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Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
Elektrik
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Elektrik
Elektrik
Elektrik
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Elektrik

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  • 1. How to Solder (Electronics)This article mainly deals with the soldering of through-hole components into printed circuit boards(PCBs). Through-hole components are those which have leads (meaning wires or tabs) that passthrough a hole in the board and are soldered to the pad (an area with metal plating) around thehole. The hole may be plated through or not.Soldering of other electrical items such as wires, lugs, have slightly different steps but the generalprinciples are the same.Steps1. Select the correct component. Many components look similar, so read the labels or check the color code carefully.2. Bend leads correctly if required, discuss stress relief. To be completed...3. Clinching leads. Discuss whether to cut leads before or after soldering based on whether heatsinking effect is required. To be completed...4. Melt a small blob of solder on end of the soldering iron. This will be used to improve the transfer of heat to your work.5. Carefully place the tip (with the blob) onto the interface of the lead and pad. The tip or blob must touch both the lead and the pad. The tip/blob should not be touching the nonmetallic pad area of the PCB (i.e the fibreglass area) as this area can be damaged by excessive heat. This should now heat the work area.6. "Feed" the solder onto the interface between the pad and lead. Do not feed the solder onto the tip! The lead and pad should be heated enough for the solder to melt on it (see previous step). If the solder does not melt onto the area, the most likely cause is insufficient heat has been transfered to it. The molten solder should "cling" to the pad and lead together by way of surface tension This is commonly referred to as [[wikipedia:Wetting|wetting]. o with practice, you will learn how to heat the joint more efficiently with the way you hold the iron onto the work o flux from the solder wire is only active for about one second maximum after melting onto the joint as it is slowly "burnt off" by heat o solder will wet a surface only if:  the surface is sufficiently heated and  there is sufficient flux present to remove oxidation from the surface and  the surface is clean and free of grease, dirt etc.7. The solder should by itself, "run around" and fill in the interface. Stop feeding the solder when the correct amount of solder has been added the the joint. The correct amount of solder is determined by: o for non plated through hole (non-PTH) PCBs (most home made PCBs are of this type) - stop feeding when the solder forms a flat fillet o for plated through hole (PTH) PCBs (most commercially manufactured PCBs) - stop feeding when a solid concave fillet can be seen o too much solder will form a "bulbous" joint with a convex shape o too little solder will form a "very concave" joint.8. More to come...Tips
  • 2. • Most soldering irons have replaceable tips. Soldering iron tips have a limited working life and also are available in different types of shapes and sizes, to suit the a variety of jobs.• The tip of a soldering iron tends to get stuck with time (if frequently used), due to oxides that build up between the copper tip and the iron sleeve. Plated tips do not usually have this problem. If the copper tip is not removed now and then, it will get stuck permanently in the soldering iron! It is then destroyed. Therefore: every 20 - 50 or so hours of use, when cold, remove the tip and move it back and forth and around so the oxide scales can come out, before locking it in place again! Now you soldering iron will last for many years of use!Warnings• Soldering irons are very hot. Do not touch the tip with your skin. Also, always use a suitable stand or holder to keep the tip up and off of your work surface.• Solders, especially lead-based solders, contain hazardous materials. Wash your hands after soldering, and be aware that items containing solder may require special handling if you dispose of them.Things Youll Need A soldering iron. Soldering irons are usually either: Fixed power - e.g 25W (small jobs) to 100W (large jobs, heavy cabling etc) Variable temperature - tip temperature can be controlled to suit the size of the job Tongs, needle-nosed pliers, or tweezers to hold the component.• A clamp or stand to hold the board.• Flux-cored solder wire. o Solder alloys.  The most common solder alloy used in electronics is Lead-Tin 60/40. This alloy is recommended if you are new to soldering.  Various lead-free alloys are becoming popular recently. These require higher soldering temperatures and do not "wet" as well as Lead-Tin alloys. Lead- free solders require more skill to produce a good quality solder joint. o Flux. Flux is an additive in solder that facilitates the soldering process by removing and preventing oxidation and by improving the wetting characteristics of the liquid solder. There are different types of flux cores available for solder wire.  Rosin is most commonly used by hobbyists. After soldering, it leaves a brown, sticky residue which is non-corrosive and non-conductive, but can be cleaned if desired with a solvent such as isopropanol (also called isopropyl alcohol or IPA). There are different grades of Rosin flux, the most commonly used is "RMA" (Rosin Mildly Activated).  No-clean fluxes leaves a clear residue after soldering, which is non- corrosive and non-conductive. This flux is designed to be left on the solder joint and surrounding areas.
