1.
Kilogram
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The kilogram or kilogramme is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype of the Kilogram. The avoirdupois pound, used in both the imperial and US customary systems, is defined as exactly 0.45359237 kg, making one kilogram approximately equal to 2.2046 avoirdupois pounds. Other traditional units of weight and mass around the world are also defined in terms of the kilogram, the gram, 1/1000 of a kilogram, was provisionally defined in 1795 as the mass of one cubic centimeter of water at the melting point of ice. The final kilogram, manufactured as a prototype in 1799 and from which the IPK was derived in 1875, had an equal to the mass of 1 dm3 of water at its maximum density. The kilogram is the only SI base unit with an SI prefix as part of its name and it is also the only SI unit that is still directly defined by an artifact rather than a fundamental physical property that can be reproduced in different laboratories. Three other base units and 17 derived units in the SI system are defined relative to the kilogram, only 8 other units do not require the kilogram in their definition, temperature, time and frequency, length, and angle. At its 2011 meeting, the CGPM agreed in principle that the kilogram should be redefined in terms of the Planck constant, the decision was originally deferred until 2014, in 2014 it was deferred again until the next meeting. There are currently several different proposals for the redefinition, these are described in the Proposed Future Definitions section below, the International Prototype Kilogram is rarely used or handled. In the decree of 1795, the term gramme thus replaced gravet, the French spelling was adopted in the United Kingdom when the word was used for the first time in English in 1797, with the spelling kilogram being adopted in the United States. In the United Kingdom both spellings are used, with kilogram having become by far the more common, UK law regulating the units to be used when trading by weight or measure does not prevent the use of either spelling. In the 19th century the French word kilo, a shortening of kilogramme, was imported into the English language where it has used to mean both kilogram and kilometer. In 1935 this was adopted by the IEC as the Giorgi system, now known as MKS system. In 1948 the CGPM commissioned the CIPM to make recommendations for a practical system of units of measurement. This led to the launch of SI in 1960 and the subsequent publication of the SI Brochure, the kilogram is a unit of mass, a property which corresponds to the common perception of how heavy an object is. Mass is a property, that is, it is related to the tendency of an object at rest to remain at rest, or if in motion to remain in motion at a constant velocity. Accordingly, for astronauts in microgravity, no effort is required to hold objects off the cabin floor, they are weightless. However, since objects in microgravity still retain their mass and inertia, the ratio of the force of gravity on the two objects, measured by the scale, is equal to the ratio of their masses. On April 7,1795, the gram was decreed in France to be the weight of a volume of pure water equal to the cube of the hundredth part of the metre
2.
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
3.
Energy
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In physics, energy is the property that must be transferred to an object in order to perform work on – or to heat – the object, and can be converted in form, but not created or destroyed. The SI unit of energy is the joule, which is the transferred to an object by the mechanical work of moving it a distance of 1 metre against a force of 1 newton. Mass and energy are closely related, for example, with a sensitive enough scale, one could measure an increase in mass after heating an object. Living organisms require available energy to stay alive, such as the humans get from food. Civilisation gets the energy it needs from energy resources such as fuels, nuclear fuel. The processes of Earths climate and ecosystem are driven by the radiant energy Earth receives from the sun, the total energy of a system can be subdivided and classified in various ways. It may also be convenient to distinguish gravitational energy, thermal energy, several types of energy, electric energy. Many of these overlap, for instance, thermal energy usually consists partly of kinetic. Some types of energy are a mix of both potential and kinetic energy. An example is energy which is the sum of kinetic. Whenever physical scientists discover that a phenomenon appears to violate the law of energy conservation. Heat and work are special cases in that they are not properties of systems, in general we cannot measure how much heat or work are present in an object, but rather only how much energy is transferred among objects in certain ways during the occurrence of a given process. Heat and work are measured as positive or negative depending on which side of the transfer we view them from, the distinctions between different kinds of energy is not always clear-cut. In contrast to the definition, energeia was a qualitative philosophical concept, broad enough to include ideas such as happiness. The modern analog of this property, kinetic energy, differs from vis viva only by a factor of two, in 1807, Thomas Young was possibly the first to use the term energy instead of vis viva, in its modern sense. Gustave-Gaspard Coriolis described kinetic energy in 1829 in its modern sense, the law of conservation of energy was also first postulated in the early 19th century, and applies to any isolated system. It was argued for years whether heat was a physical substance, dubbed the caloric, or merely a physical quantity. In 1845 James Prescott Joule discovered the link between mechanical work and the generation of heat and these developments led to the theory of conservation of energy, formalized largely by William Thomson as the field of thermodynamics
4.
