Physics is one of the oldestacademic disciplines. Over much of the past two millennia, physics,chemistry,biology, and certain branches ofmathematics were a part ofnatural philosophy, but during theScientific Revolution in the 17th century, these natural sciences branched into separate research endeavors. Physics intersects with manyinterdisciplinary areas of research, such asbiophysics andquantum chemistry, and the boundaries of physics are not rigidly defined. New ideas in physics often explain the fundamental mechanisms studied by other sciences and suggest new avenues of research in these and other academic disciplines such as mathematics and philosophy.
Many contemporary uses of uranium exploit its uniquenuclear properties. Uranium is used innuclear power plants andnuclear weapons because it is the only naturally occurring element with afissile isotope – uranium-235 – present in non-trace amounts. However, because of the low abundance of uranium-235 in natural uranium (which is overwhelmingly uranium-238), uranium needs to undergoenrichment so that enough uranium-235 is present. Uranium-238 is fissionable by fast neutrons and isfertile, meaning it can betransmuted to fissileplutonium-239 in anuclear reactor. Another fissile isotope,uranium-233, can be produced from naturalthorium and is studied for future industrial use in nuclear technology. Uranium-238 has a small probability forspontaneous fission or even induced fission with fast neutrons; uranium-235, and to a lesser degree uranium-233, have a much higher fission cross-section for slow neutrons. In sufficient concentration, these isotopes maintain a sustainednuclear chain reaction. This generates the heat innuclear power reactors and produces the fissile material for nuclear weapons. The primary civilian use for uranium harnesses the heat energy to produce electricity.Depleted uranium (238U) is used inkinetic energy penetrators andarmor plating. (Full article...)
Acomputed tomography scan (CT scan), formerly calledcomputed axial tomography scan (CAT scan), is amedical imaging technique used to obtain detailed internal images of the body. The personnel that perform CT scans are calledradiographers or radiology technologists.
CT scanners use a rotatingX-ray tube and a row of detectors placed in agantry to measure X-rayattenuations by different tissues inside the body. The multipleX-ray measurements taken from different angles are then processed on a computer usingtomographic reconstruction algorithms to producetomographic (cross-sectional) images (virtual "slices") of a body. CT scans can be used in patients with metallic implants or pacemakers, for whommagnetic resonance imaging (MRI) iscontraindicated. (Full article...)
An object in such an orbit has anorbital period equal to Earth's rotational period, onesidereal day, and so to ground observers it appears motionless, in a fixed position in the sky. The concept of a geostationary orbit was popularised by the science fiction writerArthur C. Clarke in the 1940s as a way to revolutionise telecommunications, and the firstsatellite to be placed in this kind of orbit was launched in 1963. (Full article...)
MRI is widely used in hospitals and clinics formedical diagnosis,staging and follow-up of disease. Compared to CT, MRI provides bettercontrast in images of soft tissues, e.g. in thebrain or abdomen. However, it may be perceived as less comfortable by patients, due to the usually longer and louder measurements with the subject in a long, confining tube, although "open" MRI designs mostly relieve this. Additionally,implants and other non-removable metal in the body can pose a risk and may exclude some patients from undergoing an MRI examination safely. (Full article...)
Born and raised in theAustrian Empire, Tesla first studied engineering and physics in the 1870s without receiving a degree. He then gained practical experience in the early 1880s working intelephony and atContinental Edison in the newelectric power industry. In 1884, he migrated to the United States, where he became anaturalized citizen. He worked for a short time at theEdison Machine Works in New York City before he struck out on his own. With the help of partners to finance and market his ideas, Tesla set up laboratories and companies in New York to develop a range of electrical and mechanical devices. His ACinduction motor and relatedpolyphase AC patents, licensed byWestinghouse Electric in 1888, earned him a considerable amount of money and became the cornerstone of the polyphase system, which Westinghouse marketed. (Full article...)
Levine is married toArlene Spielholz, a former NASA scientist who studied the psychological effects of astronauts spending long time periods in space, and has a daughter and grandson. (Full article...)
The proton's magnetic moment was directly measured in 1933 byOtto Stern team inUniversity of Hamburg. While the neutron was determined to have a magnetic moment by indirect methods in the mid-1930s,Luis Alvarez andFelix Bloch made the first accurate, direct measurement of the neutron's magnetic moment in 1940. The proton's magnetic moment is exploited to make measurements of molecules byproton nuclear magnetic resonance. The neutron's magnetic moment is exploited to probe the atomic structure of materials using scattering methods and to manipulate the properties of neutron beams in particle accelerators. (Full article...)