  • 3.  Water-soluble fluxes usually have a higher activity (i.e is more aggressive) that leaves a residue which must be cleaned with water. The residue is corrosive and may also damage the board or components if not cleaned correctly after use. How to Remember Electrical Resistor Color CodesResistor color codes are something that every electronics hobbyist should remember. The oldmnemonic was rather, well, disturbing, and a conscientious person would never recite it. Anyway,on with the better one! Write this down in a prominent place and youll have it committed topermanent memory in no time!Steps 1. o Heres one mnemonic "Bright Boys Rave Over Young Girls But Veto Getting Wed. Black, Brown, Red, Orange, Yellow, Green, Blue, Violet, Grey, White <=> 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. o Alternatively, most of the colors are those from the traditional rainbow. Black is 0 (as in nothing), Brown is 1, then Red through Violet, and finally Gray and White are 8 and 9.2. The multiplier band goes by the same code and can be read as "followed by N zeros", plus Gold for "divide by 10" and Silver for "divide by 100".3. Tolerances are something of a mess: Brown and Red are 1% and 2% (you usually spot them because they have an extra significant digit), Gold and Silver are 5% and 10%, and 20% doesnt even get a tolerance band (you will rarely, if ever, come across one of these).4. To read the whole thing, put the tolerance band at your right, and go like this: "green- brown-red-gold = 5-1-00-5% = 5.1K 5%". Youll get used soon enough, and a bit later youll be spotting what you want at once. Its also easy to classify them by decade: red is Ks, orange is 10Ks ...Tips• The same code is used for inductors and capacitors, only that the "units" are uH and pF. That is, orange-white-red-gold is 3.9 mH or 3.9 nF, 5%.WattFrom Wikipedia, the free encyclopediaJump to: navigation, searchFor other uses, see Watt (disambiguation).
  • 4. The watt (symbol: W) is the SI derived unit of power, equal to one joule of energy persecond. It measures a rate of energy use or production.A human climbing a flight of stairs is doing work at a rate of about 200 watts. A typicalautomobile engine produces mechanical energy at a rate of 25,000 watts (approximately33.5 horsepower) while cruising. A typical household incandescent light bulb useselectrical energy at a rate of 25 to 100 watts, while compact fluorescent lights typicallyconsume 5 to 30 watts.Contents[hide] • 1 Definition • 2 Origin and adoption as an SI unit • 3 Derived and qualified units for power distribution o 3.1 Microwatt o 3.2 Milliwatt o 3.3 Kilowatt o 3.4 Megawatt o 3.5 Gigawatt o 3.6 Terawatt o 3.7 Electrical and thermal • 4 Confusion of watts and watt-hours • 5 See also • 6 References • 7 External links DefinitionOne watt is the rate at which work is done when an object is moving at one meter persecond against a force of one newton..By the definition of the units of ampere and volt, work is done at a rate of one watt whenone ampere flows through a potential difference of one volt. [1]Origin and adoption as an SI unitThe watt is named after James Watt for his contributions to the development of the steamengine, and was adopted by the Second Congress of the British Association for theAdvancement of Science in 1889 and by the 11th General Conference on Weights andMeasures in 1960 as the unit of power incorporated in the International System of Units(or "SI"). This SI unit is named after James Watt. As with every SI unit whose name is derived
  • 5. from the proper name of a person, the first letter of its symbol is uppercase (W). When an SI unit is spelled out in English, it should always begin with a lowercase letter (watt), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase. — Based on The International System of Units, section 5.2.Derived and qualified units for power distributionMicrowattThe microwatt (symbol:μW) is equal to one millionth (10-6) of a watt.MilliwattThe milliwatt (symbol:mW) is equal to one thousandth (10-3) of a watt.KilowattThe kilowatt (symbol: kW), equal to one thousand watts, is typically used to state thepower output of engines and the power consumption of tools and machines. A kilowatt isapproximately equivalent to 1.34 horsepower. An electric heater with one heating-element might use 1 kilowatt.MegawattThe megawatt (symbol: MW) is equal to one million (106) watts.Many things can sustain the transfer or consumption of energy on this scale; some ofthese events or entities include: lightning strikes, large electric motors, naval craft (suchas aircraft carriers and submarines), engineering hardware, and some scientific researchequipment (such as the supercollider and large lasers). A large residential or retailbuilding may consume several megawatts in electric power and heating energy.The productive capacity of electrical generators operated by utility companies is oftenmeasured in MW. Modern high-powered diesel-electric railroad locomotives typicallyhave a peak power output of 3 to 5 MW, whereas U.S. nuclear power plants have netsummer capacities between about 500 and 1300 MW.[2]According to the Oxford English Dictionary, the earliest citing for "megawatt" is areference in the 1900 Websters International Dictionary of English Language. The OEDalso says "megawatt" appeared in a 28 November 1847, article in Science (506:2).
