Ampere
The ampere shortened to "amp", is the base unit of electric current in the International System of Units. It is named after André-Marie Ampère, French mathematician and physicist, considered the father of electrodynamics; the International System of Units defines the ampere in terms of other base units by measuring the electromagnetic force between electrical conductors carrying electric current. The earlier CGS measurement system had two different definitions of current, one the same as the SI's and the other using electric charge as the base unit, with the unit of charge defined by measuring the force between two charged metal plates; the ampere was defined as one coulomb of charge per second. In SI, the unit of charge, the coulomb, is defined as the charge carried by one ampere during one second. New definitions, in terms of invariant constants of nature the elementary charge, will take effect on 20 May 2019. SI defines ampere as follows: The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, placed one metre apart in vacuum, would produce between these conductors a force equal to 2×10−7 newtons per metre of length.
Ampère's force law states that there is an attractive or repulsive force between two parallel wires carrying an electric current. This force is used in the formal definition of the ampere; the SI unit of charge, the coulomb, "is the quantity of electricity carried in 1 second by a current of 1 ampere". Conversely, a current of one ampere is one coulomb of charge going past a given point per second: 1 A = 1 C s. In general, charge Q is determined by steady current I flowing. Constant and average current are expressed in amperes and the charge accumulated, or passed through a circuit over a period of time is expressed in coulombs; the relation of the ampere to the coulomb is the same as that of the watt to the joule. The ampere was defined as one tenth of the unit of electric current in the centimetre–gram–second system of units; that unit, now known as the abampere, was defined as the amount of current that generates a force of two dynes per centimetre of length between two wires one centimetre apart.
The size of the unit was chosen so that the units derived from it in the MKSA system would be conveniently sized. The "international ampere" was an early realization of the ampere, defined as the current that would deposit 0.001118 grams of silver per second from a silver nitrate solution. More accurate measurements revealed that this current is 0.99985 A. Since power is defined as the product of current and voltage, the ampere can alternatively be expressed in terms of the other units using the relationship I=P/V, thus 1 ampere equals 1 W/V. Current can be measured by a multimeter, a device that can measure electrical voltage and resistance; the standard ampere is most realized using a Kibble balance, but is in practice maintained via Ohm's law from the units of electromotive force and resistance, the volt and the ohm, since the latter two can be tied to physical phenomena that are easy to reproduce, the Josephson junction and the quantum Hall effect, respectively. At present, techniques to establish the realization of an ampere have a relative uncertainty of a few parts in 107, involve realizations of the watt, the ohm and the volt.
Rather than a definition in terms of the force between two current-carrying wires, it has been proposed that the ampere should be defined in terms of the rate of flow of elementary charges. Since a coulomb is equal to 6.2415093×1018 elementary charges, one ampere is equivalent to 6.2415093×1018 elementary charges moving past a boundary in one second. The proposed change would define 1 A as being the current in the direction of flow of a particular number of elementary charges per second. In 2005, the International Committee for Weights and Measures agreed to study the proposed change; the new definition was discussed at the 25th General Conference on Weights and Measures in 2014 but for the time being was not adopted. The current drawn by typical constant-voltage energy distribution systems is dictated by the power consumed by the system and the operating voltage. For this reason the examples given below are grouped by voltage level. Current notebook CPUs: up to 15...45 A Current high-end CPUs: up to 55...120 A Hearing aid: 700 µA USB charging adapter: 2 A A typical motor vehicle has a 12 V battery.
The various accessories that are powered by the battery might include: Instrument panel light: 166 mA Headlight: 5 A Starter motor on a smaller car: 50 A to 200 A Most Canada and United States domestic power suppliers run at 120 V. Household circuit breakers provide a maximum of 15 A or 20 A of current to a given set of outlets. USB charging adapter: 83 mA 22-inch/56-centimeter portable television: 290 mA Tungsten light bulb: 500–830 mA Toaster, kettle: 12.5 A Hair dryer: 15 A Most European domestic power supplies run at 230 V, most Commonwealth domestic power supplies run at 2
Magnetic flux
In physics electromagnetism, the magnetic flux through a surface is the surface integral of the normal component of the magnetic field B passing through that surface. The SI unit of magnetic flux is the weber, the CGS unit is the maxwell. Magnetic flux is measured with a fluxmeter, which contains measuring coils and electronics, that evaluates the change of voltage in the measuring coils to calculate the magnetic flux; the magnetic interaction is described in terms of a vector field, where each point in space is associated with a vector that determines what force a moving charge would experience at that point. Since a vector field is quite difficult to visualize at first, in elementary physics one may instead visualize this field with field lines; the magnetic flux through some surface, in this simplified picture, is proportional to the number of field lines passing through that surface. Note that the magnetic flux is the net number of field lines passing through that surface. In more advanced physics, the field line analogy is dropped and the magnetic flux is properly defined as the surface integral of the normal component of the magnetic field passing through a surface.
