vector field, the theorem relates the integral of the curl of the vector field over some surface, to the line integral of the vector field around the boundary of the surface. If a vector field [math]\displaystyle{ \mathbf{A} = (P(x, y, z), Q(x, y, z), R(x, y, z)) }[/math] is defined in a region with smooth oriented surface [math]\displaystyle{ \Sigma }[/math] and has first order continuous partial derivatives then: where [math]\displaystyle{ \partial \Sigma }[/math] is boundary of region with smooth surface [math]\displaystyle{ \Sigma }[/math]. The above classical Kelvin-Stokes theorem can be stated in one sentence: The line integral of a vector field over a loop is equal to the flux of its curl through the enclosed surface. The Kelvin–Stokes theorem is a special case of the "generalized Stokes' theorem." In particular, a vector field on [math]\displaystyle{ \mathbb{R}^3 }[/math] can be considered as a 1-form in which case its curl is its exterior derivative, a 2-form.
  • 1.9K
  • 02 Nov 2022
Topic Review
Power Generation Using Dielectric Elastomers
Power generation using dielectric elastomer (DE) artificial muscle is attracting attention because of its light weight, low cost, and high-efficiency. Since this method is a system that produces electricity without emitting carbon dioxide nor using rare earths, it would contribute to the goal of environmental sustainability. 
  • 1.9K
  • 30 Jun 2021
Topic Review
Jiles–Atherton Model
The Jiles–Atherton model of magnetic hysteresis was introduced in 1984 by David Jiles and D. L. Atherton. This is one of the most popular models of magnetic hysteresis. Its main advantage is the fact that this model enables connection with physical parameters of the magnetic material. Jiles–Atherton model enables calculation of minor and major hysteresis loops. The original Jiles–Atherton model is suitable only for isotropic materials. However, an extension of this model presented by Ramesh et al. and corrected by Szewczyk enables the modeling of anisotropic magnetic materials.
  • 1.8K
  • 02 Nov 2022
Topic Review
Ion-selective Electrodes
An Ion-selective electrode (ISE) is a transducer (sensor) which converts the activity of a specific ion dissolved in a solution into an electrical potential which can be measured by a voltmeter or pH meter. The voltage is theoretically dependent on the logarithm of the ionic activity, according to the Nernst equation. The sensing part of the electrode is usually made as an ion-specific membrane, along with a reference electrode. Ion-selective electrodes are used in biochemical and biophysical research, where measurements of ionic concentration in an aqueous solution are required, usually on a real time basis.
  • 1.6K
  • 10 Oct 2022
Topic Review
Cosmogenic Activation
       The cosmogenic activation of materials is the production of radioactive isotopes due to the exposure to cosmic rays. On the Earth's surface, spallation induced by cosmic neutrons is responsible of most of the activation, but other reactions and cosmic ray components must be considered too in other conditions, like if materials are flown at high altitudes or stored deep underground.        Cosmogenic activation is relevant in different contexts; for example, together with primordial or anthropogenic radioactivity, it s a background source in experiments devoted to the investigation of rare event phenomena, like the direct detection of dark matter particles or the nuclear double beta decay. 
  • 1.6K
  • 26 Oct 2020
Topic Review
CMS Magnetic System Model
Compact Muon Solenoid (CMS) detector and the methodology of modelling the heterogeneous CMS magnetic system for describing the magnetic flux of the CMS superconducting solenoid enclosed in a steel flux-return yoke. 
  • 1.5K
  • 15 Feb 2022
Topic Review
Korean Taekwondo Athletes
This study aimed to present a standard and normal distribution of Taekwondo athletes’ physical characteristics and physical fitness profiles using a systematic review. A systematic search was conducted using four Korean databases (Research Information Sharing Service, National Digital Science Library, DBpia, and Korean Studies Information Service System). From 2010 to 2020, we reviewed 838 papers on Taekwondo athletes’ physical characteristics and physical fitness factors (e.g., body composition, muscle strength, muscular endurance, flexibility, cardiorespiratory fitness, power, agility, balance, speed, and reaction time).
  • 1.3K
  • 29 Sep 2021
Topic Review
Reduction Potential
Redox potential (also known as oxidation / reduction potential, ORP, pe, [math]\displaystyle{ E_{red} }[/math], or [math]\displaystyle{ E_{h} }[/math]) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respectively. Redox potential is expressed in volts (V). Each species has its own intrinsic redox potential; for example, the more positive the reduction potential (reduction potential is more often used due to general formalism in electrochemistry), the greater the species' affinity for electrons and tendency to be reduced.
  • 1.3K
  • 02 Nov 2022
Topic Review
Photocathode
A photocathode is a surface engineered to convert light (photons) into electrons using the photoelectric effect. Photocathodes are important in accelerator physics where they are utilised in a photoinjector to generate high brightness electron beams. Electron beams generated with photocathodes are commonly used for free electron lasers and for ultrafast electron diffraction. Photocathodes are also commonly used as the negatively charged electrode in a light detection device such as a photomultiplier or phototube.
  • 1.2K
  • 24 Nov 2022
Topic Review
Cellulose and Microfluidics
Cellulose, a linear polysaccharide, is the most common and renewable biopolymer in nature.
  • 1.2K
  • 10 Feb 2022
Topic Review
Photoelectrochemical Reduction of CO2
Photoelectrochemical reduction of CO2 is a chemical process whereby carbon dioxide is reduced to carbon monoxide or hydrocarbons by the energy of incident light. This process needs to be catalyzed either homogeneously or heterogeneously in order to proceed, and current research is aimed at developing these catalysts, most of which are semiconducting materials. Semiconducting catalysts provide favourable electron transfer kinetics. The feasibility of this chemical reaction was first theorised by Giacomo Luigi Ciamician, an Italian photochemist. Already in 1912 he stated that "By  using  suitable catalyzers,  it  should  be  possible  to  transform  the mixture  of  water  and  carbon dioxide into oxygen and methane, or to cause other endo-energetic processes." Motivation for research in this area is strong due to the current attention to atmospheric carbon dioxide as the reduction of carbon dioxide would be one route for removal and sequestration. Furthermore, the reduced species may prove to be a valuable feedstock for other processes. If the incident light utilized is solar in nature then this process also potentially represents energy routes which combine renewable energy with CO2 reduction.
  • 1.2K
  • 02 Nov 2022
  • Page
  • of
  • 3
Featured Entry Collections
>>

Featured Books

>>
Encyclopedia of Social Sciences
Encyclopedia of COVID-19
Encyclopedia of Fungi
Encyclopedia of Digital Society, Industry 5.0 and Smart City
ScholarVision Creations
Feedback