  • 6. [edit] GigawattThe gigawatt (symbol: GW) is equal to one billion (109) watts. This unit is sometimesused with large power plants or power grids.[edit] TerawattThe terawatt (symbol: TW) is equal to one trillion (1012) watts. The average energyusage by humans (about 15 TW) is commonly measured in these units. The mostpowerful lasers from the mid 1960s to the mid 1990s produced power in terawatts, butonly for nanoseconds.[edit] Electrical and thermalIn the electric power industry, Megawatt electrical (abbreviation: MWe[citation needed] orMWe[3]) is a term that refers to electric power, while megawatt thermal (abbreviations:MWt, MWth, MWt, or MWth) refers to thermal power produced. Other SI prefixes aresometimes used, for example gigawatt electrical (GWe). [4]For example, the Embalse nuclear power plant in Argentina uses a fission reactor togenerate 2109 MWt of heat, which creates steam to drive a turbine, which generates 648MWe of electricity. The difference is heat lost to the surroundings.[edit] Confusion of watts and watt-hoursPower and energy are frequently confused in the general media. Power is the rate atwhich energy is used. A watt is one joule of energy per second. For example, if a 100watt light bulb is turned on for one hour, the energy used is 100 watt-hours or 0.1kilowatt-hour, or 360,000 joules. This same quantity of energy would light a 40 watt bulbfor 2.5 hours. A power station would be rated in watts, but its annual energy sales wouldbe in watt-hours (or kilowatt-hours or megawatt-hours). A kilowatt-hour is the amount ofenergy equivalent to a steady power of 1 kilowatt running for 1 hour: (1 kW·h)(1000 W/kW)(3600 s/h) = 3,600,000 W·s = 3,600,000 J = 3.6 MJ.[edit] See alsoEnergy portal • Volt-ampere • Conversion of units • James Watt • Declared net capacity (power plants) • Orders of magnitude (power) • Power factor • Root mean square (RMS) • Watt balance
  • 7. • Watt-hour • Wattmeter[edit] References 1. ^ "Amps, Volts, Watts, Ohms". Retrieved on 2007-04-17. 2. ^ Nuclear Regulatory Commission. (2007). 2007–2008 Information Digest. Retrieved on 2008-01-27. Appendix A. 3. ^ How Many? A Dictionary of Units of Measurement 4. ^ Megawatt electrical and megawatt thermal are not SI units,Taylor 1995, Guide for the Use of the International System of Units (SI), NIST Special Publication SP811 The International Bureau of Weights and Measures states that unit symbols should not use subscripts to provide additional information about the quantity being measured, and regards these symbols as incorrect. International Bureau of WeightsOhmFrom Wikipedia, the free encyclopediaJump to: navigation, searchThis article is about the SI derived unit. For other meanings, see Ohm (disambiguation).A multimeter can be used to measure resistance in ohms. It can also be used to measurecapacitance, voltage, current, and other electrical characteristics.Several resistors. Their resistance, in ohms, is marked using a color code.The ohm (symbol: Ω) is the SI unit of electrical impedance or, in the direct current case,electrical resistance, named after Georg Ohm.
  • 8. Contents[hide] • 1 Definition • 2 Conversions • 3 References and notes • 4 See also • 5 External links [edit] DefinitionThe ohm is the electric resistance between two points of a conductor when a constantpotential difference of 1 volt, applied to these points, produces in the conductor a currentof 1 ampere, the conductor not being the seat of any electromotive force.[1][edit] Conversions • A measurement in ohms is the reciprocal of a measurement in siemens, the SI unit of electrical conductance. Note that siemens is both singular and plural. The non- SI unit, the mho (ohm written backwards), is equivalent to siemens but is mostly obsolete and rarely used. • Ohms to watts: The power dissipated by a resistor may be calculated using resistance and voltage. The formula is a combination of Ohms law and Joules law: where P is the power in watts, R is the resistance in ohms and V is the voltage across the resistor. This method is not reliable for determining the power of an incandescent light bulb, resistance heater or electric short since all these devices operate at high temperatures and the resistance measured using a meter will not represent the operating resistance. For these conditions instead multiply V by current in amperes to get power in watts.[edit] References and notes 1. ^ BIPM SI Brochure: Appendix 1, p. 144[edit] See also • Ohms law • Resistor • Resistivity • abohm
  • 9. [edit] External links • Scanned books of Georg Simon Ohm at the library of the University of Applied Sciences Nuernberg • Official SI brochure • NIST Special Publication 811 • History of the ohm at sizes.comRetrieved from "http://en.wikipedia.org/wiki/Ohm"VoltageFrom Wikipedia, the free encyclopediaJump to: navigation, searchInternational safety symbol "Caution, risk of electric shock" (ISO 3864), colloquiallyknown as high voltage symbol.Voltage (sometimes also called electric or electrical tension) is the difference ofelectrical potential between two points of an electrical or electronic circuit, expressed involts.[1] It measures the potential energy of an electric field to cause an electric current inan electrical conductor. Depending on the difference of electrical potential it is calledextra low voltage, low voltage, high voltage or extra high voltage. Specifically Voltage isequal to energy per unit charge. It is analogous to fluid pressure in a hydraulic orpneumatic system.
