1.
System of measurement
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A system of measurement is a collection of units of measurement and rules relating them to each other. Systems of measurement have historically been important, regulated and defined for the purposes of science and commerce, systems of measurement in modern use include the metric system, the imperial system, and United States customary units. The French Revolution gave rise to the system, and this has spread around the world. In most systems, length, mass, and time are base quantities, later science developments showed that either electric charge or electric current could be added to extend the set of base quantities by which many other metrological units could be easily defined. Other quantities, such as power and speed, are derived from the set, for example. Such arrangements were satisfactory in their own contexts, the preference for a more universal and consistent system only gradually spread with the growth of science. Changing a measurement system has substantial financial and cultural costs which must be offset against the advantages to be obtained using a more rational system. However pressure built up, including scientists and engineers for conversion to a more rational. The unifying characteristic is that there was some definition based on some standard, eventually cubits and strides gave way to customary units to met the needs of merchants and scientists. In the metric system and other recent systems, a basic unit is used for each base quantity. Often secondary units are derived from the units by multiplying by powers of ten. Thus the basic unit of length is the metre, a distance of 1.234 m is 1,234 millimetres. Metrication is complete or nearly complete in almost all countries, US customary units are heavily used in the United States and to some degree in Liberia. Traditional Burmese units of measurement are used in Burma, U. S. units are used in limited contexts in Canada due to the large volume of trade, there is also considerable use of Imperial weights and measures, despite de jure Canadian conversion to metric. In the United States, metric units are used almost universally in science, widely in the military, and partially in industry, but customary units predominate in household use. At retail stores, the liter is a used unit for volume, especially on bottles of beverages. Some other standard non-SI units are still in use, such as nautical miles and knots in aviation. Metric systems of units have evolved since the adoption of the first well-defined system in France in 1795, during this evolution the use of these systems has spread throughout the world, first to non-English-speaking countries, and then to English speaking countries
2.
SI base unit
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The International System of Units defines seven units of measure as a basic set from which all other SI units can be derived. The SI base units form a set of mutually independent dimensions as required by dimensional analysis commonly employed in science, thus, the kelvin, named after Lord Kelvin, has the symbol K and the ampere, named after André-Marie Ampère, has the symbol A. Many other units, such as the litre, are not part of the SI. The definitions of the units have been modified several times since the Metre Convention in 1875. Since the redefinition of the metre in 1960, the kilogram is the unit that is directly defined in terms of a physical artifact. However, the mole, the ampere, and the candela are linked through their definitions to the mass of the platinum–iridium cylinder stored in a vault near Paris. It has long been an objective in metrology to define the kilogram in terms of a fundamental constant, two possibilities have attracted particular attention, the Planck constant and the Avogadro constant. The 23rd CGPM decided to postpone any formal change until the next General Conference in 2011
3.
Electric current
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An electric current is a flow of electric charge. In electric circuits this charge is carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in an ionised gas. The SI unit for measuring a current is the ampere. Electric current is measured using a device called an ammeter, electric currents cause Joule heating, which creates light in incandescent light bulbs. They also create magnetic fields, which are used in motors, inductors and generators, the particles that carry the charge in an electric current are called charge carriers. In metals, one or more electrons from each atom are loosely bound to the atom and these conduction electrons are the charge carriers in metal conductors. The conventional symbol for current is I, which originates from the French phrase intensité de courant, current intensity is often referred to simply as current. The I symbol was used by André-Marie Ampère, after whom the unit of current is named, in formulating the eponymous Ampères force law. The notation travelled from France to Great Britain, where it became standard, in a conductive material, the moving charged particles which constitute the electric current are called charge carriers. In other materials, notably the semiconductors, the carriers can be positive or negative. Positive and negative charge carriers may even be present at the same time, a flow of positive charges gives the same electric current, and has the same effect in a circuit, as an equal flow of negative charges in the opposite direction. Since current can be the flow of positive or negative charges. The direction of current is arbitrarily defined as the same direction as positive charges flow. This is called the direction of current I. If the current flows in the direction, the variable I has a negative value. When analyzing electrical circuits, the direction of current through a specific circuit element is usually unknown. Consequently, the directions of currents are often assigned arbitrarily
4.
International System of Units
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The International System of Units is the modern form of the metric system, and is the most widely used system of measurement. It comprises a coherent system of units of measurement built on seven base units, the system also establishes 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 was published in 1960 as the result of an initiative began in 1948. It is based on the system of units rather than any variant of the centimetre-gram-second system. The motivation for the development of the SI was the diversity of units that had sprung up within the CGS systems, the International System of Units has been adopted by most developed countries, however, the adoption has not been universal in all English-speaking countries. The metric system was first implemented during the French Revolution with just the metre and kilogram as standards of length, in the 1830s Carl Friedrich Gauss laid the foundations for a coherent system based on length, mass, and time. In the 1860s a group working under the auspices of the British Association for the Advancement of Science formulated the requirement for a coherent system of units with base units and derived units. Meanwhile, in 1875, the Treaty of the Metre passed responsibility for verification of the kilogram, in 1921, the Treaty was extended to include all physical quantities including electrical units originally defined in 1893. The units associated with these quantities were the metre, kilogram, second, ampere, kelvin, in 1971, a seventh base quantity, amount of substance represented by the mole, was added to the definition of SI. On 11 July 1792, the proposed the names metre, are, litre and grave for the units of length, area, capacity. The committee also proposed that multiples and submultiples of these units were to be denoted by decimal-based prefixes such as centi for a hundredth, on 10 December 1799, the law by which the metric system was to be definitively adopted in France was passed. Prior to this, the strength of the magnetic field had only been described in relative terms. The technique used by Gauss was to equate the torque induced on a magnet of known mass by the earth’s magnetic field with the torque induced on an equivalent system under gravity. The resultant calculations enabled him to assign dimensions based on mass, length, a French-inspired initiative for international cooperation in metrology led to the signing in 1875 of the Metre Convention. Initially the convention only covered standards for the metre and the kilogram, one of each was selected at random to become the International prototype metre and International prototype kilogram that replaced the mètre des Archives and kilogramme des Archives respectively. Each member state was entitled to one of each of the prototypes to serve as the national prototype for that country. Initially its prime purpose was a periodic recalibration of national prototype metres. The official language of the Metre Convention is French and the version of all official documents published by or on behalf of the CGPM is the French-language version
5.
