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A20

A cytoplasmic zinc finger protein (790 aa) that inhibits NFκB activity and TNF-mediated programmed cell death. The expression of the A20 mRNA is upregulated by TNFα. It is a dual function ...

noise

noise   Quick reference

A Dictionary of Physics (8 ed.)

Reference type:
Subject Reference
Current Version:
2019
Subject:
Science and technology, Physics
Length:
85 words
Illustration(s):
1

...noise 1. Any undesired sound. It is measured on a decibel scale ranging from the threshold of hearing (0 dB) to the threshold of pain (130 dB). Between these limits a whisper registers about 20 dB, heavy urban traffic about 90 dB, and a heavy hammer on steel plate about 110 dB. A high noise level (industrial or from overamplified music, for example) can cause permanent hearing impairment. 2. Any unwanted disturbance within a useful frequency band in a communication channel. Noise. Decibel scale. ...

nuclear fission

nuclear fission   Quick reference

A Dictionary of Physics (8 ed.)

Reference type:
Subject Reference
Current Version:
2019
Subject:
Science and technology, Physics
Length:
173 words
Illustration(s):
1

...n → 148 La + 85 Br + 3n The energy released is approximately 3×10 −11 J per 235 U nucleus. For 1 kg of 235 U this is equivalent to 20 000 megawatt-hours—the amount of energy produced by the combustion of 3×10 6 tonnes of coal. Nuclear fission is the process used in nuclear reactors and atom bombs ( see nuclear weapons ). https://www.nature.com/physics/looking-back/meitner/index.html Meitner and Frisch’s original paper (1939) in Nature on induced fission...

earth’s atmosphere

earth’s atmosphere   Quick reference

A Dictionary of Physics (8 ed.)

Reference type:
Subject Reference
Current Version:
2019
Subject:
Science and technology, Physics
Length:
345 words
Illustration(s):
1

...earth’s atmosphere The gas that surrounds the earth. The composition of dry air at sea level is: nitrogen 78.08%, oxygen 20.95%, argon 0.93%, carbon dioxide 0.03%, neon 0.0018%, helium 0.0005%, krypton 0.0001%, and xenon 0.00001%. In addition to water vapour, air in some localities contains sulphur compounds, hydrogen peroxide, hydrocarbons, and dust particles. The lowest level of the atmosphere, in which most of the weather occurs, is called the troposphere . Its thickness varies from about 7 km at the poles to 28 km at the equator and in this layer temperature...

electron microscope

electron microscope   Quick reference

A Dictionary of Physics (8 ed.)

Reference type:
Subject Reference
Current Version:
2019
Subject:
Science and technology, Physics
Length:
246 words
Illustration(s):
1

...scans the screen at the same frequency, consequently building up a picture of the specimen. The resolution is limited to about 10–20 nm. Electron microscope. Principle of transmission electron microscope. http://micro.magnet.fsu.edu/primer/java/electronmicroscopy/magnify1/index.html A tutorial featuring virtual scanning electron microscopy, created by the Optical Microscopy Division of the National High Magnetic Field...

Van de Graaff generator

Van de Graaff generator   Quick reference

A Dictionary of Physics (8 ed.)

Reference type:
Subject Reference
Current Version:
2019
Subject:
Science and technology, Physics
Length:
275 words
Illustration(s):
1

...of the spherical terminal. The voltage achieved by the device is proportional to the radius of the spherical terminal. A typical device with a terminal having a radius of 1 m will produce about 1 MV. However, terminals can be made smaller, for a given voltage, by enclosing the apparatus in nitrogen at a pressure of 10–20 atmospheres (1–2 MPa) to reduce sparking. Generators having a positive-ion source are fitted with an evacuated tube through which the particles can be accelerated for research purposes ( see linear accelerator ). Machines having an...

radiography

radiography   Quick reference

A Dictionary of Physics (8 ed.)