  • 10. Contents[hide] • 1 Explanation o 1.1 Voltage with respect to a common point o 1.2 Voltage between two stated points o 1.3 Addition of voltages o 1.4 Hydraulic analogy o 1.5 Mathematical definition • 2 Useful formulas o 2.1 DC circuits o 2.2 AC circuits o 2.3 AC conversions o 2.4 Total voltage o 2.5 Voltage drops • 3 Measuring instruments • 4 Safety • 5 See also • 6 References • 7 External links [edit] ExplanationBetween two points in an electric field, such as exists in an electrical circuit, thedifference in their electrical potentials is known as the electrical potential difference. Thisdifference is directly proportional to the force that tends to push electrons or othercharge-carriers from one point to the other. Electrical potential difference can be thoughtof as the ability to move electrical charge through a resistance. At a time in physics whenthe word force was used loosely, the potential difference was named the electromotiveforce or EMF—a term which is still used in certain contexts.Voltage is a property of an electric field, not individual electrons.[citation needed]An electronmoving across a voltage difference experiences a net change in energy, often measured inelectron-volts. This effect is analogous to a mass falling through a given height differencein a gravitational field. When using the term potential difference or voltage, one must beclear about the two points between which the voltage is specified or measured. There aretwo ways in which the term is used. This can lead to some confusion.[edit] Voltage with respect to a common pointOne way in which the term voltage is used is when specifying the voltage of a point in acircuit. When this is done, it is understood that the voltage is usually being specified ormeasured with respect to a stable and unchanging point in the circuit that is known asground or common. This voltage is really a voltage difference, one of the two points
  • 11. being the reference point, which is ground. A voltage can be positive or negative. "High"or "low" voltage may refer to the magnitude (the absolute value relative to the referencepoint). Thus, a large negative voltage may be referred to as a high voltage. Other authorsmay refer to a voltage that is more negative as being "lower".[edit] Voltage between two stated pointsAnother usage of the term "voltage" is in specifying how many volts are across anelectrical device (such as a resistor). In this case, the "voltage," or, more accurately, the"voltage across the device," is really the first voltage taken, relative to ground, on oneterminal of the device minus a second voltage taken, relative to ground, on the otherterminal of the device. In practice, the voltage across a device can be measured directlyand safely using a voltmeter that is isolated from ground, provided that the maximumvoltage capability of the voltmeter is not exceeded.Two points in an electric circuit that are connected by an "ideal conductor," that is, aconductor without resistance and not within a changing magnetic field, have a potentialdifference of zero. However, other pairs of points may also have a potential difference ofzero. If two such points are connected with a conductor, no current will flow through theconnection.[edit] Addition of voltagesVoltage is additive in the following sense: the voltage between A and C is the sum of thevoltage between A and B and the voltage between B and C. The various voltages in acircuit can be computed using Kirchhoffs circuit laws.When talking about alternating current (AC) there is a difference between instantaneousvoltage and average voltage. Instantaneous voltages can be added as for direct current(DC), but average voltages can be meaningfully added only when they apply to signalsthat all have the same frequency and phase.[edit] Hydraulic analogy Main article: Hydraulic analogyIf one imagines water circulating in a network of pipes, driven by pumps in the absenceof gravity, as an analogy of an electrical circuit, then the potential difference correspondsto the fluid pressure difference between two points. If there is a pressure differencebetween two points, then water flowing from the first point to the second will be able todo work, such as driving a turbine.This hydraulic analogy is a useful method of teaching a range of electrical concepts. In ahydraulic system, the work done to move water is equal to the pressure multiplied by thevolume of water moved. Similarly, in an electrical circuit, the work done to moveelectrons or other charge-carriers is equal to electrical pressure (an old term for voltage)multiplied by the quantity of electrical charge moved. Voltage is a convenient way of
  • 12. quantifying the ability to do work. In relation to electric current, the larger the gradient(voltage or hydraulic) the greater the current (assuming resistance is constant).[edit] Mathematical definitionThe electrical potential difference is defined as the amount of work needed to move a unitelectric charge from the second point to the first, or equivalently, the amount of work thata unit charge flowing from the first point to the second can perform. The potentialdifference between two points a and b is the line integral of the electric field E:[edit] Useful formulas[edit] DC circuitswhere V = potential difference (volts), I = current intensity (amps), R = resistance (ohms),P = power (watts).[edit] AC circuitsWhere V=voltage, I=current, R=resistance, P=true power, Z=impedance, φ=phasor anglebetween I and V[edit] AC conversionsWhere Vpk=peak voltage, Vppk=peak-to-peak voltage, Vavg=average voltage over a half-cycle, Vrms=effective (root mean square) voltage, and we assumed a sinusoidal wave ofthe form Vpksin(ωt − kx), with a period T = 2π / ω, and where the angle brackets (in theroot-mean-square equation) denote a time average over an entire period.[edit] Total voltageVoltage sources and drops in series:Voltage sources and drops in parallel:Where is the nth voltage source or drop[edit] Voltage dropsAcross a resistor (Resistor R):Across a capacitor (Capacitor C):Across an inductor (Inductor L):Where V=voltage, I=current, R=resistance, X=reactance.
  • 13. [edit] Measuring instrumentsA multimeter set to measure voltage.Instruments for measuring potential differences include the voltmeter, the potentiometer(measurement device), and the oscilloscope. The voltmeter works by measuring thecurrent through a fixed resistor, which, according to Ohms Law, is proportional to thepotential difference across the resistor. The potentiometer works by balancing theunknown voltage against a known voltage in a bridge circuit. The cathode-rayoscilloscope works by amplifying the potential difference and using it to deflect anelectron beam from a straight path, so that the deflection of the beam is proportional tothe potential difference.SafetyElectrical safety is discussed in the articles on High voltage (note that even low voltage,e. g. of 50 Volts, can lead to a lethal electric shock) and Electric shock.Electric currentFrom Wikipedia, the free encyclopedia (Redirected from Current (electricity))Jump to: navigation, search Electromagnetism Electricity · Magnetism [show]Electrostatics [hide]Magnetostatics
  • 14. · Ampère’s law · Electric current · Magnetic field · Magnetic flux · Biot–Savart law · Magnetic dipole moment · Gauss’s law for magnetism · [show]Electrodynamics [show]Electrical Network [show]Covariant formulation [show]Scientists This box: view • talk • editElectric current is the flow (movement) of electric charge. The SI unit of electric currentis the ampere, and electric current is measured using an ammeter. For the definition of theampere, see the Ampere article.Contents[hide] • 1 Current in a metal wire • 2 Current density • 3 The drift speed of electric charges • 4 Ohms law • 5 Conventional current • 6 Examples • 7 Electromagnetism • 8 Reference direction • 9 Electrical safety • 10 See also • 11 References • 12 External links [edit] Current in a metal wireA solid conductive metal contains a large population of mobile, or free, electrons. Theseelectrons are bound to the metal lattice but not to any individual atom. Even with noexternal electric field applied, these electrons move about randomly due to thermalenergy but, on average, there is zero net current within the metal. Given an imaginaryplane through which the wire passes, the number of electrons moving from one side tothe other in any period of time is on average equal to the number passing in the oppositedirection.
