You're using an outdated browser. Please upgrade to a modern browser for the best experience.
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 by:
Most Viewed Latest Alphabetical (A-Z) Alphabetical (Z-A)
Filter:
All Topic Review Biography Peer Reviewed Entry Video Entry
Topic Review
Thermodynamic Potential
A thermodynamic potential (or more accurately, a thermodynamic potential energy) is a scalar quantity used to represent the thermodynamic state of a system. The concept of thermodynamic potentials was introduced by Pierre Duhem in 1886. Josiah Willard Gibbs in his papers used the term fundamental functions. One main thermodynamic potential that has a physical interpretation is the internal energy U. It is the energy of configuration of a given system of conservative forces (that is why it is called potential) and only has meaning with respect to a defined set of references (or data). Expressions for all other thermodynamic energy potentials are derivable via Legendre transforms from an expression for U. In thermodynamics, external forces, such as gravity, are typically disregarded when formulating expressions for potentials. For example, while all the working fluid in a steam engine may have higher energy due to gravity while sitting on top of Mount Everest than it would at the bottom of the Mariana Trench, the gravitational potential energy term in the formula for the internal energy would usually be ignored because changes in gravitational potential within the engine during operation would be negligible. In a large system under even homogeneous external force, like the earth atmosphere under gravity, the intensive parameters ([math]\displaystyle{ p, T, \rho }[/math]) should be studied locally having even in equilibrium different values in different places far from each other (see thermodynamic models of troposphere].
  • 2.0K
  • 04 Nov 2022
Topic Review
Onium Compound
In chemistry, an onium ion is a cation formally obtained by the protonation of mononuclear parent hydride of a pnictogen (group 15 of the periodic table), chalcogen (group 16), or halogen (group 17). The oldest-known onium ion, and the namesake for the class, is ammonium, NH+4, the protonated derivative of ammonia, NH3. The name onium is also used for cations that would result from the substitution of hydrogen atoms in those ions by other groups, such as organic radicals, or halogens; such as tetraphenylphosphonium, (C6H5)4P+. The substituent groups may be divalent or trivalent, yielding ions such as iminium and nitrilium. A simple onium ion has a charge of +1. A larger ion that has two onium ion subgroups is called a double onium ion, and has a charge of +2. A triple onium ion has a charge of +3, and so on. Compounds of an onium cation and some other negative ion are known as onium compounds or onium salts. Onium ions and onium compounds are inversely analogous to -ate ions and ate complexes:
  • 1.9K
  • 16 Nov 2022
Topic Review
Multiphysics
Multiphysics is defined as the coupled processes or systems involving more than one simultaneously occurring physical field and the studies of and knowledge about these processes and systems. As an interdisciplinary study area, multiphysics spans over many science and engineering disciplines. Multiphysics is a practice built on mathematics, physics, application, and numerical analysis. The mathematics involved usually contains partial differential equations and tensor analysis. The physics refers to common types of physical processes, e.g., heat transfer (thermo-), pore water movement (hydro-), concentration field (concentro or diffuso/convecto/advecto), stress and strain (mechano-), dynamics (dyno-), chemical reactions (chemo- or chemico-), electrostatics (electro-), neutronics (neutro-), and magnetostatics (magneto-).
  • 1.7K
  • 12 Oct 2022
Topic Review
Ion-Mobility Spectrometry
Ion mobility spectrometry (IMS) is an analytical technique used to separate and identify ionized molecules in the gas phase based on their mobility in a carrier buffer gas. Though heavily employed for military or security purposes, such as detecting drugs and explosives, the technique also has many laboratory analytical applications, including the analysis of both small and large biomolecules. IMS instruments are extremely sensitive stand-alone devices, but are often coupled with mass spectrometry, gas chromatography or high-performance liquid chromatography in order to achieve a multi-dimensional separation. They come in various sizes, ranging from a few millimeters to several meters depending on the specific application, and are capable of operating under a broad range of conditions. IMS instruments such as microscale high-field asymmetric-waveform ion-mobility spectrometry can be palm-portable for use in a range of applications including volatile organic compound (VOC) monitoring, biological sample analysis, medical diagnosis and food quality monitoring. Systems operated at higher pressure (i.e. atmospheric conditions, 1 atm or 1013 hPa) are often accompanied by elevated temperature (above 100 °C), while lower pressure systems (1-20 hPa) do not require heating.
