Electric current is the flow movement of electric charge. The SI unit of electric current is the ampere, and electric current is measured using an ammeter. For the definition of the ampere, see the Ampere article.Current in a metal wireA solid conductive metal contains a large population of mobile, or free, electrons. These electrons are bound to the metal lattice but not to any individual atom. Even with no external electric field applied, these electrons move about randomly due to thermal energy but, on average, there is zero net current within the metal. Given an imaginary plane through which the wire passes, the number of electrons moving from one side to the other in any period of time is on average equal to the number passing in the opposite direction.A typical metal wire for electrical conduction is the stranded copper wire.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 as a battery, the source places an electric field across the conductor. The moment contact is made, the free electrons of the conductor are forced to drift toward the positive terminal under the influence of this field. The free electrons are therefore the current carrier in a typical solid conductor. For an electric current of ampere, coulomb of electric charge which consists of about . × electrons drifts every second through any imaginary plane through which the conductor passes.The current I in amperes can be calculated with the following equation I Q over there Q ! is the electric charge in coulombs ampere secondst ! is the time in secondsIt follows that QIt ! and t Q over IMore generally, electric current can be represented as the time rate of change of charge, or I fracdQdt.edit Current densityMain article Current densityCurrent density is a measure of the density of electrical current. It is defined as a vector whose magnitude is the electric current per crosssectional area. In SI units, the current density is measured in amperes per square meter.dit The drift speed of electric chargeshe 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 must also move together with an average drift rate. Electrons are the charge carriers in metals and they follow an erratic path, bouncing from atom to atom, but generally drifting in the direction of the electric field. The speed at which they drift can be calculated from the equation InAvQ ! where I ! is the electric currnt n ! is number of charged particles per unit volume A ! is the crosssectional area of the condutor v ! is the drift velocity, and Q ! is the charge on each particle.Electric currents in solids typically flow very slowly. For example, in a copper wire of crosssection . mm², carrying a current of A, the drift velocity of the electrons is of the order of a millimetre per second. To take a different example, in the nearvacuum inside a cathode ray tube, the electrons travel in nearstraight lines ballistically at about a tenth of the speed of light.Any accelerating electric charge, and therefore any changing electric current, gives rise to an electromagnetic wave that propagates at very high speed outside the surface of the conductor. This speed is usually a significant fraction of the speed of light, as can be deduced from Maxwells Equations, and is therefore many times faster than the drift velocity of the electrons. For example, in AC power lines, the waves of electromagnetic energy propagate through the space between the wires, moving from a source to a distant load, even though the electrons in the wires only move back and forth over a tiny distance.The ratio of the speed of the electromagnetic wave to the speed of light in free space is called the velocity factor, and depends on the electromagnetic properties of the conductor and 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 similar velocities associated with gases. The low drift velocity of charge carriers is analogous to air motion in other words, winds. The high speed of electromagnetic waves is roughly analogous to the speed of sound in a gas while the random motion of charges is analogous to heat the thermal velocity of randomly vibrating gas particles.
Ohms lawOhms law predicts the current in an ideal resistor or other ohmic device to be the applied voltage divided by resistance I frac VRwhere I is the current, measured in amperesV is the potential difference measured in volts R is the resistance measured in ohmsedit Conventional currentA flow of positive charge gives the same electric current as an opposite flow of negative charge. Thus, opposite flows of opposite charges contribute to a single electric current. For this reason, the polarity of the flowing charges can usually be ignored during measurements. All the flowing charges are assumed to have positive polarity, and this flow is called Conventional current.In solid metals such as wires, the positive charge carriers are immobile, and only the negatively charged electrons flow. Because the electron carries negative charge, the electron motion in a metal is in the direction opposite to that of conventional or electric current.Diagram showing conventional current notation. Electric charge moves from the positive side of the power source to the negative.Diagram showing conventional current notation. Electric charge moves from the positive side of the power source to the negative.In many other conductive materials, the electric current is due to the flow of both positively and negatively charged particles at the same time. In still others, the current is entirely due to positive charge flow. For example, the electric currents in electrolytes are flows of electrically charged atoms ions, which exist in both positive and negative varieties. In a common leadacid electrochemical cell, electric currents are composed of positive hydrogen ions protons flowing in one direction, and negative sulfate ions flowing in the other. Electric currents in sparks or plasma are flows of electrons as well as positive and negative ions. In ice and in certain solid electrolytes, the electric current is entirely composed of flowing protons. For conceptual simplicity, Conventional current is used to conceal these issues by summing the various currents together into a single value.There are also materials where the electric current is due to the flow of electrons, and yet it is conceptually easier to think of the current as due to the flow of positive holes the spots that should have an electron to make the conductor neutral. This is the case in a ptype semiconductor.