Electrodynamics
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The theory provides an excellent description of electromagnetic phenomena whenever the relevant length scales and field strengths are large enough that quantum mechanical effects are negligible. For small distances and low field strengths, such interactions are described by quantum electrodynamics. Fundamental physical aspects of classical electrodynamics are presented in texts, such as those by Feynman, Leighton and Sands, Griffiths, Panofsky and Phillips. The physical phenomena that electromagnetism describes have been studied as separate fields since antiquity, for example, there were many advances in the field of optics centuries before light was understood to be an electromagnetic wave. For a detailed account, consult Pauli, Whittaker, Pais. The above equation illustrates that the Lorentz force is the sum of two vectors, one is the cross product of the velocity and magnetic field vectors. Based on the properties of the product, this produces a vector that is perpendicular to both the velocity and magnetic field vectors. The other vector is in the direction as the electric field. The sum of two vectors is the Lorentz force. In the absence of a field, the force is perpendicular to the velocity of the particle. If both electric and magnetic fields are present, the Lorentz force is the sum of both of these vectors, the electric field E is defined such that, on a stationary charge, F = q 0 E where q0 is what is known as a test charge. The size of the charge doesnt really matter, as long as it is small enough not to influence the field by its mere presence. What is plain from this definition, though, is that the unit of E is N/C and this unit is equal to V/m, see below. In electrostatics, where charges are not moving, around a distribution of point charges, both of the above equations are cumbersome, especially if one wants to determine E as a function of position. A scalar function called the potential can help. Electric potential, also called voltage, is defined by the line integral φ = − ∫ C E ⋅ d l where φ is the electric potential, unfortunately, this definition has a caveat. From Maxwells equations, it is clear that ∇ × E is not always zero, as a result, one must add a correction factor, which is generally done by subtracting the time derivative of the A vector potential described below. Whenever the charges are quasistatic, however, this condition will be essentially met, the scalar φ will add to other potentials as a scalar
6.
Electric charge
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Electric charge is the physical property of matter that causes it to experience a force when placed in an electromagnetic field. There are two types of charges, positive and negative. Like charges repel and unlike attract, an absence of net charge is referred to as neutral. An object is charged if it has an excess of electrons. The SI derived unit of charge is the coulomb. In electrical engineering, it is common to use the ampere-hour. The symbol Q often denotes charge, early knowledge of how charged substances interact is now called classical electrodynamics, and is still accurate for problems that dont require consideration of quantum effects. The electric charge is a conserved property of some subatomic particles. Electrically charged matter is influenced by, and produces, electromagnetic fields, the interaction between a moving charge and an electromagnetic field is the source of the electromagnetic force, which is one of the four fundamental forces. 602×10−19 coulombs. The proton has a charge of +e, and the electron has a charge of −e, the study of charged particles, and how their interactions are mediated by photons, is called quantum electrodynamics. Charge is the property of forms of matter that exhibit electrostatic attraction or repulsion in the presence of other matter. Electric charge is a property of many subatomic particles. The charges of free-standing particles are integer multiples of the charge e. Michael Faraday, in his electrolysis experiments, was the first to note the discrete nature of electric charge, robert Millikans oil drop experiment demonstrated this fact directly, and measured the elementary charge. By convention, the charge of an electron is −1, while that of a proton is +1, charged particles whose charges have the same sign repel one another, and particles whose charges have different signs attract. The charge of an antiparticle equals that of the corresponding particle, quarks have fractional charges of either −1/3 or +2/3, but free-standing quarks have never been observed. The electric charge of an object is the sum of the electric charges of the particles that make it up. An ion is an atom that has lost one or more electrons, giving it a net charge, or that has gained one or more electrons
7.
Coulomb
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The coulomb is the International System of Units unit of electric charge. 242×1018 protons, and −1 C is equivalent to the charge of approximately 6. 242×1018 electrons. This SI unit is named after Charles-Augustin de Coulomb, as with every International System of Units unit named for a person, the first letter of its symbol is upper case. Note that degree Celsius conforms to this rule because the d is lowercase. — Based on The International System of Units, the SI system defines the coulomb in terms of the ampere and second,1 C =1 A ×1 s. The second is defined in terms of a frequency emitted by caesium atoms. The ampere is defined using Ampères force law, the definition relies in part on the mass of the prototype kilogram. In practice, the balance is used to measure amperes with the highest possible accuracy. One coulomb is the magnitude of charge in 6. 24150934×10^18 protons or electrons. The inverse of this gives the elementary charge of 1. 6021766208×10−19 C. The magnitude of the charge of one mole of elementary charges is known as a faraday unit of charge. In terms of Avogadros number, one coulomb is equal to approximately 1.036 × NA×10−5 elementary charges, one ampere-hour =3600 C,1 mA⋅h =3.6 C. One statcoulomb, the obsolete CGS electrostatic unit of charge, is approximately 3. 3356×10−10 C or about one-third of a nanocoulomb, the elementary charge, the charge of a proton, is approximately 1. 6021766208×10−19 C. In SI, the charge in coulombs is an approximate value. However, in other systems, the elementary charge has an exact value by definition. Specifically, e90 = / C exactly, SI itself may someday change its definitions in a similar way. For example, one possible proposed redefinition is the ampere. is such that the value of the charge e is exactly 1. 602176487×10−19 coulombs. This proposal is not yet accepted as part of the SI, the charges in static electricity from rubbing materials together are typically a few microcoulombs. The amount of charge that travels through a lightning bolt is typically around 15 C, the amount of charge that travels through a typical alkaline AA battery from being fully charged to discharged is about 5 kC =5000 C ≈1400 mA⋅h. The hydraulic analogy uses everyday terms to illustrate movement of charge, the analogy equates charge to a volume of water, and voltage to pressure
8.
Metre
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The metre or meter, is the base unit of length in the International System of Units. The metre is defined as the length of the path travelled by light in a vacuum in 1/299792458 seconds, the metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole. In 1799, it was redefined in terms of a metre bar. In 1960, the metre was redefined in terms of a number of wavelengths of a certain emission line of krypton-86. In 1983, the current definition was adopted, the imperial inch is defined as 0.0254 metres. One metre is about 3 3⁄8 inches longer than a yard, Metre is the standard spelling of the metric unit for length in nearly all English-speaking nations except the United States and the Philippines, which use meter. Measuring devices are spelled -meter in all variants of English, the suffix -meter has the same Greek origin as the unit of length. This range of uses is found in Latin, French, English. Thus calls for measurement and moderation. In 1668 the English cleric and philosopher John Wilkins proposed in an essay a decimal-based unit of length, as a result of the French Revolution, the French Academy of Sciences charged a commission with determining a single scale for all measures. In 1668, Wilkins proposed using Christopher Wrens suggestion of defining the metre using a pendulum with a length which produced a half-period of one second, christiaan Huygens had observed that length to be 38 Rijnland inches or 39.26 English inches. This is the equivalent of what is now known to be 997 mm, no official action was taken regarding this suggestion. In the 18th century, there were two approaches to the definition of the unit of length. One favoured Wilkins approach, to define the metre in terms of the length of a pendulum which produced a half-period of one second. The other approach was to define the metre as one ten-millionth of the length of a quadrant along the Earths meridian, that is, the distance from the Equator to the North Pole. This means that the quadrant would have defined as exactly 10000000 metres at that time. To establish a universally accepted foundation for the definition of the metre, more measurements of this meridian were needed. This portion of the meridian, assumed to be the length as the Paris meridian, was to serve as the basis for the length of the half meridian connecting the North Pole with the Equator
9.