  • 15. A typical metal wire for electrical conduction is the stranded copper wire.When a metal wire is connected across the two terminals of a DC voltage source such asa battery, the source places an electric field across the conductor. The moment contact ismade, the free electrons of the conductor are forced to drift toward the positive terminalunder the influence of this field. The free electrons are therefore the current carrier in atypical solid conductor. For an electric current of 1 ampere, 1 coulomb of electric charge(which consists of about 6.242 × 1018 electrons) drifts every second through anyimaginary plane through which the conductor passes.The current I in amperes can be calculated with the following equation:where is the electric charge in coulombs (ampere seconds) is the time in secondsIt follows that: andMore generally, electric current can be represented as the time rate of change of charge,or .[edit] Current density Main article: Current densityCurrent density is a measure of the density of electrical current. It is defined as a vectorwhose magnitude is the electric current per cross-sectional area. In SI units, the currentdensity is measured in amperes per square meter.
  • 16. [edit] The drift speed of electric chargesThe mobile charged particles within a conductor move constantly in random directions,like the particles of a gas. In order for there to be a net flow of charge, the particles mustalso move together with an average drift rate. Electrons are the charge carriers in metalsand they follow an erratic path, bouncing from atom to atom, but generally drifting in thedirection of the electric field. The speed at which they drift can be calculated from theequation:where is the electric current is number of charged particles per unit volume is the cross-sectional area of the conductor is the drift velocity, and is the charge on each particle.Electric currents in solids typically flow very slowly. For example, in a copper wire ofcross-section 0.5 mm², carrying a current of 5 A, the drift velocity of the electrons is ofthe order of a millimetre per second. To take a different example, in the near-vacuuminside a cathode ray tube, the electrons travel in near-straight lines ("ballistically") atabout a tenth of the speed of light.Any accelerating electric charge, and therefore any changing electric current, gives rise toan electromagnetic wave that propagates at very high speed outside the surface of theconductor. This speed is usually a significant fraction of the speed of light, as can bededuced from Maxwells Equations, and is therefore many times faster than the driftvelocity of the electrons. For example, in AC power lines, the waves of electromagneticenergy propagate through the space between the wires, moving from a source to a distantload, even though the electrons in the wires only move back and forth over a tinydistance.The ratio of the speed of the electromagnetic wave to the speed of light in free space iscalled the velocity factor, and depends on the electromagnetic properties of the conductorand the insulating materials surrounding it, and on their shape and size.The nature of these three velocities can be illustrated by an analogy with the three similarvelocities associated with gases. The low drift velocity of charge carriers is analogous toair motion; in other words, winds. The high speed of electromagnetic waves is roughlyanalogous to the speed of sound in a gas; while the random motion of charges isanalogous to heat - the thermal velocity of randomly vibrating gas particles.[edit] Ohms lawOhms law predicts the current in an (ideal) resistor (or other ohmic device) to be theapplied voltage divided by resistance:
  • 17. where I is the current, measured in amperes V is the potential difference measured in volts R is the resistance measured in ohms[edit] Conventional currentConventional current was defined early in the history of electrical science as a flow ofpositive charge. In solid metals, like wires, the positive charge carriers are immobile,and only the negatively charged electrons flow. Because the electron carries negativecharge, the electron current is in the direction opposite to that of conventional (orelectric) current.Diagram showing conventional current notation. Electric charge moves from the positiveside of the power source to the negative.In other conductive materials, the electric current is due to the flow of charged particlesin both directions at the same time. Electric currents in electrolytes are flows ofelectrically charged atoms (ions), which exist in both positive and negative varieties. Forexample, an electrochemical cell may be constructed with salt water (a solution ofsodium chloride) on one side of a membrane and pure water on the other. The membranelets the positive sodium ions pass, but not the negative chloride ions, so a net currentresults. Electric currents in plasma are flows of electrons as well as positive and negativeions. In ice and in certain solid electrolytes, flowing protons constitute the electriccurrent. To simplify this situation, the original definition of conventional current stillstands.There are also materials where the electric current is due to the flow of electrons and yetit is conceptually easier to think of the current as due to the flow of positive "holes" (thespots that should have an electron to make the conductor neutral). This is the case in a p-type semiconductor.[edit] ExamplesNatural examples include lightning and the solar wind, the source of the polar auroras(the aurora borealis and aurora australis). The artificial form of electric current is the flowof conduction electrons in metal wires, such as the overhead power lines that deliverelectrical energy across long distances and the smaller wires within electrical andelectronic equipment. In electronics, other forms of electric current include the flow ofelectrons through resistors or through the vacuum in a vacuum tube, the flow of ionsinside a battery, and the flow of holes within a semiconductor.