dit 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 flow of conduction electrons in metal wires, such as the overhead power lines that deliver electrical energy across long distances and the smaller wires within electrical and electronic equipment. In electronics, other forms of electric current include the flow of electrons through resistors or through the vacuum in a vacuum tube, the flow of ions inside a battery, and the flow of holes within a semiconductor.According to Ampères law, an electric current produces a magnetic field.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 a pattern of circular field lines surrounding the wire.Electric current can be directly measured with a galvanometer, but this method involves breaking the circuit, which is sometimes inconvenient. Current can also be measured without 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.
When solving electrical circuits, the actual direction of current through a specific circuit element is usually unknown. Consequently, each circuit element is assigned a current variable with an arbitrarily chosen reference direction. When the circuit is solved, the circuit element currents may have positive or negative values. A negative value means that the actual direction of current through that circuit element is opposite that of the chosen reference direction.edit Electrical safetyThe most obvious hazard is electrical shock, where a current passes through part of the body. It is the amount of current passing through the body that determines the effect, and this depends on the nature of the contact, the condition of the body part, the current path through the body and the voltage of the source. While a very small amount can cause a slight tingle, too much can cause severe burns if it passes through the skin or even cardiac arrest if enough passes through the heart. The effect also varies considerably from individual to individual. For approximate figures see Shock Effects under electric shock.
Due to this and the fact that passing current cannot be easily predicted in most practical circumstances, any supply of over volts should be considered a possible source of dangerous electric shock. In particular, note that volts a minimum voltage at which AC mains power is distributed in much of the Americas, and other countries, mostly in Asia 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 arc welding machine has a voltage between the electrodes of just a few tens of volts, are very hot and emit ultraviolet UV and infrared radiation IR. Proximity to an electric arc can therefore cause severe thermal burns, and UV is damaging to unprotected eyes and skin.Accidental electric heating can also be dangerous. An overloaded power cable is a frequent cause of fire. A battery as small as an AA cell placed in a pocket with metal coins 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 a very high current due to their low internal resistance.Current densityFrom Wikipedia, the free encyclopediaJump to navigation, searchThis page is mainly about the electric current density in electromagnetism. For the probability current denity in quantum mechanics, see Current density quantum mechanics.Current density is a measure of the density of flow of a conserved charge. Usually the charge is the electric charge, in which case the associated current density is the electric current per unit area of cross section, but the term current density can also be applied to other conserved quantities. It is defined as a vector whose magnitude is the current per crosssectional area.In SI units, the electric current density is measured in amperes per square metre. is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutatortype electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also be through semiconductors, insulators, or even through a vacuum as in electron or ion beams. In direct current, the electric charges flow in the same direction, distinguishing it from alternating current AC. A term formerly used for direct current was Galvanic current.Types of direct currentTypes of direct currentDirect current may be obtained from an alternating current supply by use of a currentswitching arrangement called a rectifier, which contains electronic elements usually or electromechanical elements historically that allow current to flow only in one direction. Direct current may be made into alternating current with an inverter or a motorgenerator set.The first commercial electric power transmission developed by Thomas Edison in the late nineteenth century used direct current. Because of the advantage of alternating current over direct current in transforming and transmission, electric power distribution today is nearly all alternating current. For applications requiring direct current, such as third rail power systems, alternating current is distributed to a substation, which utilizes a rectifier to convert the power to direct current. See War of Currents.
