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Thermal Diffusion
Thermal diffusion may refer to: * A thermal force on a gas due to a temperature gradient, also called ''thermal diffusion'' or Thermal transpiration. ** It is used to drive a gas pump with no moving parts called a Knudsen pump. ** It is the currently accepted theory for the rotation of the Crookes radiometer. * Diffusion in a temperature gradient, also called ''thermodiffusion'' or thermophoresis. ** It can be used as an obsolete method of making enriched uranium (see Enriched_Uranium#Thermal_diffusion, enriched uranium § thermal diffusion). ** Thermal diffusion can be used to measure fluid flow, including perfusion and rCBF at one location over time. * Brownian motion (at a constant ''non-zero absolute temperature''). See also: * Molecular diffusion * Heat conduction (i.e., diffusion of heat) {{bca ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Temperature Gradient
A temperature gradient is a physical quantity that describes in which direction and at what rate the temperature changes the most rapidly around a particular location. The temperature spatial gradient is a vector quantity with Dimensional analysis, dimension of temperature difference per unit length. The International System of Units, SI Units of measurement, unit is kelvin per meter (K/m). Temperature gradients in the Earth's atmosphere, atmosphere are important in the atmospheric sciences (meteorology, climatology and related fields). Mathematical description Assuming that the temperature ''T'' is an intensive quantity, i.e., a single-valued, Continuous function, continuous and Derivative, differentiable Function (mathematics), function of three-dimensional space (often called a scalar field), i.e., that :T=T(x,y,z) where ''x'', ''y'' and ''z'' are the Cartesian coordinate system, coordinates of the location of interest, then the temperature gradient is the vector (geometric) ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Thermal Transpiration
Thermal transpiration (or thermal diffusion) refers to the thermal force on a gas due to a temperature difference. Thermal transpiration causes a flow of gas in the absence of any other pressure difference, and is able to maintain a certain pressure difference called thermomolecular pressure difference in a steady state. The effect is strongest when the mean free path of the gas molecules is comparable to the dimensions of the gas container. Thermal transpiration appears as an important correction in the readings of vapor pressure thermometers, and the effect is historically famous as being an explanation for the rotation of the Crookes radiometer. See also * Knudsen pump — a gas pump with no moving parts which functions via thermal transpiration. *Thermophoresis Thermophoresis (also thermomigration, thermodiffusion, the Soret effect, or the Ludwig–Soret effect) is a phenomenon observed in mixtures of mobile particles where the different particle types exhibit different r ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Knudsen Pump
The Knudsen pump also referred to as "thermal transpiration pump" or "Knudsen compressor" is a gas pump that utilizes no moving parts. Instead it uses thermal transpiration, the phenomenon that gas molecules drift from the hot end to the cold end of a narrow channel. Thus a hot vacuum chamber "transpires" into the cold chamber. This thermal transpiration flow is induced when the boundary walls of the pump have a temperature gradient. Because the pump is based simply on temperature differences and has no moving parts, it could provide reliable and precise control of gas flow for a variety of applications, such as gas-sensing breath analyzers, chemical weapons detectors, and in satellite control. It is named after Martin Knudsen Martin Hans Christian Knudsen (15 February 1871 in Hasmark on Funen – 27 May 1949 in Copenhagen) was a Danish physicist who taught and conducted research at the Technical University of Denmark. He is primarily known for his study of molecul ..., a Dan ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Crookes Radiometer
The Crookes radiometer (also known as a light mill) consists of an airtight glass bulb containing a partial vacuum, with a set of vanes which are mounted on a spindle inside. The vanes rotate when exposed to light, with faster rotation for more intense light, providing a quantitative measurement of electromagnetic radiation intensity. The reason for the rotation was a cause of much scientific debate in the ten years following the invention of the device, but in 1879 the currently accepted explanation for the rotation was published. Today the device is mainly used in physics education as a demonstration of a heat engine run by light energy. It was invented in 1873 by the chemist William Crookes, Sir William Crookes as the by-product of some chemical research. In the course of very accurate quantitative chemical work, he was weighing samples in a partially evacuated chamber to reduce the effect of air currents, and noticed the weighings were disturbed when sunlight shone on the ba ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Thermophoresis
Thermophoresis (also thermomigration, thermodiffusion, the Soret effect, or the Ludwig–Soret effect) is a phenomenon observed in mixtures of mobile particles where the different particle types exhibit different responses to the force of a temperature gradient. This phenomenon tends to move light molecules to hot regions and heavy molecules to cold regions. The term ''thermophoresis'' most often applies to aerosol mixtures whose mean free path \lambda is comparable to its characteristic length scale L, but may also commonly refer to the phenomenon in all phases of matter. The term ''Soret effect'' normally applies to liquid mixtures, which behave according to different, less well-understood mechanisms than gaseous mixtures. Thermophoresis may not apply to thermomigration in solids, especially multi-phase alloys. Thermophoretic force The phenomenon is observed at the scale of one millimeter or less. An example that may be observed by the naked eye with good lighting is when the h ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Enriched Uranium