  • 1.7K
  • 30 Nov 2022
Topic Review
Valence-shell Electron-pair Repulsion Model
There are the following main assumptions of the Valence-shell Electron-pair Repulsion (VSEPR) model. - The arrangement of covalent bonds of the atom centre analyzed depends on the number of electron pairs in its valence shell: bonds and nonbonding pairs as lone electron pairs. - The arrangement of valence electron pairs around the centre considered is to maximize their distances apart. - The non-valence electrons - inner electrons with nucleus (i.e. the core) possess the spherical symmetry (or at least it is in force for the main groups elements). It is worth to note that the intra- and intermolecular interactions influence on electronic and molecular structures in accordance with this VSEPR model.
  • 1.7K
  • 06 Sep 2021
Topic Review
Defects and Heteroatoms and Supported Graphene Layers
The possibility of using graphene-based materials as “metal-free” catalysts is attracting enormous interest, since it reduces the need for precious or rare elements currently used in heterogeneous catalysis. However, free standing  and perfect graphene is known to be “perfectly inert”, while it is now well established that there is an essential role of defects and dopants in activating its chemical properties.
  • 1.6K
  • 02 Apr 2022
Topic Review
Levitated Dipole Experiment
The Levitated Dipole Experiment (LDX) was an experiment investigating the generation of fusion power using the concept of a levitated dipole. The device was the first of its kind to test the levitated dipole concept and was funded by the US Department of Energy. The machine was also part of a collaboration between the MIT Plasma Science and Fusion Center and Columbia University, where another levitated dipole experiment, the Collisionless Terrella Experiment (CTX), was located. LDX ceased operations in November 2011 when its funding from the Department of Energy ended as resources were being diverted to tokamak research.
  • 1.6K
  • 22 Nov 2022
Topic Review
Neutral Beam Injection
Neutral beam injection (NBI) is one method used to heat plasma inside a fusion device consisting in a beam of high-energy neutral particles that can enter the magnetic confinement field. When these neutral particles are ionized by collision with the plasma particles, they are kept in the plasma by the confining magnetic field, and can transfer most of their energy by further collisions with the plasma. By tangential injection in the torus, neutral beams also provide momentum to the plasma and current drive, one essential feature for long pulses of burning plasmas. Neutral beam injection is a flexible and reliable technique, which has been the main heating system on a large variety of fusion devices. To date, all NBI systems were based on positive precursor ion beams. In the 90s there has been impressive progress in negative ion sources and accelerators with the construction of multi-megawatt negative ion based NBI systems at LHD (H0, 180 keV) and JT-60U (D0, 500 keV). The NBI designed for ITER is a substantial challenge (D0, 1MeV, 40A) and a prototype is being constructed to optimize its performance in view of the ITER future operations. Other ways to heat plasma for nuclear fusion include RF heating, electron cyclotron resonance heating (ECRH), and ion cyclotron resonance heating (ICRH), Lower hybrid resonance heating (LH).
  • 1.6K
  • 15 Nov 2022
Topic Review
Two-Dimensional Materials in Nonlinear Optical Devices
All-optical signal processing based on nonlinear optical devices is promising for ultrafast information processing in optical communication systems. Recent advances in two-dimensional (2D) layered materials with unique structures and distinctive properties have opened up new avenues for nonlinear optics and the fabrication of related devices with high performance. 
  • 1.6K
  • 13 Feb 2023
Topic Review
Defining Equation
In physics, defining equations are equations that define new quantities in terms of base quantities. This article uses the current SI system of units, not natural or characteristic units.
  • 1.5K
  • 08 Oct 2022
Topic Review
Gordon Decomposition
In mathematical physics, the Gordon-decomposition (named after Walter Gordon one of the discoverers of the Klein-Gordon equation) of the Dirac current is a splitting of the charge or particle-number current into a part that arises from the motion of the center of mass of the particles and a part that arises from gradients of the spin density. It makes explicit use of the Dirac equation and so it applies only to "on-shell" solutions of the Dirac equation.
  • 1.5K
  • 26 Oct 2022
Topic Review
Ray Tracing
In physics, ray tracing is a method for calculating the path of waves or particles through a system with regions of varying propagation velocity, absorption characteristics, and reflecting surfaces. Under these circumstances, wavefronts may bend, change direction, or reflect off surfaces, complicating analysis. Ray tracing solves the problem by repeatedly advancing idealized narrow beams called rays through the medium by discrete amounts. Simple problems can be analyzed by propagating a few rays using simple mathematics. More detailed analysis can be performed by using a computer to propagate many rays. When applied to problems of electromagnetic radiation, ray tracing often relies on approximate solutions to Maxwell's equations that are valid as long as the light waves propagate through and around objects whose dimensions are much greater than the light's wavelength. Ray theory does not describe phenomena such as interference and diffraction, which require wave theory (involving the phase of the wave).