Newton (unit)
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The newton is the International System of Units derived unit of force. It is named after Isaac Newton in recognition of his work on classical mechanics, see below for the conversion factors. One newton is the force needed to one kilogram of mass at the rate of one metre per second squared in direction of the applied force. In 1948, the 9th CGPM resolution 7 adopted the name newton for this force, the MKS system then became the blueprint for todays SI system of units. The newton thus became the unit of force in le Système International dUnités. This SI unit is named after Isaac Newton, as with every International System of Units unit named for a person, the first letter of its symbol is upper case. Note that degree Celsius conforms to this rule because the d is lowercase. — Based on The International System of Units, section 5.2. Newtons second law of motion states that F = ma, where F is the applied, m is the mass of the object receiving the force. The newton is therefore, where the symbols are used for the units, N for newton, kg for kilogram, m for metre. In dimensional analysis, F = M L T2 where F is force, M is mass, L is length, at average gravity on earth, a kilogram mass exerts a force of about 9.8 newtons. An average-sized apple exerts about one newton of force, which we measure as the apples weight, for example, the tractive effort of a Class Y steam train and the thrust of an F100 fighter jet engine are both around 130 kN. One kilonewton,1 kN, is 102.0 kgf,1 kN =102 kg ×9.81 m/s2 So for example, a platform rated at 321 kilonewtons will safely support a 32,100 kilograms load. Specifications in kilonewtons are common in safety specifications for, the values of fasteners, Earth anchors. Working loads in tension and in shear, thrust of rocket engines and launch vehicles clamping forces of the various moulds in injection moulding machines used to manufacture plastic parts
10.
Battery (electricity)
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An electric battery is a device consisting of one or more electrochemical cells with external connections provided to power electrical devices such as flashlights, smartphones, and electric cars. When a battery is supplying power, its positive terminal is the cathode. The terminal marked negative is the source of electrons that when connected to a circuit will flow. It is the movement of ions within the battery which allows current to flow out of the battery to perform work. Historically the term specifically referred to a device composed of multiple cells. Primary batteries are used once and discarded, the materials are irreversibly changed during discharge. Common examples are the battery used for flashlights and a multitude of portable electronic devices. Secondary batteries can be discharged and recharged multiple times using mains power from a wall socket, examples include the lead-acid batteries used in vehicles and lithium-ion batteries used for portable electronics such as laptops and smartphones. According to a 2005 estimate, the battery industry generates US$48 billion in sales each year. Batteries have much lower energy than common fuels such as gasoline. This is somewhat offset by the efficiency of electric motors in producing mechanical work. The usage of battery to describe a group of electrical devices dates to Benjamin Franklin, alessandro Volta built and described the first electrochemical battery, the voltaic pile, in 1800. This was a stack of copper and zinc plates, separated by brine-soaked paper disks, Volta did not understand that the voltage was due to chemical reactions. Although early batteries were of value for experimental purposes, in practice their voltages fluctuated. It consisted of a pot filled with a copper sulfate solution, in which was immersed an unglazed earthenware container filled with sulfuric acid. These wet cells used liquid electrolytes, which were prone to leakage and spillage if not handled correctly, many used glass jars to hold their components, which made them fragile and potentially dangerous. These characteristics made wet cells unsuitable for portable appliances, near the end of the nineteenth century, the invention of dry cell batteries, which replaced the liquid electrolyte with a paste, made portable electrical devices practical. Batteries convert chemical energy directly to electrical energy, a battery consists of some number of voltaic cells
11.
Joule
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The joule, symbol J, is a derived unit of energy in the International System of Units. It is equal to the transferred to an object when a force of one newton acts on that object in the direction of its motion through a distance of one metre. It is also the energy dissipated as heat when a current of one ampere passes through a resistance of one ohm for one second. It is named after the English physicist James Prescott Joule, one joule can also be defined as, The work required to move an electric charge of one coulomb through an electrical potential difference of one volt, or one coulomb volt. This relationship can be used to define the volt, the work required to produce one watt of power for one second, or one watt second. This relationship can be used to define the watt and this SI unit is named after James Prescott Joule. As with every International System of Units unit named for a person, note that degree Celsius conforms to this rule because the d is lowercase. — Based on The International System of Units, section 5.2. The CGPM has given the unit of energy the name Joule, the use of newton metres for torque and joules for energy is helpful to avoid misunderstandings and miscommunications. The distinction may be also in the fact that energy is a scalar – the dot product of a vector force. By contrast, torque is a vector – the cross product of a distance vector, torque and energy are related to one another by the equation E = τ θ, where E is energy, τ is torque, and θ is the angle swept. Since radians are dimensionless, it follows that torque and energy have the same dimensions, one joule in everyday life represents approximately, The energy required to lift a medium-size tomato 1 m vertically from the surface of the Earth. The energy released when that same tomato falls back down to the ground, the energy required to accelerate a 1 kg mass at 1 m·s−2 through a 1 m distance in space. The heat required to raise the temperature of 1 g of water by 0.24 °C, the typical energy released as heat by a person at rest every 1/60 s. The kinetic energy of a 50 kg human moving very slowly, the kinetic energy of a 56 g tennis ball moving at 6 m/s. The kinetic energy of an object with mass 1 kg moving at √2 ≈1.4 m/s, the amount of electricity required to light a 1 W LED for 1 s. Since the joule is also a watt-second and the unit for electricity sales to homes is the kW·h. For additional examples, see, Orders of magnitude The zeptojoule is equal to one sextillionth of one joule,160 zeptojoules is equivalent to one electronvolt. The nanojoule is equal to one billionth of one joule, one nanojoule is about 1/160 of the kinetic energy of a flying mosquito
12.
Silver nitrate
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Silver nitrate is an inorganic compound with chemical formula AgNO3. This compound is a precursor to many other silver compounds. It is far less sensitive to light than the halides and it was once called lunar caustic because silver was called luna by the ancient alchemists, who believed that silver was associated with the moon. In solid silver nitrate, the ions are three-coordinated in a trigonal planar arrangement. Albertus Magnus, in the 13th century, documented the ability of nitric acid to separate gold, Magnus noted that the resulting solution of silver nitrate could blacken skin. Silver nitrate can be prepared by reacting silver, such as a silver bullion or silver foil, with acid, resulting in silver nitrate, water. Reaction byproducts depend upon the concentration of acid used. 3 Ag +4 HNO3 →3 AgNO3 +2 H2O + NO Ag +2 HNO3 → AgNO3 + H2O + NO2 This is performed under a fume hood because of nitrogen oxide evolved during the reaction. A typical reaction with silver nitrate is to suspend a rod of copper in a solution of silver nitrate, silver nitrate is the least expensive salt of silver, it offers several other advantages as well. It is non-hygroscopic, in contrast to silver fluoroborate and silver perchlorate and it is relatively stable to light. Finally, it dissolves in solvents, including water. The nitrate can be replaced by other ligands, rendering AgNO3 versatile. Treatment with solutions of halide ions gives a precipitate of AgX, similarly, silver nitrate is used to prepare some silver-based explosives, such as the fulminate, azide, or acetylide, through a precipitation reaction. This reaction is used in inorganic chemistry to abstract halides, Ag+ + X− → AgX where X− = Cl−, Br−. Other silver salts with non-coordinating anions, namely silver tetrafluoroborate and silver hexafluorophosphate are used for demanding applications. Similarly, this reaction is used in chemistry to confirm the presence of chloride, bromide. Samples are typically acidified with nitric acid to remove interfering ions, e. g. carbonate ions. This step avoids confusion of silver sulfide or silver carbonate precipitates with that of silver halides, the color of precipitate varies with the halide, white, pale yellow/cream, yellow
13.