  • 18. According to Ampères law, an electric current produces a magnetic field.[edit] ElectromagnetismElectric current produces a magnetic field. The magnetic field can be visualized as apattern of circular field lines surrounding the wire.Electric current can be directly measured with a galvanometer, but this method involvesbreaking the circuit, which is sometimes inconvenient. Current can also be measuredwithout breaking the circuit by detecting the magnetic field associated with the current.Devices used for this include Hall effect sensors, current clamps, current transformers,and Rogowski coils.[edit] Reference directionWhen solving electrical circuits, the actual direction of current through a specific circuitelement is usually unknown. Consequently, each circuit element is assigned a currentvariable with an arbitrarily chosen reference direction. When the circuit is solved, thecircuit element currents may have positive or negative values. A negative value meansthat the actual direction of current through that circuit element is opposite that of thechosen reference direction.[edit] Electrical safetyThe most obvious hazard is electrical shock, where a current passes through part of thebody. It is the amount of current passing through the body that determines the effect, andthis depends on the nature of the contact, the condition of the body part, the current paththrough the body and the voltage of the source. While a very small amount can cause aslight tingle, too much can cause severe burns if it passes through the skin or even cardiacarrest if enough passes through the heart. The effect also varies considerably fromindividual to individual. (For approximate figures see Shock Effects under electricshock.)Due to this and the fact that passing current cannot be easily predicted in most practicalcircumstances, any supply of over 50 volts should be considered a possible source ofdangerous electric shock. In particular, note that 110 volts (a minimum voltage at whichAC mains power is distributed in much of the Americas, and 4 other countries, mostly inAsia) can certainly cause a lethal amount of current to pass through the body.Electric arcs, which can occur with supplies of any voltage (for example, a typical arcwelding machine has a voltage between the electrodes of just a few tens of volts), arevery hot and emit ultra-violet (UV) and infra-red radiation (IR). Proximity to an electricarc can therefore cause severe thermal burns, and UV is damaging to unprotected eyesand skin.
  • 19. Accidental electric heating can also be dangerous. An overloaded power cable is afrequent cause of fire. A battery as small as an AA cell placed in a pocket with metalcoins can lead to a short circuit heating the battery and the coins which may inflict burns.NiCad, NiMh cells, and lithium batteries are particularly risky because they can deliver avery high current due to their low internal resistance.[edit] See alsoElectronics portal • Alternating current • Current density • Direct current • Electrical conduction for more information on the physical mechanism of current flow in materials • Four-current • History of electrical engineering • Hydraulic analogy • SI electromagnetism unitsAmpereFrom Wikipedia, the free encyclopediaJump to: navigation, searchFor other uses, see Ampere (disambiguation).Current can be measured by a galvanometer, via the deflection of a magnetic needle inthe magnetic field created by the current.The ampere, in practice often shortened to amp, (symbol: A) is a unit of electric current,or amount of electric charge per second. The ampere is an SI base unit, and is namedafter André-Marie Ampère, one of the main discoverers of electromagnetism.
  • 20. Contents[hide] • 1 Definition • 2 History • 3 Realization • 4 Proposed future definition o 4.1 CIPM recommendation • 5 See also • 6 References • 7 External links [edit] DefinitionOne ampere is defined to be the constant current which will produce a force of 2×10–7newton per metre of length between two straight, parallel conductors of infinite lengthand negligible circular cross section placed one metre apart in a vacuum.[1][2] Thedefinition is based on Ampères force law .[3] . The ampere is a base unit, along with themetre, kelvin, second, mole, candela and the kilogram: it is defined without reference tothe quantity of electric charge.The S.I. unit of charge, the coulomb, is defined to be the quantity of charge displaced bya one ampere per second.[4] Conversely, an ampere is one coulomb of charge goingpast a given point in the duration of one second; that is, in general, charge Q isdetermined by steady current I flowing per unit time t as:[edit] HistoryThe ampere was originally defined as one tenth of the CGS system electromagnetic unitof current (now known as the abampere), the amount of current which generates a forceof two dynes per centimetre of length between two wires one centimetre apart. [5] - thesize of the unit was chosen so that the units derived from it in the MKSA system wouldbe conveniently sized.The "international ampere" was an early realization of the ampere, defined as the currentthat would deposit 0.001118000 grams of silver per second from a silver nitrate solution.[6] Later, more accurate measurements revealed that this current is 0.99985 A.[edit] RealizationThe ampere is most accurately realized using a watt balance, but is in practice maintainedvia Ohms Law from the units of EMF and resistance, the volt and the ohm, since thelatter two can be tied to physical phenomena that are relatively easy to reproduce, theJosephson junction and the quantum Hall effect, respectively. The official realization of a