If the magnetic field is constant, the magnetic flux passing through a surface of vector area S is Φ B = B ⋅ S = B S cos θ, where B is the magnitude of the magnetic field having the unit of Wb/m2, S is the area of the surface, θ is the angle between the magnetic field lines and the normal to S. For a varying magnetic field, we first consider the magnetic flux through an infinitesimal area element dS, where we may consider the field to be constant: d Φ B = B ⋅ d S. A generic surface, S, can be broken into infinitesimal elements and the total magnetic flux through the surface is the surface integral Φ B = ∬ S B ⋅ d S. From the definition of the magnetic vector potential A and the fundamental theorem of the curl the magnetic flux may be defined as: Φ B = ∮ ∂ S A ⋅ d ℓ, where the line integral is taken over the boundary of the surface S, denoted ∂S. Gauss's law for magnetism, one of the four Maxwell's equations, states that the total magnetic flux through a closed surface is equal to zero.
This law is a consequence of the empirical observation that magnetic monopoles have never been found. In other words, Gauss's law for magnetism is the statement: Φ B = S B ⋅ d S = 0 for any closed surface S. While the magnetic flux through a closed surface is always zero, the magnetic flux through an open surface need not be zero and is an important quantity in electromagnetism. For example, a change in the magnetic flux passing through a loop of conductive wire will cause an electromotive force, therefore an electric current, in the loop; the relationship is given by Faraday's law: E = ∮ ∂ Σ ⋅ d ℓ = − d Φ B d t, where E is the electromotive force, ΦB is the magnetic flux through the open surface Σ, ∂Σ is the boundary of the open surface Σ. The electromotive force is induced along this boundary. Dℓ is an infinitesimal vector element of the contour ∂Σ, v is the velocity of the boundary ∂Σ, E is the electric field, B is the magnetic field; the two equations for the EMF are, the work per unit charge done against the Lorentz force in moving a test charge around the surface boundary ∂Σ and, secondly, as the change of magnetic flux through the open surface Σ.
This equation is the principle behind an electrical generator. By way of contrast, Gauss's law for electric fields, another of Maxwell's equations, is Φ E = S {\displaystyle \scripts
Oliver Heaviside
Oliver Heaviside FRS was an English self-taught electrical engineer and physicist who adapted complex numbers to the study of electrical circuits, invented mathematical techniques for the solution of differential equations, reformulated Maxwell's field equations in terms of electric and magnetic forces and energy flux, independently co-formulated vector analysis. Although at odds with the scientific establishment for most of his life, Heaviside changed the face of telecommunications and science for years to come. Heaviside was born in London, at 55 Kings Street, he was a short and red-headed child, suffered from scarlet fever when young, which left him with a hearing impairment. A small legacy enabled the family to move to a better part of Camden when he was thirteen and he was sent to Camden House Grammar School, he was a good student, placed fifth out of five hundred students in 1865, but his parents could not keep him at school after he was 16, so he continued studying for a year by himself and had no further formal education.
Heaviside's uncle by marriage was Sir Charles Wheatstone, an internationally celebrated expert in telegraphy and electromagnetism, the original co-inventor of the first commercially successful telegraph in the mid-1830s. Wheatstone took a strong interest in his nephew's education and in 1867 sent him north to work with his own, older brother Arthur, managing one of Wheatstone's telegraph companies in Newcastle-upon-Tyne. Two years he took a job as a telegraph operator with the Danish Great Northern Telegraph Company laying a cable from Newcastle to Denmark using British contractors, he soon became an electrician. Heaviside continued to study while working, by the age of 22 he published an article in the prestigious Philosophical Magazine on'The Best Arrangement of Wheatstone's Bridge for measuring a Given Resistance with a Given Galvanometer and Battery' which received positive comments from physicists who had unsuccessfully tried to solve this algebraic problem, including Sir William Thomson, to whom he gave a copy of the paper, James Clerk Maxwell.