Resistor
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A resistor is a passive two-terminal electrical component that implements electrical resistance as a circuit element. In electronic circuits, resistors are used to reduce current flow, adjust signal levels, to divide voltages, bias active elements, and terminate transmission lines, among other uses. High-power resistors that can dissipate many watts of power as heat may be used as part of motor controls, in power distribution systems. Fixed resistors have resistances that only slightly with temperature, time or operating voltage. Variable resistors can be used to adjust circuit elements, or as sensing devices for heat, light, humidity, force, Resistors are common elements of electrical networks and electronic circuits and are ubiquitous in electronic equipment. Practical resistors as discrete components can be composed of various compounds, Resistors are also implemented within integrated circuits. The electrical function of a resistor is specified by its resistance, the nominal value of the resistance falls within the manufacturing tolerance, indicated on the component. Two typical schematic diagram symbols are as follows, The notation to state a resistors value in a circuit diagram varies, one common scheme is the letter and digit code for resistance values following IEC60062. It avoids using a separator and replaces the decimal separator with a letter loosely associated with SI prefixes corresponding with the parts resistance. For example, 8K2 as part marking code, in a diagram or in a bill of materials indicates a resistor value of 8.2 kΩ. Additional zeros imply a tighter tolerance, for example 15M0 for three significant digits, when the value can be expressed without the need for a prefix, an R is used instead of the decimal separator. For example, 1R2 indicates 1.2 Ω, and 18R indicates 18 Ω, for example, if a 300 ohm resistor is attached across the terminals of a 12 volt battery, then a current of 12 /300 =0.04 amperes flows through that resistor. Practical resistors also have some inductance and capacitance which affect the relation between voltage and current in alternating current circuits, the ohm is the SI unit of electrical resistance, named after Georg Simon Ohm. An ohm is equivalent to a volt per ampere, since resistors are specified and manufactured over a very large range of values, the derived units of milliohm, kilohm, and megohm are also in common usage. The total resistance of resistors connected in series is the sum of their resistance values. R e q = R1 + R2 + ⋯ + R n, the total resistance of resistors connected in parallel is the reciprocal of the sum of the reciprocals of the individual resistors. 1 R e q =1 R1 +1 R2 + ⋯ +1 R n. For example, a 10 ohm resistor connected in parallel with a 5 ohm resistor, a resistor network that is a combination of parallel and series connections can be broken up into smaller parts that are either one or the other
5.
Capacitor
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A capacitor is a passive two-terminal electrical component that stores electrical energy in an electric field. The effect of a capacitor is known as capacitance, a capacitor was therefore historically first known as an electric condenser. The physical form and construction of practical capacitors vary widely and many types are in common use. Most capacitors contain at least two electrical conductors often in the form of plates or surfaces separated by a dielectric medium. A conductor may be a foil, thin film, sintered bead of metal, the nonconducting dielectric acts to increase the capacitors charge capacity. Materials commonly used as dielectrics include glass, ceramic, plastic film, paper, mica, Capacitors are widely used as parts of electrical circuits in many common electrical devices. Unlike a resistor, a capacitor does not dissipate energy. No current actually flows through the dielectric, instead, the effect is a displacement of charges through the source circuit, if the condition is maintained sufficiently long, this displacement current through the battery ceases. However, if a voltage is applied across the leads of the capacitor. Capacitance is defined as the ratio of the charge on each conductor to the potential difference between them. The unit of capacitance in the International System of Units is the farad, capacitance values of typical capacitors for use in general electronics range from about 1 pF to about 1 mF. The capacitance of a capacitor is proportional to the area of the plates. In practice, the dielectric between the plates passes a small amount of leakage current and it has an electric field strength limit, known as the breakdown voltage. The conductors and leads introduce an undesired inductance and resistance, Capacitors are widely used in electronic circuits for blocking direct current while allowing alternating current to pass. In analog filter networks, they smooth the output of power supplies, in resonant circuits they tune radios to particular frequencies. In electric power systems, they stabilize voltage and power flow. The property of energy storage in capacitors was exploited as dynamic memory in digital computers. Von Kleists hand and the water acted as conductors, and the jar as a dielectric, von Kleist found that touching the wire resulted in a powerful spark, much more painful than that obtained from an electrostatic machine
6.