Power (physics)
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In physics, power is the rate of doing work. It is the amount of energy consumed per unit time, having no direction, it is a scalar quantity. In the SI system, the unit of power is the joule per second, known as the watt in honour of James Watt, another common and traditional measure is horsepower. Being the rate of work, the equation for power can be written, because this integral depends on the trajectory of the point of application of the force and torque, this calculation of work is said to be path dependent. As a physical concept, power requires both a change in the universe and a specified time in which the change occurs. This is distinct from the concept of work, which is measured in terms of a net change in the state of the physical universe. The output power of a motor is the product of the torque that the motor generates. The power involved in moving a vehicle is the product of the force of the wheels. The dimension of power is divided by time. The SI unit of power is the watt, which is equal to one joule per second, other units of power include ergs per second, horsepower, metric horsepower, and foot-pounds per minute. One horsepower is equivalent to 33,000 foot-pounds per minute, or the required to lift 550 pounds by one foot in one second. Other units include dBm, a logarithmic measure with 1 milliwatt as reference, food calories per hour, Btu per hour. This shows how power is an amount of energy consumed per unit time. If ΔW is the amount of work performed during a period of time of duration Δt and it is the average amount of work done or energy converted per unit of time. The average power is simply called power when the context makes it clear. The instantaneous power is then the value of the average power as the time interval Δt approaches zero. P = lim Δ t →0 P a v g = lim Δ t →0 Δ W Δ t = d W d t. In the case of constant power P, the amount of work performed during a period of duration T is given by, W = P t
14.
Kibble balance
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A watt balance is an experimental electromechanical weight measuring instrument that measures the weight of a test object very precisely by the strength of an electric current and a voltage. In 2016, metrologists agreed to rename watt balances as Kibble balances, in honour of and it is being developed as a metrological instrument that may one day provide a definition of the kilogram unit of mass based on electronic units, a so-called electronic or electrical kilogram. The name watt balance comes from the fact that the weight of the test mass is proportional to the product of the current and the voltage, which is measured in units of watts. In this new application, the balance will be used in the opposite sense, the weight of the kilogram is then used to compute the mass of the kilogram by accurately determining the local gravitational acceleration. This will define the mass of a kilogram in terms of a current, the principle that is used in the watt balance was proposed by B. P. Kibble of the UK National Physical Laboratory in 1975 for measurement of the gyromagnetic ratio. The main weakness of the balance method is that the result depends on the accuracy with which the dimensions of the coils are measured. The watt balance method has an extra step in which the effect of the geometry of the coils is eliminated. This extra step involves moving the force coil through a magnetic flux at a known speed. This step was done in 1990, in 2014, NRC researchers published the most accurate measurement of the Planck constant to date, with a relative uncertainty of 1. 8×10−8. A conducting wire of length L that carries an electric current I perpendicular to a field of strength B will experience a Laplace force equal to BLI. In the watt balance, the current is varied so that this force exactly counteracts the weight w of a mass m. This is also the principle behind the ampere balance, W is given by the mass m multiplied by the local gravitational acceleration g. Kibbles watt balance avoids the problems of measuring B and L with a calibration step. The same wire is moved through the magnetic field at a known speed v. By Faradays law of induction, a potential difference U is generated across the ends of the wire. The unknown product BL can be eliminated from the equations to give U I = m g v. With U, I, g, and v accurately measured, this gives an accurate value for m. Both sides of the equation have the dimensions of power, measured in watts in the International System of Units, the current watt balance experiments are equivalent to measuring the value of the conventional watt in SI units. The importance of measurements is that they are also a direct measurement of the Planck constant h, h =4 K J2 R K. The principle of the kilogram would be to define the value of the Planck constant in the same way that the meter is defined by the speed of light
15.
Ohm's law
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Ohms law states that the current through a conductor between two points is directly proportional to the voltage across the two points. More specifically, Ohms law states that the R in this relation is constant, independent of the current and he presented a slightly more complex equation than the one above to explain his experimental results. The above equation is the form of Ohms law. In physics, the term Ohms law is used to refer to various generalizations of the law originally formulated by Ohm. This reformulation of Ohms law is due to Gustav Kirchhoff, in January 1781, before Georg Ohms work, Henry Cavendish experimented with Leyden jars and glass tubes of varying diameter and length filled with salt solution. He measured the current by noting how strong a shock he felt as he completed the circuit with his body, Cavendish wrote that the velocity varied directly as the degree of electrification. He did not communicate his results to other scientists at the time, francis Ronalds delineated “intensity” and “quantity” for the dry pile – a high voltage source – in 1814 using a gold-leaf electrometer. He found for a dry pile that the relationship between the two parameters was not proportional under certain meteorological conditions, Ohm did his work on resistance in the years 1825 and 1826, and published his results in 1827 as the book Die galvanische Kette, mathematisch bearbeitet. He drew considerable inspiration from Fouriers work on heat conduction in the explanation of his work. For experiments, he initially used voltaic piles, but later used a thermocouple as this provided a stable voltage source in terms of internal resistance. He used a galvanometer to measure current, and knew that the voltage between the terminals was proportional to the junction temperature. He then added test wires of varying length, diameter, from this, Ohm determined his law of proportionality and published his results. Ohms law was probably the most important of the early descriptions of the physics of electricity. We consider it almost obvious today, when Ohm first published his work, this was not the case, critics reacted to his treatment of the subject with hostility. They called his work a web of naked fancies and the German Minister of Education proclaimed that a professor who preached such heresies was unworthy to teach science, also, Ohms brother Martin, a mathematician, was battling the German educational system. These factors hindered the acceptance of Ohms work, and his work did not become widely accepted until the 1840s, fortunately, Ohm received recognition for his contributions to science well before he died. While the old term for electrical conductance, the mho, is used, a new name. The siemens is preferred in formal papers, Ohms work long preceded Maxwells equations and any understanding of frequency-dependent effects in AC circuits
16.
Electromotive force
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Electromotive force, also called emf, is the voltage developed by any source of electrical energy such as a battery or dynamo. It is generally defined as the potential for a source in a circuit. A device that supplies electrical energy is called electromotive force or emf, emfs convert chemical, mechanical, and other forms of energy into electrical energy. The product of such a device is known as emf. The word force in case is not used to mean mechanical force, measured in newtons. In electromagnetic induction, emf can be defined around a loop as the electromagnetic work that would be done on a charge if it travels once around that loop. This potential difference can drive a current if a circuit is attached to the terminals. Devices that can provide emf include electrochemical cells, thermoelectric devices, solar cells, photodiodes, electrical generators, transformer, in nature, emf is generated whenever magnetic field fluctuations occur through a surface. The shifting of the Earths magnetic field during a geomagnetic storm, … By chemical, mechanical or other means, the source of emf performs work dW on that charge to move it to the high potential terminal. The emf ℰ of the source is defined as the work dW done per charge dq, in the open-circuit case, charge separation continues until the electrical field from the separated charges is sufficient to arrest the reactions. Again the emf is countered by the voltage due to charge separation. If a load is attached, this voltage can drive a current, the general principle governing the emf in such electrical machines is Faradays law of induction. Electromotive force is often denoted by E or ℰ, in a device without internal resistance, if an electric charge Q passes through that device, and gains an energy W, the net emf for that device is the energy gained per unit charge, or J/Q. Like other measures of energy per charge, emf has SI units of volts, Electromotive force in electrostatic units is the statvolt. Inside a source of emf that is open-circuited, the electrostatic field created by separation of charge exactly cancels the forces producing the emf. Thus, the emf has the same value but opposite sign as the integral of the field aligned with an internal path between two terminals A and B of a source of emf in open-circuit condition. This equation applies only to locations A and B that are terminals and this equation involves the electrostatic electric field due to charge separation Ecs and does not involve any non-conservative component of electric field due to Faradays law of induction. The electrostatic field does not contribute to the net emf around a circuit because the portion of the electric field is conservative
17.