When he published an article on the duplex method of using a telegraph cable, he poked fun at R. S. Culley, the engineer in chief of the Post Office telegraph system, dismissing duplex as impractical. In 1873 his application to join the Society of Telegraph Engineers was turned down with the comment that "they didn't want telegraph clerks"; this riled Heaviside, who asked Thomson to sponsor him, along with support of the society's president he was admitted "despite the P. O. snobs". In 1873 Heaviside had encountered Maxwell's newly published, famous, two-volume Treatise on Electricity and Magnetism. In his old age Heaviside recalled: I remember my first look at the great treatise of Maxwell's when I was a young man… I saw that it was great and greatest, with prodigious possibilities in its power… I was determined to master the book and set to work. I was ignorant. I had no knowledge of mathematical analysis and thus my work was laid out for me, it took me several years before I could understand as much as I could.
I set Maxwell aside and followed my own course. And I progressed much more quickly… It will be understood that I preach the gospel according to my interpretation of Maxwell. Undertaking research from home, he helped develop transmission line theory. Heaviside showed mathematically that uniformly distributed inductance in a telegraph line would diminish both attenuation and distortion, that, if the inductance were great enough and the insulation resistance not too high, the circuit would be distortionless while currents of all frequencies would have equal speeds of propagation. Heaviside's equations helped further the implementation of the telegraph. From 1882 to 1902, except for three years, he contributed regular articles to the trade paper The Electrician, which wished to improve its standing, for which he was paid £40 per year; this was hardly enough to live on, but his demands were small and he was doing what he most wanted to. Between 1883 and 1887 these averaged 2–3 articles per month and these articles formed the bulk of his Electromagnetic Theory and Electrical Papers.
In 1880, Heaviside researched the skin effect in telegraph transmission lines. That same year he patented, in the coaxial cable. In 1884 he recast Maxwell's mathematical analysis from its original cumbersome form to its modern vector terminology, thereby reducing twelve of the original twenty equations in twenty unknowns down to the four differential equations in two unknowns we now know as Maxwell's equations; the four re-formulated Maxwell's equations describe the nature of electric charges, magnetic fields, the relationship between the two, namely electromagnetic fields. Between 1880 and 1887, Heaviside developed the operational calculus using p for the differential operator, giving a method of solving differential equations by direct solution as algebraic equations; this caused a great deal of controversy, owing to its lack of rigour. He famously said, "Mathematics is an experimental science, definitions do not come first, but on." On another occasion he asked somewhat more defensively, "Shall I refuse my dinner because I do not understand the process of digestion?"In 1887, Heaviside worked with his brother Arthur on a paper entitled "The Bridge System of Telephony".
However the paper was blocked by Arthur's
British Science Association
The British Science Association is a charity and learned society founded in 1831 to aid in the promotion and development of science. Until 2009 it was known as the British Association for the Advancement of Science; the Chief Executive is Katherine Mathieson. In the present, the British Science Association's mission is to get more people engaged in the field of science by coordinating and overseeing different projects that are suited to achieve these goals. To maintain this vision of a world that puts science in the heart of today's culture and society, the British Science Association partners with many national and local organizations that share their vision. Diversifying the people involved in science increases the potential of being able to solve some of the world's biggest challenges in science and to do this the British Science Association are putting together a strategy for 2018-2020 to help them achieve their goals; these key components include: 1. Championing diversity and inclusion, 2.