Inductor
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An inductor, also called a coil or reactor, is a passive two-terminal electrical component that stores electrical energy in a magnetic field when electric current is flowing through it. An inductor typically consists of a conductor, such as a wire. When the current flowing through an inductor changes, the magnetic field induces a voltage in the conductor. According to Lenzs law, the direction of induced electromotive force opposes the change in current that created it, as a result, inductors oppose any changes in current through them. An inductor is characterized by its inductance, which is the ratio of the voltage to the rate of change of current, in the International System of Units, the unit of inductance is the henry. Inductors have values that range from 1 µH to 1 H. Many inductors have a core made of iron or ferrite inside the coil. Along with capacitors and resistors, inductors are one of the three passive linear circuit elements that make up electronic circuits, Inductors are widely used in alternating current electronic equipment, particularly in radio equipment. They are used to block AC while allowing DC to pass and they are also used in electronic filters to separate signals of different frequencies, and in combination with capacitors to make tuned circuits, used to tune radio and TV receivers. An electric current flowing through a conductor generates a magnetic field surrounding it, any changes of current and therefore in the magnetic flux through the cross-section of the inductor creates an opposing electromotive force in the conductor. An inductor is a component consisting of a wire or other conductor shaped to increase the flux through the circuit. Winding the wire into a coil increases the number of times the magnetic flux lines link the circuit, increasing the field, the more turns, the higher the inductance. The inductance also depends on the shape of the coil, separation of the turns, by adding a magnetic core made of a ferromagnetic material like iron inside the coil, the magnetizing field from the coil will induce magnetization in the material, increasing the magnetic flux. The high permeability of a core can increase the inductance of a coil by a factor of several thousand over what it would be without it. Any change in the current through an inductor creates a changing flux, for example, an inductor with an inductance of 1 henry produces an EMF of 1 volt when the current through the inductor changes at the rate of 1 ampere per second. This is usually taken to be the relation of the inductor. The dual of the inductor is the capacitor, which stores energy in a field rather than a magnetic field. Its current-voltage relation is obtained by exchanging current and voltage in the inductor equations, the polarity of the induced voltage is given by Lenzs law, which states that it will be such as to oppose the change in current
7.
Multimeter
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A multimeter or a multitester, also known as a VOM, is an electronic measuring instrument that combines several measurement functions in one unit. A typical multimeter can measure voltage, current, and resistance, analog multimeters use a microammeter with a moving pointer to display readings. Digital multimeters have a display, and may also show a graphical bar representing the measured value. Digital multimeters are now far more due to their cost and precision. A multimeter can be a device useful for basic fault finding and field service work. Multimeters are available in a range of features and prices. Cheap multimeters can cost less than US$10, while models with certified calibration can cost more than US$5,000. The first moving-pointer current-detecting device was the galvanometer in 1820 and these were used to measure resistance and voltage by using a Wheatstone bridge, and comparing the unknown quantity to a reference voltage or resistance. While useful in the lab, the devices were very slow and these galvanometers were bulky and delicate. The DArsonval/Weston meter movement uses a coil which carries a pointer. The coil rotates in a permanent magnetic field and is restrained by fine spiral springs which also serve to carry current into the moving coil and it gives proportional measurement rather than just detection, and deflection is independent of the orientation of the meter. Instead of balancing a bridge, values could be read off the instruments scale. The basic moving coil meter is only for direct current measurements. It is easily adapted to read heavier currents by using shunts or to voltage using series resistances known as multipliers. To read alternating currents or voltages, a rectifier is needed, multimeters were invented in the early 1920s as radio receivers and other vacuum tube electronic devices became more common. Macadie invented an instrument which could measure amperes, volts and ohms, the meter comprised a moving coil meter, voltage and precision resistors, and switches and sockets to select the range. The Automatic Coil Winder and Electrical Equipment Company was set up to manufacture the Avometer, although a shareholder of ACWEECO, Mr MacAdie continued to work for the Post Office until his retirement in 1933. His son, Hugh S. MacAdie, joined ACWEECO in 1927, Automatic Coil Winder and Electrical Equipment Company
8.
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
9.
Voltage
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Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential energy between two points per unit electric charge. The voltage between two points is equal to the work done per unit of charge against an electric field to move the test charge between two points. This is measured in units of volts, voltage can be caused by static electric fields, by electric current through a magnetic field, by time-varying magnetic fields, or some combination of these three. A voltmeter can be used to measure the voltage between two points in a system, often a reference potential such as the ground of the system is used as one of the points. A voltage may represent either a source of energy or lost, used, given two points in space, x A and x B, voltage is the difference in electric potential between those two points. Electric potential must be distinguished from electric energy by noting that the potential is a per-unit-charge quantity. Like mechanical potential energy, the zero of electric potential can be chosen at any point, so the difference in potential, i. e. the voltage, is the quantity which is physically meaningful. The voltage between point A to point B is equal to the work which would have to be done, per unit charge, against or by the electric field to move the charge from A to B. The voltage between the two ends of a path is the energy required to move a small electric charge along that path. Mathematically this is expressed as the integral of the electric field. In the general case, both an electric field and a dynamic electromagnetic field must be included in determining the voltage between two points. Historically this quantity has also called tension and pressure. Pressure is now obsolete but tension is used, for example within the phrase high tension which is commonly used in thermionic valve based electronics. Voltage is defined so that negatively charged objects are pulled towards higher voltages, therefore, the conventional current in a wire or resistor always flows from higher voltage to lower voltage. Current can flow from lower voltage to higher voltage, but only when a source of energy is present to push it against the electric field. This is the case within any electric power source, for example, inside a battery, chemical reactions provide the energy needed for ion current to flow from the negative to the positive terminal. The electric field is not the only factor determining charge flow in a material, the electric potential of a material is not even a well defined quantity, since it varies on the subatomic scale. A more convenient definition of voltage can be found instead in the concept of Fermi level, in this case the voltage between two bodies is the thermodynamic work required to move a unit of charge between them
10.