Volt
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The volt is the derived unit for electric potential, electric potential difference, and electromotive force. One volt is defined as the difference in potential between two points of a conducting wire when an electric current of one ampere dissipates one watt of power between those points. It is also equal to the difference between two parallel, infinite planes spaced 1 meter apart that create an electric field of 1 newton per coulomb. Additionally, it is the difference between two points that will impart one joule of energy per coulomb of charge that passes through it. It can also be expressed as amperes times ohms, watts per ampere, or joules per coulomb, for the Josephson constant, KJ = 2e/h, the conventional value KJ-90 is used, K J-90 =0.4835979 GHz μ V. This standard is typically realized using an array of several thousand or tens of thousands of junctions. Empirically, several experiments have shown that the method is independent of device design, material, measurement setup, etc. in the water-flow analogy sometimes used to explain electric circuits by comparing them with water-filled pipes, voltage is likened to difference in water pressure. Current is proportional to the diameter of the pipe or the amount of water flowing at that pressure. A resistor would be a reduced diameter somewhere in the piping, the relationship between voltage and current is defined by Ohms Law. Ohms Law is analogous to the Hagen–Poiseuille equation, as both are linear models relating flux and potential in their respective systems, the voltage produced by each electrochemical cell in a battery is determined by the chemistry of that cell. Cells can be combined in series for multiples of that voltage, mechanical generators can usually be constructed to any voltage in a range of feasibility. High-voltage electric power lines,110 kV and up Lightning, Varies greatly. Volta had determined that the most effective pair of metals to produce electricity was zinc. In 1861, Latimer Clark and Sir Charles Bright coined the name volt for the unit of resistance, by 1873, the British Association for the Advancement of Science had defined the volt, ohm, and farad. In 1881, the International Electrical Congress, now the International Electrotechnical Commission and they made the volt equal to 108 cgs units of voltage, the cgs system at the time being the customary system of units in science. At that time, the volt was defined as the difference across a conductor when a current of one ampere dissipates one watt of power. The international volt was defined in 1893 as 1/1.434 of the emf of a Clark cell and this definition was abandoned in 1908 in favor of a definition based on the international ohm and international ampere until the entire set of reproducible units was abandoned in 1948. Prior to the development of the Josephson junction voltage standard, the volt was maintained in laboratories using specially constructed batteries called standard cells
18.
Ohm
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The ohm is the SI derived unit of electrical resistance, named after German physicist Georg Simon Ohm. The definition of the ohm was revised several times, today the definition of the ohm is expressed from the quantum Hall effect. In many cases the resistance of a conductor in ohms is approximately constant within a range of voltages, temperatures. In alternating current circuits, electrical impedance is also measured in ohms, the siemens is the SI derived unit of electric conductance and admittance, also known as the mho, it is the reciprocal of resistance in ohms. The power dissipated by a resistor may be calculated from its resistance, non-linear resistors have a value that may vary depending on the applied voltage. The rapid rise of electrotechnology in the last half of the 19th century created a demand for a rational, coherent, consistent, telegraphers and other early users of electricity in the 19th century needed a practical standard unit of measurement for resistance. Two different methods of establishing a system of units can be chosen. Various artifacts, such as a length of wire or a standard cell, could be specified as producing defined quantities for resistance, voltage. This latter method ensures coherence with the units of energy, defining a unit for resistance that is coherent with units of energy and time in effect also requires defining units for potential and current. Some early definitions of a unit of resistance, for example, the absolute-units system related magnetic and electrostatic quantities to metric base units of mass, time, and length. These units had the advantage of simplifying the equations used in the solution of electromagnetic problems. However, the CGS units turned out to have impractical sizes for practical measurements, various artifact standards were proposed as the definition of the unit of resistance. In 1860 Werner Siemens published a suggestion for a reproducible resistance standard in Poggendorffs Annalen der Physik und Chemie and he proposed a column of pure mercury, of one square millimetre cross section, one metre long, Siemens mercury unit. However, this unit was not coherent with other units, one proposal was to devise a unit based on a mercury column that would be coherent – in effect, adjusting the length to make the resistance one ohm. Not all users of units had the resources to carry out experiments to the required precision. The BAAS in 1861 appointed a committee including Maxwell and Thomson to report upon Standards of Electrical Resistance, in the third report of the committee,1864, the resistance unit is referred to as B. A. unit, or Ohmad. By 1867 the unit is referred to as simply Ohm, the B. A. ohm was intended to be 109 CGS units but owing to an error in calculations the definition was 1. 3% too small. The error was significant for preparation of working standards, on September 21,1881 the Congrès internationale délectriciens defined a practical unit of Ohm for the resistance, based on CGS units, using a mercury column at zero deg
19.
Josephson effect
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The Josephson effect is the phenomenon of supercurrent—i. e. A current that flows indefinitely long without any voltage applied—across a device known as a Josephson junction, the weak link can consist of a thin insulating barrier, a short section of non-superconducting metal, or a physical constriction that weakens the superconductivity at the point of contact. The Josephson effect is an example of a quantum phenomenon. It is named after the British physicist Brian David Josephson, who predicted in 1962 the mathematical relationships for the current, the first paper to claim the discovery of Josephsons effect, and to make the requisite experimental checks, was that of Philip Anderson and John Rowell. These authors were awarded patents on the effects that were never enforced, before Josephsons prediction, it was only known that normal electrons can flow through an insulating barrier, by means of quantum tunneling. Josephson was the first to predict the tunneling of superconducting Cooper pairs, for this work, Josephson received the Nobel Prize in Physics in 1973. Josephson junctions have important applications in quantum-mechanical circuits, such as SQUIDs, superconducting qubits, the NIST standard for one volt is achieved by an array of 20,208 Josephson junctions in series. Types of Josephson junction include the pi Josephson junction, varphi Josephson junction, long Josephson junction, a Dayem bridge is a thin-film variant of the Josephson junction in which the weak link consists of a superconducting wire with dimensions on the scale of a few micrometres or less. The Josephson junction count of a device is used as a benchmark for its complexity and they are widely used in science and engineering. In precision metrology, the Josephson effect provides an exactly reproducible conversion between frequency and voltage, however, BIPM has not changed the official SI unit definition. Single-electron transistors are often constructed of superconducting materials, allowing use to be made of the Josephson effect to achieve novel effects, the resulting device is called a superconducting single-electron transistor. The Josephson effect is used for the most precise measurements of elementary charge in terms of the Josephson constant. RSFQ digital electronics is based on shunted Josephson junctions, Josephson junctions are integral in superconducting quantum computing as qubits such as in a flux qubit or others schemes where the phase and charge act as the conjugate variables. Superconducting tunnel junction detectors may become a replacement for CCDs for use in astronomy. These devices are effective across a spectrum from ultraviolet to infrared. The technology has been tried out on the William Herschel Telescope in the SCAM instrument, quiterons and similar superconducting switching devices. Josephson effect has also observed in SHeQUIDs, the superfluid helium analog of a dc-SQUID. The critical current is an important phenomenological parameter of the device that can be affected by temperature as well as by a magnetic field
20.