Improving science education, 3. Influencing and convening stakeholders. Located in the Wellcome Wolfson Building, the BSA's professional team of staff works on creating and delivering a range of projects and events that both recognize and encourage people involved in science; these include the British Science Festival, British Science Week, the CREST Awards, Huxley Summit, Youth Pannel, Media Fellowships Scheme, along with regional and local events. The Association was founded in 1831 and modelled on the German Gesellschaft Deutscher Naturforscher und Ärzte, it was founded during post-war reconstruction after the Peninsula war to improve the advancement of science in England. The prime mover was Reverend William Vernon Harcourt, following a suggestion by Sir David Brewster, disillusioned with the elitist and conservative attitude of the Royal Society. Charles Babbage, William Whewell and J. F. W. Johnston are considered to be founding members; the first meeting was held in York on Tuesday 27 September 1831 with various scientific papers being presented on the following days.
It was chaired by Viscount Milton, President of the Yorkshire Philosophical Society, "upwards of 300 gentlemen" attended the meeting. The Preston Mercury recorded that those gathered consisted of "persons of distinction from various parts of the kingdom, together with several of the gentry of Yorkshire and the members of philosopher societies in this country"; the newspaper published the names of over a hundred of those attending and these included, amongst others, eighteen clergymen, eleven doctors, four knights, two Viscounts and one Lord. From that date onwards a meeting was held annually at a place chosen at a previous meeting. In 1832, for example, the meeting was held in Oxford, chaired by Reverend Dr William Buckland. By this stage the Association had four sections: Physics, Chemistry and Natural History. During this second meeting, the first objects and rules of the Association were published. Objects included systematically directing the acquisition of scientific knowledge, spreading this knowledge as well as discussion between scientists across the world, to focus on furthering science by removing obstacles to progress.
The rules established included what constituted a member of the Association, the fee to remain a member, the process for future meetings. They include dividing the members into different committees; these committees separated members into their preferred subject matter, were to recommend investigations into areas of interest report on these findings, as well as progress in their science at the annual meetings. Additional sections were added throughout the years by either splitting off part of an original section, like making Geography and Ethnology its own section apart from Geology in 1851, or by defining a new subject area of discussion, such as Anthropology in 1869. A important decision in the Association's history was made in 1842 when it was resolved to create a “physical observatory”. A building that became well known as the Kew Observatory was taken on for the purpose and Francis Ronalds was chosen as the inaugural Honorary Director. Kew Observatory became one of the most renowned meteorological and geomagnetic observatories in the world.
The Association relinquished control of the Kew Observatory in 1871 to the management of the Royal Society, after a large donation to grant the observatory its independence. In 1872, the Association purchased its first central office in London, acquiring four rooms at 22 Albemarle Street; this office was intended to be a resource for members of the Association. One of the most famous events linked to the Association Meeting was an exchange between Thomas Henry Huxley and Bishop Samuel Wilberforce in 1860. Although it is described as a "debate", the exchange occurred after the presentation of a paper by Prof Draper of New York, on the intellectual development of Europe with relation to Darwin's theory and the subsequent discussion involved a number of other participants. Although a number of newspapers made passing references to the exchange, it was not until that it was accorded greater significance in the evolution debate. One of the most important contributions of the British Association was the establishment of standards for electrical usage: the ohm as the unit of electrical resistance, the volt as the unit of electrical potential, the ampere as the unit of electrical current.
A need for standards a
International System of Units
The International System of Units is the modern form of the metric system, is the most used system of measurement. It comprises a coherent system of units of measurement built on seven base units, which are the ampere, second, kilogram, mole, a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units; the system specifies names for 22 derived units, such as lumen and watt, for other common physical quantities. The base units are derived from invariant constants of nature, such as the speed of light in vacuum and the triple point of water, which can be observed and measured with great accuracy, one physical artefact; the artefact is the international prototype kilogram, certified in 1889, consisting of a cylinder of platinum-iridium, which nominally has the same mass as one litre of water at the freezing point. Its stability has been a matter of significant concern, culminating in a revision of the definition of the base units in terms of constants of nature, scheduled to be put into effect on 20 May 2019.