  • 21. standard ampere is discussed in NIST Special publication 330 Barry N Taylor (editor)Appendix 2, p. 56.[edit] Proposed future definitionSince a coulomb is approximately equal to 6.24150948×1018 elementary charges, oneampere is approximately equivalent to 6.24150948×1018 elementary charges, such aselectrons, moving past a boundary in one second.As with other SI base units, there have been proposals to redefine the kilogram in such away as to define some presently measured physical constants to fixed values. Oneproposed definition of the kilogram is:The kilogram is the mass which would be accelerated at precisely 2×10-7 m/s2 if subjectedto the per metre force between two straight parallel conductors of infinite length, ofnegligible circular cross section, placed 1 metre apart in vacuum, through which flow aconstant current of exactly 6 241 509 479 607 717 888 elementary charges per second.This redefinition of the kilogram has the effect of fixing the elementary charge to be e =1.60217653×10-19 C and would result in a functionally equivalent definition for thecoulomb as being the sum of exactly 6 241 509 479 607 717 888 elementary charges andthe ampere as being the electrical current of exactly 6 241 509 479 607 717 888elementary charges per second. This is consistent with the current 2002 CODATA valuefor the elementary charge which is 1.60217653×10-19 ± 0.00000014×10-19 C.[edit] CIPM recommendationInternational Committee for Weights and Measures (CIPM) Recommendation 1 (CI-2005): Preparative steps towards new definitions of the kilogram, the ampere, the kelvinand the mole in terms of fundamental constantsThe International Committee for Weights and Measures (CIPM), • approve in principle the preparation of new definitions and mises en pratique of the kilogram, the ampere and the kelvin so that if the results of experimental measurements over the next few years are indeed acceptable, all having been agreed with the various Consultative Committees and other relevant bodies, the CIPM can prepare proposals to be put to Member States of the Metre Convention in time for possible adoption by the 24th CGPM in 2011; • give consideration to the possibility of redefining, at the same time, the mole in terms of a fixed value of the Avogadro constant; • prepare a Draft Resolution that may be put to the 23rd CGPM in 2007 to alert Member States to these activities; This SI unit is named after André-Marie Ampère. As with every SI unit whose name is derived from the proper name of a person, the first letter of its symbol is uppercase
  • 22. (A). When an SI unit is spelled out in English, it should always begin with a lowercase letter (ampere), except where any word would be capitalized, such as at the beginning of a sentence or in capitalized material such as a title. Note that "degree Celsius" conforms to this rule because the "d" is lowercase. — Based on The International System of Units, section 5.2.[edit] See also • International System of Units • Ohms Law • Hydraulic analogy • Electric shock • Ampères force law • Ammeter • Coulomb • Magnetic constant[edit] References 1. ^ BIPM official definition 2. ^ Paul M. S. Monk, Physical Chemistry: Understanding our Chemical World, John Wiley and Sons, 2004 online. 3. ^ Raymond A Serway & Jewett JW (2006). Serways principles of physics: a calculus based text, Fourth Edition, Belmont, CA: Thompson Brooks/Cole, p. 746. ISBN 053449143X. 4. ^ BIPM Table 3 5. ^ A short history of the SI units in electricity 6. ^ History of the ampereElectric currentFrom Wikipedia, the free encyclopediaJump to: navigation, search Electromagnetism Electricity · Magnetism [show]Electrostatics [hide]Magnetostatics · Ampère’s law · Electric current · Magnetic field · Magnetic flux · Biot–Savart law · Magnetic dipole moment · Gauss’s law
  • 23. for magnetism · [show]Electrodynamics [show]Electrical Network [show]Covariant formulation [show]Scientists This box: view • talk • editElectric current is the flow (movement) of electric charge. The SI unit of electric currentis the ampere, and electric current is measured using an ammeter. For the definition of theampere, see the Ampere article.Contents[hide] • 1 Current in a metal wire • 2 Current density • 3 The drift speed of electric charges • 4 Ohms law • 5 Conventional current • 6 Examples • 7 Electromagnetism • 8 Reference direction • 9 Electrical safety • 10 See also • 11 References • 12 External links [edit] Current in a metal wireA solid conductive metal contains a large population of mobile, or free, electrons. Theseelectrons are bound to the metal lattice but not to any individual atom. Even with noexternal electric field applied, these electrons move about randomly due to thermalenergy but, on average, there is zero net current within the metal. Given an imaginaryplane through which the wire passes, the number of electrons moving from one side tothe other in any period of time is on average equal to the number passing in the oppositedirection.
  • 24. A typical metal wire for electrical conduction is the stranded copper wire.When a metal wire is connected across the two terminals of a DC voltage source such asa battery, the source places an electric field across the conductor. The moment contact ismade, the free electrons of the conductor are forced to drift toward the positive terminalunder the influence of this field. The free electrons are therefore the current carrier in atypical solid conductor. For an electric current of 1 ampere, 1 coulomb of electric charge(which consists of about 6.242 × 1018 electrons) drifts every second through anyimaginary plane through which the conductor passes.The current I in amperes can be calculated with the following equation:where is the electric charge in coulombs (ampere seconds) is the time in secondsIt follows that: andMore generally, electric current can be represented as the time rate of change of charge,or .[edit] Current density Main article: Current densityCurrent density is a measure of the density of electrical current. It is defined as a vectorwhose magnitude is the electric current per cross-sectional area. In SI units, the currentdensity is measured in amperes per square meter.