Enriched uranium is a type of uranium in which the percent composition of uranium-235 (written 235U) has been increased through the process of isotope separation. Naturally occurring uranium is composed of three major isotopes: uranium-238 (238U with 99.2732–99.2752% natural abundance), uranium-235 (235U, 0.7198–0.7210%), and uranium-234 (234U, 0.0049–0.0059%). 235U is the only nuclide existing in nature (in any appreciable amount) that is fissile with thermal neutrons. Enriched uranium is a critical component for both civil nuclear power generation and military nuclear weapons. Low-enriched uranium (20% 235U, typically >85%) is used for the cores of many nuclear weapons, as well as compact reactors for naval propulsion and research, as well as breeder reactors. There are about 2,000 tonnes of highly enriched uranium in the world. Enrichment methods were first developed on a large scale by the Manhattan Project. Its gaseous diffusion method was used in the 194 ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
RCBF
Cerebral circulation is the movement of blood through a network of cerebral arteries and veins supplying the brain. The rate of cerebral blood flow in an adult human is typically 750 milliliters per minute, or about 15% of cardiac output. Arteries deliver oxygenated blood, glucose and other nutrients to the brain. Veins carry "used or spent" blood back to the heart, to remove carbon dioxide, lactic acid, and other metabolic products. The neurovascular unit regulates cerebral blood flow so that activated neurons can be supplied with energy in the right amount and at the right time. Because the brain would quickly suffer damage from any stoppage in blood supply, the cerebral circulatory system has safeguards including autoregulation of the blood vessels. The failure of these safeguards may result in a stroke. The volume of blood in circulation is called the cerebral blood flow. Sudden intense accelerations change the gravitational forces perceived by bodies and can severely impai ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Brownian Motion
Brownian motion is the random motion of particles suspended in a medium (a liquid or a gas). The traditional mathematical formulation of Brownian motion is that of the Wiener process, which is often called Brownian motion, even in mathematical sources. This motion pattern typically consists of Randomness, random fluctuations in a particle's position inside a fluid sub-domain, followed by a relocation to another sub-domain. Each relocation is followed by more fluctuations within the new closed volume. This pattern describes a fluid at thermal equilibrium, defined by a given temperature. Within such a fluid, there exists no preferential direction of flow (as in transport phenomena). More specifically, the fluid's overall Linear momentum, linear and Angular momentum, angular momenta remain null over time. The Kinetic energy, kinetic energies of the molecular Brownian motions, together with those of molecular rotations and vibrations, sum up to the caloric component of a fluid's in ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Absolute Temperature
Thermodynamic temperature, also known as absolute temperature, is a physical quantity which measures temperature starting from absolute zero, the point at which particles have minimal thermal motion. Thermodynamic temperature is typically expressed using the Kelvin scale, where the unit of measurement is the ''kelvin'' (unit symbol: K). The Kelvin scale uses the same degree interval as the Celsius scale but is offset so that 0 K corresponds to absolute zero. For comparison, a temperature of 295 K corresponds to 21.85 °C and 71.33 °F. Another absolute scale of temperature is the Rankine scale, which is based on the Fahrenheit degree interval. Historically, thermodynamic temperature was defined by Lord Kelvin in terms of a macroscopic relation between Work (thermodynamics), thermodynamic work and Heat, heat transfer as defined in thermodynamics, but the kelvin was redefined by international agreement in 2019 in terms of phenomena that are now understood as man ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
Molecular Diffusion
Molecular diffusion is the motion of atoms, molecules, or other particles of a gas or liquid at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid, size and density (or their product, mass) of the particles. This type of diffusion explains the net flux of molecules from a region of higher concentration to one of lower concentration. Once the concentrations are equal the molecules continue to move, but since there is no concentration gradient the process of molecular diffusion has ceased and is instead governed by the process of self-diffusion, originating from the random motion of the molecules. The result of diffusion is a gradual mixing of material such that the distribution of molecules is uniform. Since the molecules are still in motion, but an equilibrium has been established, the result of molecular diffusion is called a "dynamic equilibrium". In a Phase (matter), phase with uniform temperature, absent external n ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] [Amazon] |