Quantum Hall effect
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The prefactor, ν is known as the filling factor, and can take on either integer or fractional values. The quantum Hall effect is referred to as the integer or fractional quantum Hall effect depending on whether ν is an integer or fraction, the striking feature of the integer quantum Hall effect is the persistence of the quantization as the electron density is varied. The fractional quantum Hall effect is more complicated, as its existence relies fundamentally on electron–electron interactions, although the microscopic origins of the fractional quantum Hall effect are unknown, there are several phenomenological approaches that provide accurate approximations. For example, the effect can be thought of as an integer quantum Hall effect, not of electrons, in 1988, it was proposed that there was quantum Hall effect without Landau levels. This quantum Hall effect is referred to as the quantum anomalous Hall effect, there is also a new concept of the quantum spin Hall effect which is an analogue of the quantum Hall effect, where spin currents flow instead of charge currents. The quantization of the Hall conductance has the important property of being exceedingly precise, actual measurements of the Hall conductance have been found to be integer or fractional multiples of e2/h to nearly one part in a billion. This phenomenon, referred to as exact quantization, has shown to be a subtle manifestation of the principle of gauge invariance. It has allowed for the definition of a new standard for electrical resistance. This is named after Klaus von Klitzing, the discoverer of exact quantization, since 1990, a fixed conventional value RK-90 is used in resistance calibrations worldwide. The quantum Hall effect also provides an extremely precise independent determination of the structure constant. Several researchers subsequently observed the effect in experiments carried out on the layer of MOSFETs. For this finding, von Klitzing was awarded the 1985 Nobel Prize in Physics, the link between exact quantization and gauge invariance was subsequently found by Robert Laughlin, who connected the quantized conductivity to the quantized charge transport in Thouless charge pump. Most integer quantum Hall experiments are now performed on gallium arsenide heterostructures, in 2007, the integer quantum Hall effect was reported in graphene at temperatures as high as room temperature, and in the oxide ZnO-MgxZn1−xO. In two dimensions, when electrons are subjected to a magnetic field they follow circular cyclotron orbits. When the system is treated quantum mechanically, these orbits are quantized, the energy levels of these quantized orbitals take on discrete values, E n = ℏ ω c, where ωc = eB/m is the cyclotron frequency. For strong magnetic fields, each Landau level is highly degenerate, the integers that appear in the Hall effect are examples of topological quantum numbers. They are known in mathematics as the first Chern numbers and are related to Berrys phase. A striking model of much interest in this context is the Azbel-Harper-Hofstadter model whose quantum phase diagram is the Hofstadter butterfly shown in the figure, the vertical axis is the strength of the magnetic field and the horizontal axis is the chemical potential, which fixes the electron density
21.
Relative uncertainty
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The approximation error in some data is the discrepancy between an exact value and some approximation to it. An approximation error can occur because the measurement of the data is not precise due to the instruments, or approximations are used instead of the real data. In the mathematical field of analysis, the numerical stability of an algorithm in numerical analysis indicates how the error is propagated by the algorithm. One commonly distinguishes between the error and the absolute error. Given some value v and its approximation vapprox, the error is ϵ = | v − v approx |. In words, the error is the magnitude of the difference between the exact value and the approximation. The relative error is the absolute error divided by the magnitude of the exact value, the percent error is the relative error expressed in terms of per 100. These definitions can be extended to the case when v and v approx are n-dimensional vectors, by replacing the absolute value with an n-norm. As an example, if the value is 50 and the approximation is 49.9, then the absolute error is 0.1. Another example would be if, in measuring a 6mL beaker, the correct reading being 6mL, this means the percent error in that particular situation is, rounded,16. 7%.003 and in the second it is only 0.000003. There are two features of relative error that should be kept in mind, firstly, relative error is undefined when the true value is zero as it appears in the denominator. Secondly, relative error only makes sense when measured on a ratio scale, otherwise it would be sensitive to the measurement units. For example, when an error in a temperature measurement given in Celsius scale is 1 °C, and the true value is 2 °C, the relative error is 0.5. Celsius temperature is measured on a scale, whereas the Kelvin scale has a true zero. In most indicating instruments, the accuracy is guaranteed to a percentage of full-scale reading. The limits of deviations from the specified values are known as limiting errors or guarantee errors
22.
Proposed redefinition of SI base units
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The metric system was originally conceived as a system of measurement that was derivable from nature. When the metric system was first introduced in France in 1799 technical limitations necessitated the use of such as the prototype metre. In 1960 the metre was redefined in terms of the wavelength of light from a source, making it derivable from nature. If the proposed redefinition is accepted, the system will, for the first time. The proposal can be summarised as follows, There will still be the seven base units. The second, metre and candela are already defined by physical constants, the new definitions will improve the SI without changing the size of any units, thus ensuring continuity with present measurements. Further details are found in the chapter of the Ninth SI Units Brochure. The last major overhaul of the system was in 1960 when the International System of Units was formally published as a coherent set of units of measure. SI is structured around seven base units that have apparently arbitrary definitions, although the set of units form a coherent system, the definitions do not. The proposal before the CIPM seeks to remedy this by using the quantities of nature as the basis for deriving the base units. This will mean, amongst other things, that the prototype kilogram will cease to be used as the replica of the kilogram. The second and the metre are already defined in such a manner, the basic structure of SI was developed over a period of about 170 years. Since 1960 technological advances have made it possible to address weaknesses in SI. Specifically, the metre was defined as one ten-millionth of the distance from the North Pole to the Equator, although these definitions were chosen so that nobody would own the units, they could not be measured with sufficient convenience or precision for practical use. Instead copies were created in the form of the mètre des Archives, in 1875, by which time the use of the metric system had become widespread in Europe and in Latin America, twenty industrially developed nations met for the Convention of the Metre. They were, CGPM —The Conference meets every four to six years, CIPM —The Committee consists of eighteen eminent scientists, each from a different country, nominated by the CGPM. The CIPM meets annually and is tasked to advise the CGPM, the CIPM has set up a number of sub-committees, each charged with a particular area of interest. One of these, the Consultative Committee for Units, amongst other things, the first CGPM formally approved the use of 40 prototype metres and 40 prototype kilograms from the British firm Johnson Matthey as the standards mandated by the Convention of the Metre
23.