Reference type:
Subject Reference
Current Version:
2019
Subject:
Science and technology, Physics
Length:
864 words
Illustration(s):
1

... 30keV–20MeV pair production rises slowly with E ∝ Z 2 above 20 MeV Air has a higher average Z than soft tissue, which consists of mostly carbon ( 6 C) and hydrogen ( 1 H). However, the low average density of air means that it does not attenuate X-rays strongly. Barium, on the other hand, has an atomic number of 56, and thus attenuates very strongly, which is why it is used as a contrast medium in barium meals and barium enemas. Diagnostic radiography requires high-quality images of internal structures. This means that a high contrast...

calculus

calculus   Quick reference

A Dictionary of Physics (8 ed.)

Reference type:
Subject Reference
Current Version:
2019
Subject:
Science and technology, Physics
Length:
281 words
Illustration(s):
1

...curves and volumes and other problems involving the summation of infinitesimals. For problems such as Brownian motion and fractals , in which functions that are not smoothly varying appear, a rigorous mathematical treatment requires a more general concept of calculus. Such a concept was developed by Norbert Wiener (1894–1964) and others in the first half of the 20th...

big-bang theory

big-bang theory   Quick reference

A Dictionary of Physics (8 ed.)

Reference type:
Subject Reference
Current Version:
2019
Subject:
Science and technology, Physics
Length:
979 words
Illustration(s):
1

... background corresponding to a black-body temperature a few degrees above absolute zero. This microwave background was discovered 20 years later. The big-bang theory also explains the amount of helium in the universe. In 1992 , the COBE satellite discovered that there were very small variations in the microwave background. This discovery helped to explain why the universe formed into galaxies and stars. The nonuniformities that began the nucleation of galactic matter in the early universe now appear as the small variations in the microwave background. In 1998...

quantum mechanics

quantum mechanics   Quick reference

A Dictionary of Physics (8 ed.)

Reference type:
Subject Reference
Current Version:
2019
Subject:
Science and technology, Physics
Length:
1,612 words
Illustration(s):
3

...quantum mechanics A system of mechanics based on quantum theory , which arose out of the failure of classical mechanics and electromagnetic theory to provide a consistent explanation of both electromagnetic waves and atomic structure. Many phenomena at the atomic level puzzled physicists at the beginning of the 20th century because there seemed to be no way of explaining them without making use of incompatible concepts. One such phenomenon was the emission of electrons from the surface of a metal illuminated by light. Einstein realized that the...

horse

horse   Quick reference

A Dictionary of Weights, Measures, and Units

Reference type:
Subject Reference
Current Version:
2004
Subject:
Science and technology, Mathematics and Computer Science, Physics
Length:
244 words
Illustration(s):
1

...In contrast, the horse power figure used for taxing cars in the UK up to 1947 was of the order of 8 to 20; this was a notional calculation, proportional to the aggregate cross-sectional area of the cylinders, i.e. = 0.4 × D 2 ⋅ N where D is the cylinder diameter in inches and N the number of cylinders. If P is the maximal pressure on the pistons in pounds per square inch and S is the stroke length in inches, then Besides the original unit, there is the rounded metric horse power = 75 kg-f⋅m⋅s -1 (0.986 320∼ hp, 0.735 499∼ kWand several...

logarithm

logarithm   Quick reference

A Dictionary of Weights, Measures, and Units

Reference type:
Subject Reference
Current Version:
2004
Subject:
Science and technology, Mathematics and Computer Science, Physics
Length:
569 words
Illustration(s):
1

...late in the 20th century of the electronic calculator, logarithms were the essential means for any extensive calculation with many-digit numbers. Even with the powerful assistance of the computer, logarithms remain the means for elaborate root taking, using the fact that With electronic calculators, the machine calculates any required logarithm, as it does for cosines and other trigonometric values, using the various mathematical formulae. However, prior to the calculator, all such values were obtained by table look-up and interpolation, a process laborious...

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