Alessandro Volta
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He invented the Voltaic pile in 1799, and reported the results of his experiments in 1800 in a two-part letter to the President of the Royal Society. With this invention Volta proved that electricity could be generated chemically, Voltas invention sparked a great amount of scientific excitement and led others to conduct similar experiments which eventually led to the development of the field of electrochemistry. Alessandro Volta also drew admiration from Napoleon Bonaparte for his invention, Volta enjoyed a certain amount of closeness with the Emperor throughout his life and he was conferred numerous honours by him. Alessandro Volta held the chair of physics at the University of Pavia for nearly 40 years and was widely idolised by his students. Despite his professional success, Volta tended to be a person inclined towards domestic life, at this time he tended to live secluded from public life and more for the sake of his family until his eventual death in 1827 from a series of illnesses which began in 1823. The SI unit of electric potential is named in his honour as the volt, Volta was born in Como, a town in present-day northern Italy, on 18 February 1745. In 1794, Volta married a lady also from Como, Teresa Peregrini. His father, Filippo Volta, was of noble lineage and his mother, Donna Maddalena, came from the family of the Inzaghis. In 1774, he became a professor of physics at the Royal School in Como, a year later, he improved and popularised the electrophorus, a device that produced static electricity. In 1777, he travelled through Switzerland, there he befriended H. B. de Saussure. In the years between 1776 and 1778, Volta studied the chemistry of gases and he researched and discovered methane after reading a paper by Benjamin Franklin of the United States on flammable air. In November 1776, he found methane at Lake Maggiore, and he devised experiments such as the ignition of methane by an electric spark in a closed vessel. Volta also studied what we now call electrical capacitance, developing separate means to both electrical potential and charge, and discovering that for a given object, they are proportional. This is called Voltas Law of Capacitance, and it was for work the unit of electrical potential has been named the volt. In 1779 he became a professor of physics at the University of Pavia. Luigi Galvani, an Italian physicist, discovered something he named animal electricity when two different metals were connected in series with a leg and to one another. Volta realised that the frogs leg served as both a conductor of electricity and as a detector of electricity and he replaced the frogs leg with brine-soaked paper, and detected the flow of electricity by other means familiar to him from his previous studies. This may be called Voltas Law of the electrochemical series, Volta had determined that the most effective pair of dissimilar metals to produce electricity was zinc and copper
11.
Pressure
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Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the relative to the ambient pressure. Various units are used to express pressure, Pressure may also be expressed in terms of standard atmospheric pressure, the atmosphere is equal to this pressure and the torr is defined as 1⁄760 of this. Manometric units such as the centimetre of water, millimetre of mercury, Pressure is the amount of force acting per unit area. The symbol for it is p or P, the IUPAC recommendation for pressure is a lower-case p. However, upper-case P is widely used. The usage of P vs p depends upon the field in one is working, on the nearby presence of other symbols for quantities such as power and momentum. Mathematically, p = F A where, p is the pressure, F is the normal force and it relates the vector surface element with the normal force acting on it. It is incorrect to say the pressure is directed in such or such direction, the pressure, as a scalar, has no direction. The force given by the relationship to the quantity has a direction. If we change the orientation of the element, the direction of the normal force changes accordingly. Pressure is distributed to solid boundaries or across arbitrary sections of normal to these boundaries or sections at every point. It is a parameter in thermodynamics, and it is conjugate to volume. The SI unit for pressure is the pascal, equal to one newton per square metre and this name for the unit was added in 1971, before that, pressure in SI was expressed simply in newtons per square metre. Other units of pressure, such as pounds per square inch, the CGS unit of pressure is the barye, equal to 1 dyn·cm−2 or 0.1 Pa. Pressure is sometimes expressed in grams-force or kilograms-force per square centimetre, but using the names kilogram, gram, kilogram-force, or gram-force as units of force is expressly forbidden in SI. The technical atmosphere is 1 kgf/cm2, since a system under pressure has potential to perform work on its surroundings, pressure is a measure of potential energy stored per unit volume. It is therefore related to density and may be expressed in units such as joules per cubic metre. Similar pressures are given in kilopascals in most other fields, where the prefix is rarely used