Proton
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A proton is a subatomic particle, symbol p or p+, with a positive electric charge of +1e elementary charge and mass slightly less than that of a neutron. Protons and neutrons, each with masses of one atomic mass unit, are collectively referred to as nucleons. One or more protons are present in the nucleus of every atom, the number of protons in the nucleus is the defining property of an element, and is referred to as the atomic number. Since each element has a number of protons, each element has its own unique atomic number. The word proton is Greek for first, and this name was given to the nucleus by Ernest Rutherford in 1920. In previous years, Rutherford had discovered that the nucleus could be extracted from the nuclei of nitrogen by atomic collisions. Protons were therefore a candidate to be a particle, and hence a building block of nitrogen. In the modern Standard Model of particle physics, protons are hadrons, and like neutrons, although protons were originally considered fundamental or elementary particles, they are now known to be composed of three valence quarks, two up quarks and one down quark. The rest masses of quarks contribute only about 1% of a protons mass, the remainder of a protons mass is due to quantum chromodynamics binding energy, which includes the kinetic energy of the quarks and the energy of the gluon fields that bind the quarks together. At sufficiently low temperatures, free protons will bind to electrons, however, the character of such bound protons does not change, and they remain protons. A fast proton moving through matter will slow by interactions with electrons and nuclei, the result is a protonated atom, which is a chemical compound of hydrogen. In vacuum, when electrons are present, a sufficiently slow proton may pick up a single free electron, becoming a neutral hydrogen atom. Such free hydrogen atoms tend to react chemically with other types of atoms at sufficiently low energies. When free hydrogen atoms react with other, they form neutral hydrogen molecules. Protons are spin-½ fermions and are composed of three quarks, making them baryons. Protons have an exponentially decaying positive charge distribution with a mean square radius of about 0.8 fm. Protons and neutrons are both nucleons, which may be together by the nuclear force to form atomic nuclei. The nucleus of the most common isotope of the atom is a lone proton
24.
Electron
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The electron is a subatomic particle, symbol e− or β−, with a negative elementary electric charge. Electrons belong to the first generation of the lepton particle family, the electron has a mass that is approximately 1/1836 that of the proton. Quantum mechanical properties of the include a intrinsic angular momentum of a half-integer value, expressed in units of the reduced Planck constant. As it is a fermion, no two electrons can occupy the same state, in accordance with the Pauli exclusion principle. Like all elementary particles, electrons exhibit properties of particles and waves, they can collide with other particles and can be diffracted like light. Since an electron has charge, it has an electric field. Electromagnetic fields produced from other sources will affect the motion of an electron according to the Lorentz force law, electrons radiate or absorb energy in the form of photons when they are accelerated. Laboratory instruments are capable of trapping individual electrons as well as electron plasma by the use of electromagnetic fields, special telescopes can detect electron plasma in outer space. Electrons are involved in applications such as electronics, welding, cathode ray tubes, electron microscopes, radiation therapy, lasers, gaseous ionization detectors. Interactions involving electrons with other particles are of interest in fields such as chemistry. The Coulomb force interaction between the positive protons within atomic nuclei and the negative electrons without, allows the composition of the two known as atoms, ionization or differences in the proportions of negative electrons versus positive nuclei changes the binding energy of an atomic system. The exchange or sharing of the electrons between two or more atoms is the cause of chemical bonding. In 1838, British natural philosopher Richard Laming first hypothesized the concept of a quantity of electric charge to explain the chemical properties of atoms. Irish physicist George Johnstone Stoney named this charge electron in 1891, electrons can also participate in nuclear reactions, such as nucleosynthesis in stars, where they are known as beta particles. Electrons can be created through beta decay of isotopes and in high-energy collisions. The antiparticle of the electron is called the positron, it is identical to the electron except that it carries electrical, when an electron collides with a positron, both particles can be totally annihilated, producing gamma ray photons. The ancient Greeks noticed that amber attracted small objects when rubbed with fur, along with lightning, this phenomenon is one of humanitys earliest recorded experiences with electricity. In his 1600 treatise De Magnete, the English scientist William Gilbert coined the New Latin term electricus, both electric and electricity are derived from the Latin ēlectrum, which came from the Greek word for amber, ἤλεκτρον
25.
Circuit breaker
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A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by excess current, typically resulting from an overload or short circuit. Its basic function is to interrupt current flow after a fault is detected, unlike a fuse, which operates once and then must be replaced, a circuit breaker can be reset to resume normal operation. The generic function of a breaker, RCD or a fuse. An early form of circuit breaker was described by Thomas Edison in an 1879 patent application and its purpose was to protect lighting circuit wiring from accidental short circuits and overloads. A modern miniature circuit breaker similar to the now in use was patented by Brown. Hugo Stotz, an engineer who had sold his company to BBC, was credited as the inventor on DRP458392, stotzs invention was the forerunner of the modern thermal-magnetic breaker commonly used in household load centers to this day. All circuit breaker systems have features in their operation, but details vary substantially depending on the voltage class, current rating. The circuit breaker must firstly detect a fault condition, in small mains and low voltage circuit breakers, this is usually done within the device itself. Typically, the heating and/or magnetic effects of current are employed. Circuit breakers for large currents or high voltages are usually arranged with protective relay pilot devices to sense a fault condition, Circuit breakers may also use the higher current caused by the fault to separate the contacts, such as thermal expansion or a magnetic field. The circuit breaker contacts must carry the current without excessive heating. Contacts are made of copper or copper alloys, silver alloys, service life of the contacts is limited by the erosion of contact material due to arcing while interrupting the current. Miniature and molded-case circuit breakers are usually discarded when the contacts have worn, when a high current or voltage is interrupted, an arc is generated. The length of the arc is generally proportional to the voltage while the intensity is proportional to the current and this arc must be contained, cooled and extinguished in a controlled way, so that the gap between the contacts can again withstand the voltage in the circuit. Different circuit breakers use vacuum, air, insulating gas, or oil as the medium the arc forms in, finally, once the fault condition has been cleared, the contacts must again be closed to restore power to the interrupted circuit. Low-voltage miniature circuit breaker uses air alone to extinguish the arc and these circuit breakers contain so-called arc chutes, a stack of mutually insulated parallel metal plates which divide and cool the arc. By splitting the arc into smaller arcs the arc is cooled down while the arc voltage is increased, the number of plates in the arc chute is dependent on the short-circuit rating and nominal voltage of the circuit breaker. In larger ratings, oil circuit breakers rely upon vaporization of some of the oil to blast a jet of oil through the arc
26.
Commonwealth of Nations
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The Commonwealth of Nations, also known as simply the Commonwealth, is an intergovernmental organisation of 52 member states that are mostly former territories of the British Empire. The Commonwealth dates back to the century with the decolonisation of the British Empire through increased self-governance of its territories. It was formally constituted by the London Declaration in 1949, which established the states as free. The symbol of free association is Queen Elizabeth II who is the Head of the Commonwealth. The Queen is also the monarch of 16 members of the Commonwealth, the other Commonwealth members have different heads of state,31 members are republics and five are monarchies with a different monarch. Member states have no obligation to one another. Instead, they are united by language, history, culture and their values of democracy, free speech, human rights. These values are enshrined in the Commonwealth Charter and promoted by the quadrennial Commonwealth Games, the Commonwealth covers more than 29,958,050 km2, 20% of the worlds land area, and spans all six inhabited continents. She declared, So, it marks the beginning of that free association of independent states which is now known as the Commonwealth of Nations. As long ago as 1884, however, Lord Rosebery, while visiting Australia, had described the changing British Empire—as some of its colonies became more independent—as a Commonwealth of Nations. Conferences of British and colonial prime ministers occurred periodically from the first one in 1887, the Commonwealth developed from the imperial conferences. Newfoundland never did, as on 16 February 1934, with the consent of its parliament, Newfoundland later joined Canada as its 10th province in 1949. Australia and New Zealand ratified the Statute in 1942 and 1947 respectively, after World War II ended, the British Empire was gradually dismantled. Most of its components have become independent countries, whether Commonwealth realms or republics, there remain the 14 British overseas territories still held by the United Kingdom. In April 1949, following the London Declaration, the word British was dropped from the title of the Commonwealth to reflect its changing nature, burma and Aden are the only states that were British colonies at the time of the war not to have joined the Commonwealth upon independence. Hoped for success was reinforced by such achievements as climbing Mount Everest in 1953, breaking the four minute mile in 1954, however, the humiliation of the Suez Crisis of 1956 badly hurt morale of Britain and the Commonwealth as a whole. More broadly, there was the loss of a role of the British Empire. That role was no longer militarily or financially feasible, as Britains withdrawal from Greece in 1947 painfully demonstrated, Britain itself was now just one part of the NATO military alliance in which the Commonwealth had no role apart from Canada
27.