Direct current is used to charge batteries, and in nearly all electronic systems as the power supply. Very large quantities of directcurrent power are used in production of aluminum and other electrochemical processes. Direct current is used for some railway propulsion, especially in urban areas. High voltage direct current is used to transmit large amounts of power from remote generation sites or to interconnect alternating current power grids.ContentsVarious definitionsWithin electrical engineering, the term DC is used to refer to power systems that use only one polarity of voltage or current, and to refer to the constant, zerofrequency, or slowly varying local mean value of a voltage or current. For example, the voltage across a DC voltage source is constant as is the current through a DC current source. The DC solution of an electric circuit is the solution where all voltages and currents are constant. It can be shown that any stationary voltage or current waveform can be decomposed into a sum of a DC component and a zeromean timevarying component the DC component is defined to be the expected value, or the average value of the voltage or current over all time.Although DC stands for Direct Current, DC sometimes refers to constant polarity. With this definition, DC voltages can vary in time, such as the raw output of a rectifier or the fluctuating voice signal on a telephone line.ome forms of DC such as that produced by a voltage regulator have almost no variations in voltage, but may still have variations in output power and current.edit ApplicationsDirectcurrent installations usually have different types of sockets, switches, and fixtures, mostly due to the low voltages used, from those suitable for alternating current. It is usually important with a directcurrent appliance not to reverse polarity unless the device has a diode bridge to correct for this most batterypowered devices do not.This symbol is found on many electronic devices that either require or produce direct currentThis symbol is found on many electronic devices that either require or produce direct currentDC is commonly found in many lowvoltage applications, especially where these are powered by batteries, which can produce only DC, or solar power systems, since solar cells can produce only DC. Most automotive applications use DC, although the alternator is an AC device which uses a rectifier to produce DC. Most electronic circuits require a DC power supply. Applications using fuel cells mixing hydrogen and oxygen together with a catalyst to produce electricity and water as byproducts also produce only DC.Many telephones connect to a twisted pair of wires, and internally separate the AC component of the voltage between the two wires the audio signal from the DC component of the voltage between the two wires used to power the phone.Telephone exchange communication equipment, such as DSLAM, uses standard V DC power supply. The negative polarity is achieved by grounding the positive terminal of power supply system and the battery bank. This is done to prevent electrolysis depositions.An electrified third rail can be used to power both underground subway and overground trains.An alternating current AC is an electric current whose direction reverses cyclically, as opposed to direct current, whose direction remains constant. The usual waveform of an AC power circuit is a sine wave, as this results in the most efficient transmission of energy. However in certain applications different waveforms are used, such as triangular or square waves.sed generically, AC refers to the form in which electricity is delivered to businesses and residences. However, audio and radio signals carried on electrical wires are also examples of alternating current. In these applications, an important goal is often the recovery of information encoded or modulated onto the AC signal.istoryWestinghouse Early AC System U.S. Patent , Westinghouse Early AC System U.S. Patent , William Stanley, Jr. designed one of the first practical devices to transfer AC power efficiently between isolated circuits. Using pairs of coils wound on a common iron core, his design, called an induction coil, was an early transformer. The AC power system used today developed rapidly after , and includes key concepts by Nikola Tesla, who subsequently sold his patent to George Westinghouse. Lucien Gaulard, John Dixon Gibbs, Carl Wilhelm Siemens and others contributed subsequently to this field. AC systems overcame the limitations of the direct current system used by Thomas Edison to distribute electricity efficiently over long distances.The first modern commercial power plant using threephase alternating current was at the Mill Creek hydroelectric plant near Redlands, California in designed by Almirian Decker. Deckers design incorporated , volt threephase transmission and established the standards for the complete system of generation, transmission and motors used today.Alternating current circuit theory evolved rapidly in the latter part of the th and early th century. Notable contributors to the theoretical basis of alternating current calculations include Charles Steinmetz, James Clerk Maxwell, Oliver Heaviside, and many others. Calculations in unbalanced threephase systems were simplified by the symmetrical components methods discussed by Charles Legeyt Fortescue in .
Transmission, distribution, and domestic power supply
Main article Electricity distributionAC power can be increased or decreased in voltage with a transformer. Use of a higher voltage leads to significantly more efficient transmission of power. The power losses in a conductor are a product of the square of the current and the resistance of the conductor, described by the formula PI^ cdot R ,! . This means that when transmitting a fixed power on a given wire, if the current is doubled, the power loss will be four times greater.Since the power transmitted is equal to the product of the current, the voltage and the cosine of the phase difference f P IVcosf, the same amount of power can be transmitted with a lower current by increasing the voltage. Therefore it is advantageous when transmitting large amounts of power to distribute the power with high voltages often hundreds of kilovolts.However, high voltages also have disadvantages, the main one being the increased insulation required, and generally increased difficulty in their safe handling. In a power plant, power is generated at a convenient voltage for the design of a generator, and then stepped up to a high voltage for transmission. Near the loads, the transmission voltage is stepped down to the voltages used by equipment. Consumer voltages vary depending on the country and size of load, but generally motors and lighting are built to use up to a few hundred volts between phases.The utilization voltage delivered to equipment such as lighting and motor loads is standardized, with an allowable range of voltage over which equipment is expected to operate. Standard power utilization voltages and percentage tolerance vary in the different mains power systems found in the world.Modern highvoltage, directcurrent electric power transmission systems contrast with the more common alternatingcurrent systems as a means for the bulk transmission of electrical power over long distances. HVDC systems tend to be more expensive and less efficient than transformers. Transmission with high voltage direct current was not feasible when Edison, Westinghouse and Tesla were designing their power systems, since there was then no way to economically convert AC power to DC and back again at the necessary voltages.Threephase electrical generation is very common. Three separate coils in the generator stator are physically offset by an angle of ° to each other. Three current waveforms are produced that are equal in magnitude and ° out of phase to each other.If the load on a threephase system is balanced equally among the phases, no current flows through the neutral point. Even in the worstcase unbalanced linear load, the neutral current will not exceed the highest of the phase currents. It is noteworthy that nonlinear loads e.g. computers may require an oversized neutral bus and neutral conductor in the upstream distribution panel to handle harmonics. Harmonics can cause neutral conductor current levels to exceed that of one or all phase conductors.For threephase at utilization voltages a fourwire system is often used. When stepping down threephase, a transformer with a Delta primary and a Star secondary is often used so there is no need for a neutral on the supply side.