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Matteucci's frog battery, 1845 (top left); Aldini's frog battery, 1818 (bottom); apparatus for controlled exposure of gases to frog battery (top right). Afrog battery is anelectrochemical battery consisting of a number of dead frogs (or sometimes live ones), which form thecells of the battery connected in aseries arrangement. It is a kind ofbiobattery. It was used in early scientific investigations of electricity and academic demonstrations.
The principle behind the battery is theinjury potential created in a muscle when it is damaged, although this was not fully understood in the 18th and 19th centuries; thepotential being caused incidentally due to the dissection of the frog's muscles. (Full article...)
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The state of avibrating string can be modeled as a point in a Hilbert space. The decomposition of a vibrating string into its vibrations in distinctovertones is given by the projection of the point onto thecoordinate axes in the space.
Incoming wave (red) reflected at the wall produces the outgoing wave (blue), both being overlaid resulting in the clapotis (black).
Inhydrodynamics, aclapotis (fromFrench for "lapping of water") is a non-breakingstanding wave pattern, caused for example, by the reflection of a traveling surfacewave train from a near vertical shoreline like abreakwater,seawall or steepcliff. The resultingclapotic wave does not travel horizontally, but has a fixed pattern ofnodes and antinodes. These waves promote erosion at the toe of the wall, and can cause severe damage to shore structures. The term was coined in 1877 byFrenchmathematician andphysicistJoseph Valentin Boussinesq who called these waves 'le clapotis' meaning "the lapping".
In the idealized case of "full clapotis" where a purelymonotonic incoming wave is completely reflectednormal to a solid vertical wall, the standingwave height is twice the height of the incoming waves at a distance of one half wavelength from the wall. In this case, the circular orbits of the water particles in thedeep-water wave are converted to purely linear motion, with vertical velocities at the antinodes, and horizontal velocities at the nodes. (Full article...)
Physically, carbon–oxygen white dwarfs with a low rate of rotation are limited to below 1.44 solar masses (M☉). Beyond this "critical mass", they reignite and in some cases trigger a supernova explosion; this critical mass is often referred to as the Chandrasekhar mass, but is marginally different from the absoluteChandrasekhar limit, whereelectron degeneracy pressure is unable to prevent catastrophic collapse. If a white dwarf gradually accretes mass from a binary companion, or merges with a second white dwarf, the general hypothesis is that a white dwarf's core will reach the ignition temperature forcarbon fusion as it approaches the Chandrasekhar mass. Within a few seconds of initiation of nuclear fusion, a substantial fraction of the matter in the white dwarf undergoes arunaway reaction, releasing enough energy (1×1044J) to unbind the star in a supernova explosion. (Full article...)
Image 18Classical physics is usually concerned with everyday conditions: speeds are much lower than thespeed of light, sizes are much greater than that of atoms, yet very small in astronomical terms. Modern physics, however, is concerned with high velocities, small distances, and very large energies. (fromModern physics)
Image 22The Hindu-Arabic numeral system. The inscriptions on theedicts of Ashoka (3rd century BCE) display this number system being used by the ImperialMauryas. (fromHistory of physics)
Image 31Computer simulation ofnanogears made offullerene molecules. It is hoped that advances in nanoscience will lead to machines working on the molecular scale. (fromCondensed matter physics)
Image 32One possible signature of a Higgs boson from a simulatedproton–proton collision. It decays almost immediately into two jets ofhadrons and two electrons, visible as lines. (fromHistory of physics)
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![Image 1 Hilde Levi Hilde Levi (9 May 1909 – 26 July 2003) was a German-Danish physicist. She was a pioneer of the use of radioactive isotopes in biology and medicine, notably the techniques of radiocarbon dating and autoradiography. In later life she became a scientific historian, and published a biography of George de Hevesy. Born into a non-religious Jewish family in Frankfurt, Germany, Levi entered the University of Munich in 1929. She carried out her doctoral studies at the Kaiser Wilhelm Institute for Physical Chemistry and Electrochemistry at Berlin-Dahlem, writing her thesis on the spectra of alkali metal halides under the supervision of Peter Pringsheim [de] and Fritz Haber. By the time she completed it in 1934, the Nazi Party had been elected to office in Germany, and Jews were no longer allowed to be hired for academic positions. She went to Denmark where she found a position at the Niels Bohr Institute of Theoretical Physics at the University of Copenhagen. Working with James Franck and George de Hevesy, she published a number of papers on the use of radioactive substances in biology. (Full article...)](/mt/?noimg=&dark=on&url=https%3a%2f%2fen.wikipedia.org%2fwiki%2fFile%3aBlank.png)
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