Ammeter
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An ammeter is a measuring instrument used to measure the current in a circuit. Electric currents are measured in amperes, hence the name, instruments used to measure smaller currents, in the milliampere or microampere range, are designated as milliammeters or microammeters. Early ammeters were laboratory instruments which relied on the Earths magnetic field for operation, by the late 19th century, improved instruments were designed which could be mounted in any position and allowed accurate measurements in electric power systems. The tangent galvanometer was used to measure using this effect. This made these instruments usable only when aligned with the Earths field, sensitivity of the instrument was increased by using additional turns of wire to multiply the effect – the instruments were called multipliers. The DArsonval galvanometer is a moving coil ammeter and it uses magnetic deflection, where current passing through a coil placed in the magnetic field of a permanent magnet causes the coil to move. The modern form of instrument was developed by Edward Weston. The uniform air gap between the core and the permanent magnet poles make the deflection of the meter linearly proportional to current. Basic meter movements can have full-scale deflection for currents from about 25 microamperes to 10 milliamperes, because the magnetic field is polarised, the meter needle acts in opposite directions for each direction of current. A DC ammeter is thus sensitive to which way round it is connected, most are marked with a positive terminal, a moving coil meter indicates the average of a varying current through it, which is zero for AC. For this reason moving-coil meters are only usable directly for DC and this type of meter movement is extremely common for both ammeters and other meters derived from them, such as voltmeters and ohmmeters. Moving magnet ammeters operate on essentially the same principle as moving coil, except that the coil is mounted in the case. Indeed, some Ammeters of this type do not have hairsprings at all, an electrodynamic movement uses an electromagnet instead of the permanent magnet of the dArsonval movement. This instrument can respond to both alternating and direct current and also indicates true RMS for AC, see Wattmeter for an alternative use for this instrument. Moving iron ammeters use a piece of iron which moves when acted upon by the force of a fixed coil of wire. The moving-iron meter was invented by Austrian engineer Friedrich Drexler in 1884 and this type of meter responds to both direct and alternating currents. The iron element consists of a vane attached to a pointer. The deflection of a moving iron meter is proportional to the square of the current, consequently, such meters would normally have a non linear scale, but the iron parts are usually modified in shape to make the scale fairly linear over most of its range
28.
Hydraulic analogy
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The electronic–hydraulic analogy is the most widely used analogy for electron fluid in a metal conductor. Since electric current is invisible and the processes at play in electronics are difficult to demonstrate. Electricity was originally understood to be a kind of fluid, as with all analogies, it demands an intuitive and competent understanding of the baseline paradigms. There is no unique paradigm for establishing this analogy, two paradigms can be used to introduce the concept to students, Version with pressure induced by gravity. Large tanks of water are held up high, or are filled to differing levels. This is reminiscent of electrical diagrams with an up arrow pointing to +V, grounded pins that otherwise are not shown connecting to anything and this has the advantage of associating electric potential with gravitational potential. Completely enclosed version with pumps providing pressure only, no gravity and this is reminiscent of a circuit diagram with a voltage source shown and the wires actually completing a circuit. This paradigm is further discussed below, Hydraulic ohms are the units of hydraulic impedance, which is defined as the ratio of pressure to volume flow rate. The pressure and volume flow variables are treated as phasors in this definition, a slightly different paradigm is used in acoustics, where acoustic impedance is defined as a relationship between pressure and air speed. In this paradigm, a cavity with a hole is analogous to a capacitor that stores compressional energy when the time-dependent pressure deviates from atmospheric pressure. A hole is analogous to an inductor that stores kinetic energy associated with the flow of air, electric potential In general, this is equivalent to hydraulic head. This model assumes that the water is flowing horizontally, so that the force of gravity can be ignored, in this case electric potential is equivalent to pressure. The voltage is a difference in pressure between two points, electric potential and voltage are usually measured in volts. Current Equivalent to a volume flow rate, that is. Electric charge Equivalent to a quantity of water, conducting wire A relatively wide pipe completely filled with water is equivalent to a piece of wire. When comparing to a piece of wire, the pipe should be thought of as having semi-permanent caps on the ends, connecting one end of a wire to a circuit is equivalent to un-capping one end of the pipe and attaching it to another pipe. With few exceptions, a wire with one end attached to a circuit will do nothing, the pipe remains capped on the free end. Resistor A constriction in the bore of the pipe which requires more pressure to pass the same amount of water, all pipes have some resistance to flow, just as all wires have some resistance to current
29.
Portable Document Format
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The Portable Document Format is a file format used to present documents in a manner independent of application software, hardware, and operating systems. Each PDF file encapsulates a complete description of a fixed-layout flat document, including the text, fonts, graphics, PDF was developed in the early 1990s as a way to share computer documents, including text formatting and inline images. It was among a number of competing formats such as DjVu, Envoy, Common Ground Digital Paper, Farallon Replica, in those early years before the rise of the World Wide Web and HTML documents, PDF was popular mainly in desktop publishing workflows. Adobe Systems made the PDF specification available free of charge in 1993 and these proprietary technologies are not standardized and their specification is published only on Adobe’s website. Many of them are not supported by popular third-party implementations of PDF. So when organizations publish PDFs which use proprietary technologies, they present accessibility issues for some users. In 2014, ISO TC171 voted to deprecate XFA for ISO 32000-2, on January 9,2017, the final draft for ISO 32000-2 was published, thus reaching the approval stage. The PDF combines three technologies, A subset of the PostScript page description programming language, for generating the layout, a font-embedding/replacement system to allow fonts to travel with the documents. A structured storage system to bundle these elements and any associated content into a single file, PostScript is a page description language run in an interpreter to generate an image, a process requiring many resources. It can handle graphics and standard features of programming such as if. PDF is largely based on PostScript but simplified to remove flow control features like these, often, the PostScript-like PDF code is generated from a source PostScript file. The graphics commands that are output by the PostScript code are collected and tokenized, any files, graphics, or fonts to which the document refers also are collected. Then, everything is compressed to a single file, therefore, the entire PostScript world remains intact. PDF supports graphic transparency, PostScript does not, PostScript is an interpreted programming language with an implicit global state, so instructions accompanying the description of one page can affect the appearance of any following page. Therefore, all preceding pages in a PostScript document must be processed to determine the appearance of a given page. A PDF file is a 7-bit ASCII file, except for elements that may have binary content. A PDF file starts with a header containing the magic number, the format is a subset of a COS format. A COS tree file consists primarily of objects, of which there are eight types, Boolean values, representing true or false Numbers Strings, enclosed within parentheses, objects may be either direct or indirect