For smaller customers just how small varies by country and age of the installation only a single phase and the neutral or two phases and the neutral are taken to the property. For larger installations all three phases and the neutral are taken to the main distribution panel. From the threephase main panel, both single and threephase circuits may lead off.Threewire single phase systems, with a single centretapped transformer giving two live conductors, is a common distribution scheme for residential and small commercial buildings in North America. This arrangement is sometimes incorrectly referred to as two phase. A similar method is used for a different reason on construction sites in the UK. Small power tools and lighting are supposed to be supplied by a local centertapped transformer with a voltage of V between each power conductor and the earth. This significantly reduces the risk of electric shock in the event that one of the live conductors becomes exposed through an equipment fault whilst still allowing a reasonable voltage for running the tools.A third wire, called the bond wire, is often connected between noncurrentcarrying metal enclosures and earth ground. This conductor provides protection from electric shock due to accidental contact of circuit conductors with the metal chassis of portable appliances and tools. Bonding all noncurrentcarrying metal parts into one complete system ensures there is always a low impedance path to ground sufficient to carry any fault current for as long as it takes for the system to clear the fault. This low impedance path allows the maximum amount of fault current, causing the overcurrent protection device Breakers, fuses to trip or burn out as quickly as possible, returning the electrical system to a safe state. All bond wires are bonded to ground at the main service panel, as is the NeutralIdentified conductor if present.edit AC power supply frequenciesThe frequency of the electrical system varies by country most electric power is generated at either or Hz. See List of countries with mains power plugs, voltages and frequencies. Some countries have a mixture of Hz and Hz supplies, notably Japan.A low frequency eases the design of low speed electric motors, particularly for hoisting, crushing and rolling applications, and commutatortype traction motors for applications such as railways, but also causes a noticeable flicker in incandescent lighting and objectionable flicker of fluorescent lamps. ? Hz power is still used in some European rail systems, such as in Austria, Germany, Norway, Sweden and Switzerland. The use of lower frequencies also provided the advantage of lower impedance losses, which are proportional to frequency. The original Niagara Falls generators were built to produce Hz power, as a compromise between low frequency for traction and heavy induction motors, while still allowing incandescent lighting to operate although with noticeable flicker most of the Hz residential and commercial customers for Niagara Falls power were converted to Hz by the late s, although some Hz industrial customers still existed as of the start of the st century.Offshore,military, textile industry, marine, computer mainframe, aircraft, and spacecraft applications sometimes use Hz, for benefits of reduced weight of apparatus or higher motor speeds.