  • 25. [edit] The drift speed of electric chargesThe mobile charged particles within a conductor move constantly in random directions,like the particles of a gas. In order for there to be a net flow of charge, the particles mustalso move together with an average drift rate. Electrons are the charge carriers in metalsand they follow an erratic path, bouncing from atom to atom, but generally drifting in thedirection of the electric field. The speed at which they drift can be calculated from theequation:where is the electric current is number of charged particles per unit volume is the cross-sectional area of the conductor is the drift velocity, and is the charge on each particle.Electric currents in solids typically flow very slowly. For example, in a copper wire ofcross-section 0.5 mm², carrying a current of 5 A, the drift velocity of the electrons is ofthe order of a millimetre per second. To take a different example, in the near-vacuuminside a cathode ray tube, the electrons travel in near-straight lines ("ballistically") atabout a tenth of the speed of light.Any accelerating electric charge, and therefore any changing electric current, gives rise toan electromagnetic wave that propagates at very high speed outside the surface of theconductor. This speed is usually a significant fraction of the speed of light, as can bededuced from Maxwells Equations, and is therefore many times faster than the driftvelocity of the electrons. For example, in AC power lines, the waves of electromagneticenergy propagate through the space between the wires, moving from a source to a distantload, even though the electrons in the wires only move back and forth over a tinydistance.The ratio of the speed of the electromagnetic wave to the speed of light in free space iscalled the velocity factor, and depends on the electromagnetic properties of the conductorand the insulating materials surrounding it, and on their shape and size.The nature of these three velocities can be illustrated by an analogy with the three similarvelocities associated with gases. The low drift velocity of charge carriers is analogous toair motion; in other words, winds. The high speed of electromagnetic waves is roughlyanalogous to the speed of sound in a gas; while the random motion of charges isanalogous to heat - the thermal velocity of randomly vibrating gas particles.[edit] Ohms lawOhms law predicts the current in an (ideal) resistor (or other ohmic device) to be theapplied voltage divided by resistance:
  • 26. where I is the current, measured in amperes V is the potential difference measured in volts R is the resistance measured in ohms[edit] Conventional currentConventional current was defined early in the history of electrical science as a flow ofpositive charge. In solid metals, like wires, the positive charge carriers are immobile,and only the negatively charged electrons flow. Because the electron carries negativecharge, the electron current is in the direction opposite to that of conventional (orelectric) current.Diagram showing conventional current notation. Electric charge moves from the positiveside of the power source to the negative.In other conductive materials, the electric current is due to the flow of charged particlesin both directions at the same time. Electric currents in electrolytes are flows ofelectrically charged atoms (ions), which exist in both positive and negative varieties. Forexample, an electrochemical cell may be constructed with salt water (a solution ofsodium chloride) on one side of a membrane and pure water on the other. The membranelets the positive sodium ions pass, but not the negative chloride ions, so a net currentresults. Electric currents in plasma are flows of electrons as well as positive and negativeions. In ice and in certain solid electrolytes, flowing protons constitute the electriccurrent. To simplify this situation, the original definition of conventional current stillstands.There are also materials where the electric current is due to the flow of electrons and yetit is conceptually easier to think of the current as due to the flow of positive "holes" (thespots that should have an electron to make the conductor neutral). This is the case in a p-type semiconductor.[edit] ExamplesNatural examples include lightning and the solar wind, the source of the polar auroras(the aurora borealis and aurora australis). The artificial form of electric current is the flowof conduction electrons in metal wires, such as the overhead power lines that deliverelectrical energy across long distances and the smaller wires within electrical andelectronic equipment. In electronics, other forms of electric current include the flow ofelectrons through resistors or through the vacuum in a vacuum tube, the flow of ionsinside a battery, and the flow of holes within a semiconductor.
  • 27. According to Ampères law, an electric current produces a magnetic field.[edit] ElectromagnetismElectric current produces a magnetic field. The magnetic field can be visualized as apattern of circular field lines surrounding the wire.Electric current can be directly measured with a galvanometer, but this method involvesbreaking the circuit, which is sometimes inconvenient. Current can also be measuredwithout breaking the circuit by detecting the magnetic field associated with the current.Devices used for this include Hall effect sensors, current clamps, current transformers,and Rogowski coils.[edit] Reference directionWhen solving electrical circuits, the actual direction of current through a specific circuitelement is usually unknown. Consequently, each circuit element is assigned a currentvariable with an arbitrarily chosen reference direction. When the circuit is solved, thecircuit element currents may have positive or negative values. A negative value meansthat the actual direction of current through that circuit element is opposite that of thechosen reference direction.[edit] Electrical safetyThe most obvious hazard is electrical shock, where a current passes through part of thebody. It is the amount of current passing through the body that determines the effect, andthis depends on the nature of the contact, the condition of the body part, the current paththrough the body and the voltage of the source. While a very small amount can cause aslight tingle, too much can cause severe burns if it passes through the skin or even cardiacarrest if enough passes through the heart. The effect also varies considerably fromindividual to individual. (For approximate figures see Shock Effects under electricshock.)Due to this and the fact that passing current cannot be easily predicted in most practicalcircumstances, any supply of over 50 volts should be considered a possible source ofdangerous electric shock. In particular, note that 110 volts (a minimum voltage at whichAC mains power is distributed in much of the Americas, and 4 other countries, mostly inAsia) can certainly cause a lethal amount of current to pass through the body.Electric arcs, which can occur with supplies of any voltage (for example, a typical arcwelding machine has a voltage between the electrodes of just a few tens of volts), arevery hot and emit ultra-violet (UV) and infra-red radiation (IR). Proximity to an electricarc can therefore cause severe thermal burns, and UV is damaging to unprotected eyesand skin.
  • 28. Accidental electric heating can also be dangerous. An overloaded power cable is afrequent cause of fire. A battery as small as an AA cell placed in a pocket with metalcoins can lead to a short circuit heating the battery and the coins which may inflict burns.NiCad, NiMh cells, and lithium batteries are particularly risky because they can deliver avery high current due to their low internal resistance.[edit] See alsoElectronics portal • Alternating current • Current density • Direct current • Electrical conduction for more information on the physical mechanism of current flow in materials • Four-current • History of electrical engineering • Hydraulic analogy • SI electromagnetism units

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