Derived units may be defined in terms of other derived units. They are adopted to facilitate measurement of diverse quantities; the SI is intended to be an evolving system. The most recent derived unit, the katal, was defined in 1999; the reliability of the SI depends not only on the precise measurement of standards for the base units in terms of various physical constants of nature, but on precise definition of those constants. The set of underlying constants is modified as more stable constants are found, or may be more measured. For example, in 1983 the metre was redefined as the distance that light propagates in vacuum in a given fraction of a second, thus making the value of the speed of light in terms of the defined units exact; the motivation for the development of the SI was the diversity of units that had sprung up within the centimetre–gram–second systems and the lack of coordination between the various disciplines that used them. The General Conference on Weights and Measures, established by the Metre Convention of 1875, brought together many international organisations to establish the definitions and standards of a new system and standardise the rules for writing and presenting measurements.
The system was published in 1960 as a result of an initiative that began in 1948. It is based on the metre–kilogram–second system of units rather than any variant of the CGS. Since the SI has been adopted by all countries except the United States and Myanmar; the International System of Units consists of a set of base units, derived units, a set of decimal-based multipliers that are used as prefixes. The units, excluding prefixed units, form a coherent system of units, based on a system of quantities in such a way that the equations between the numerical values expressed in coherent units have the same form, including numerical factors, as the corresponding equations between the quantities. For example, 1 N = 1 kg × 1 m/s2 says that one newton is the force required to accelerate a mass of one kilogram at one metre per second squared, as related through the principle of coherence to the equation relating the corresponding quantities: F = m × a. Derived units apply to derived quantities, which may by definition be expressed in terms of base quantities, thus are not independent.
Other useful derived quantities can be specified in terms of the SI base and derived units that have no named units in the SI system, such as acceleration, defined in SI units as m/s2. The SI base units are the building blocks of the system and all the other units are derived from them; when Maxwell first introduced the concept of a coherent system, he identified three quantities that could be used as base units: mass and time. Giorgi identified the need for an electrical base unit, for which the unit of electric current was chosen for SI. Another three base units were added later; the early metric systems defined a unit of weight as a base unit, while the SI defines an analogous unit of mass. In everyday use, these are interchangeable, but in scientific contexts the difference matters. Mass the inertial mass, represents a quantity of matter, it relates the acceleration of a body to the applied force via Newton's law, F = m × a: force equals mass times acceleration. A force of 1 N applied to a mass of 1 kg will accelerate it at 1 m/s2.
This is true whether the object is floating in space or in a gravity field e.g. at the Earth's surface. Weight is the force exerted on a body by a gravitational field, hence its weight depends on the strength of the gravitational field. Weight of a 1 kg mass at the Earth's surface is m × g. Since the acceleration due to gravity is local and varies by location and altitude on the Earth, weight is unsuitable for precision
Henry (unit)
The henry is the SI derived unit of electrical inductance. If a current of 1 ampere flowing through the coil produces flux linkage of 1 weber turn, the coil has a self inductance of 1 henry. The unit is named after Joseph Henry, the American scientist who discovered electromagnetic induction independently of and at about the same time as Michael Faraday in England. The inductance of an electric circuit is one henry when an electric current, changing at one ampere per second results in an electromotive force of one volt across the inductor: V = L d I d t,where V denotes the resulting voltage across the circuit, I is the current through the circuit, L is the inductance of the circuit; the henry is a derived unit based on four of the seven base units of the International System of Units: kilogram, metre and ampere. Expressed in combinations of SI units, the henry is: H = kg ⋅ m 2 s 2 ⋅ A 2 = kg ⋅ m 2 C 2 = J A 2 = T ⋅ m 2 A = Wb A = V ⋅ s A = s 2 F = Ω Hz = Ω ⋅ s, in which the following additional derived units occur: coulomb, joule, tesla, volt and ohm.
The International System of Units specifies to write the symbol of a unit named for a person with an initial capital letter, while the name is not capitalized in sentence text, except when any word in that position would be capitalized, such as at the beginning of a sentence or in material using title case. The United States National Institute of Standards and Technology recommends users writing in English to use the plural as henries; the inductance of a coil depends on its size, the number of turns, the permeability of the material within and surrounding the coil. Formulas can be used to calculate the inductance of many common arrangements of conductors, such as parallel wires, or a solenoid. A small air-core coil used for broadcast AM radio tuning might have an inductance of a few tens of microhenries. A large motor winding with many turns around an iron core may have an inductance of scores or hundreds of henries; the physical size of an inductance is related to its current carrying and voltage withstand ratings.
Inductor