  • 1.5K
  • 03 Nov 2022
Topic Review
Chemical Beam Epitaxy
Chemical beam epitaxy (CBE) forms an important class of deposition techniques for semiconductor layer systems, especially III-V semiconductor systems. This form of epitaxial growth is performed in an ultrahigh vacuum system. The reactants are in the form of molecular beams of reactive gases, typically as the hydride or a metalorganic. The term CBE is often used interchangeably with metal-organic molecular beam epitaxy (MOMBE). The nomenclature does differentiate between the two (slightly different) processes, however. When used in the strictest sense, CBE refers to the technique in which both components are obtained from gaseous sources, while MOMBE refers to the technique in which the group III component is obtained from a gaseous source and the group V component from a solid source.
  • 1.5K
  • 30 Nov 2022
Topic Review
Statistical Ensemble (Mathematical Physics)
In physics, specifically statistical mechanics, an ensemble (also statistical ensemble) is an idealization consisting of a large number of virtual copies (sometimes infinitely many) of a system, considered all at once, each of which represents a possible state that the real system might be in. In other words, a statistical ensemble is a probability distribution for the state of the system. The concept of an ensemble was introduced by J. Willard Gibbs in 1902. A thermodynamic ensemble is a specific variety of statistical ensemble that, among other properties, is in statistical equilibrium (defined below), and is used to derive the properties of thermodynamic systems from the laws of classical or quantum mechanics.
  • 1.4K
  • 07 Nov 2022
Topic Review
Chemical Ionization
Chemical ionization (CI) is a soft ionization technique used in mass spectrometry. This was first introduced by Burnaby Munson and Frank H. Field in 1966. This technique is a branch of gaseous ion-molecule chemistry. Reagent gas molecules (often methane or ammonia) are ionized by electron ionization to form reagent ions, which subsequently react with analyte molecules in the gas phase to create analyte ions for analysis by mass spectrometry. Negative chemical ionization (NCI), charge-exchange chemical ionization, atmospheric-pressure chemical ionization (APCI) and atmospheric pressure photoionization (APPI) are some of the common variants of the technique. CI mass spectrometry finds general application in the identification, structure elucidation and quantitation of organic compounds as well as some utility in biochemical analysis. Samples to be analyzed must be in vapour form, or else (in the case of liquids or solids), must be vapourized before introduction into the source.
  • 1.4K
  • 28 Nov 2022
Topic Review
Optical Polarization-Based Measurement for Peptides and Amino Acids
Polarization-based optical measurement methods are very useful in the analysis of the molecular orientations of materials, and, thus, these methods are implemented in numerous material-science studies, including into the characterization of amino acids’ (SAPA)  micro- and nanostructures.
  • 1.4K
  • 21 Mar 2022
Topic Review
Lockheed Martin CFR
The Lockheed Martin Compact Fusion Reactor (CFR) is a proposed nuclear fusion reactor project at Lockheed Martin’s Skunk Works. Its high-beta configuration, which implies that the ratio of plasma pressure to magnetic pressure is greater than or equal to 1 (compared to tokamak designs' 0.05), allows a compact fusion reactor (CFR) design and expedited development. The CFR chief designer and technical team lead, Thomas McGuire studied fusion as a source of space propulsion in response to a NASA desire to improve travel times to Mars.
  • 1.4K
  • 18 Oct 2022
Topic Review
Earth Hour
Earth Hour is a worldwide movement organized by the World Wildlife Fund (WWF). The event is held annually, encouraging individuals, communities, and businesses to turn off non-essential electric lights, for one hour, from 8:00 to 9:00 p.m. on the last Saturday of March, as a symbol of commitment to the planet. It was started as a lights-off event in Sydney, Australia, in 2007. Occasionally, in years when Holy Saturday falls on the last Saturday of March, Earth Hour is moved a week early rather than its usual date.
  • 1.3K
  • 14 Oct 2022
Topic Review
Garlic Volatile Diallyl Disulfide
Diallyl disulfide (DADS) is a major allelochemical of the volatile organic compounds in garlic.
  • 1.3K
  • 24 Dec 2021
Topic Review
Biological Effects of Non-Ionising UVR and Ionising Radiation
Non-ionising ultraviolet radiation (UVR) and ionising radiation differ in their interactions with biomolecules, resulting in varied consequences. Here describing the underlying molecular interactions of radiation in the context of biological systems and their outcomes from exposure. 
  • 1.3K
  • 24 Nov 2021
  • Page
  • of
  • 5
Featured Entry Collections
>>

Featured Books

>>
Encyclopedia of COVID-19
Encyclopedia of Engineering
Encyclopedia of Social Sciences
Encyclopedia of Digital Society, Industry 5.0 and Smart City
Academic Video Service
Feedback