Effects at high frequencies
A direct, constant current flows uniformly throughout the crosssection of the uniform wire that carries it. With alternating current of any frequency, the current is forced towards the outer surface of the wire, and away from the center. This is because an electric charge which accelerates as is the case of an alternating current radiates electromagnetic waves, and materials of high conductivity the metal which makes up the wire do not allow propagation of electromagnetic waves. This phenomenon is called skin effect.At very high frequencies the current no longer flows in the wire, but effectively flows on the surface of the wire, within a thickness of a few skin depths. The skin depth is the thickness at which the current density is reduced by %. Even at relatively low frequencies used for high power transmission – Hz, nonuniform distribution of current still occurs in sufficiently thick conductors. For example, the skin depth of a copper conductor is approximately . mm at Hz, so high current conductors are usually hollow to reduce their mass and cost.Since the current tends to flow in the periphery of conductors, the effective crosssection of the conductor is reduced. This increases the effective AC resistance of the conductor, since resistance is inversely proportional to the crosssectional area in which the current actually flows. The AC resistance often is many times higher than the DC resistance, causing a much higher energy loss due to ohmic heating also called IR loss.edit Techniques for reducing AC resistanceFor low to medium frequencies, conductors can be divided into stranded wires, each insulated from one other, and the individual strands specially arranged to change their relative position within the conductor bundle. Wire constructed using this technique is called Litz wire. This measure helps to partially mitigate skin effect by forcing more equal current flow throughout the total cross section of the stranded conductors. Litz wire is used for making high Q inductors, reducing losses in flexible conductors carrying very high currents at power frequencies, and in the windings of devices carrying higher radio frequency current up to hundreds of kilohertz, such as switchmode power supplies and radio frequency transformers.edit Techniques for reducing radiation lossAs written above, an alternating current is made of electric charge under periodic acceleration, which causes radiation of electromagnetic waves. Energy that is radiated represents a loss. Depending on the frequency, different techniques are used to minimize the loss due to radiation.edit Twisted pairsAt frequencies up to about GHz, wires are paired together in cabling to form a twisted pair in order to reduce losses due to electromagnetic radiation and inductive coupling. A twisted pair must be used with a balanced signalling system, where the two wires carry equal but opposite currents. The result is that each wire in the twisted pair radiates a signal that is effectively cancelled by the other wire, resulting in almost no electromagnetic radiation.
Coaxial cablesAt frequencies above GHz, unshielded wires of practical dimensions lose too much energy to radiation, so coaxial cables are used instead. A coaxial cable has a conductive wire inside a conductive tube. The current flowing on the inner conductor is equal and opposite to the current flowing on the inner surface of the outer tube. This causes the electromagnetic field to be completely contained within the tube, and ideally no energy is radiated or coupled outside the tube. Coaxial cables have acceptably small losses for frequencies up to about GHz. For microwave frequencies greater than GHz, the dielectric losses due mainly to the dissipation factor of the dielectric layer which separates the inner wire from the outer tube become too large, making waveguides a more efficient medium for transmitting energy.dit WaveguidesWaveguides are similar to coax cables, as both consist of tubes, with the biggest difference being that the waveguide has no inner conductor. Waveguides can have any arbitrary cross section, but rectangular cross sections are the most common. Because waveguides do not have an inner conductor to carry a return current, waveguides cannot deliver energy by means of an electric current, but rather by means of a guided electromagnetic field. Although surface currents do flow on the inner walls of the waveguides, those surface currents do not carry power. Power is carried by the guided electromagnetic fields. The surface currents are set up by the guided electromagnetic fields and have the effect of keeping the fields inside the waveguide and preventing leakage of the fields to the space outside the waveguide.Waveguides have dimensions comparable to the wavelength of the alternating current to be transmitted, so they are only feasible at microwave frequencies. In addition to this mechanical feasability, electrical resistance of the nonideal metals forming the walls of the waveguide causes dissipation of power surface currents flowing on lossy conductors dissipate power. At higher frequencies, the power lost to this dissipation becomes unacceptably large.edit Fiber opticsAt frequencies greater than GHz, waveguide dimensions become impractically small, and the ohmic losses in the waveguide walls become large. Instead, fiber optics, which are a form of dielectric waveguides, can be used. For such frequencies, the concepts of voltages and currents are no longer used.edit Mathematics of AC voltagesA sine wave, over one cycle °. The dashed line represents the root mean square RMS value at about .A sine wave, over one cycle °. The dashed line represents the root mean square RMS value at about .Alternating currents are accompanied or caused by alternating voltages. An AC voltage v can be described mathematically as a function of time by the following equation vtV_mathrmpeakcdotsinomega t, displaystyle V_rm peak is the peak voltage unit volt, displaystyle omega is the angular frequency unit radians per secondThe angular frequency is related to the physical frequency, displaystyle f, which represents the number of oscillations per second unit hertz, by the equation omega ,pi, f. displaystyle t is the time unit second.The peaktopeak value of an AC voltage is defined as the difference between its positive peak and its negative peak. Since the maximum value of displaystyle sinx is + and the minimum value is -, an AC voltage swings between displaystyle+V_rm peak and displaystyleV_rm peak. The peaktopeak voltage, usually written as displaystyle V_rm pp or displaystyle V_rm PP, is therefore V_rm peak leftV_rm peakright times V_rm peak.
Power and root mean square
The relationship between voltage and power is displaystyle Pt fracV^tR where displaystyle R represents a load resistance.ather than using instantaneous power, displaystyle Plefttright, it is more practical to use a time averaged power where the averaging is performed over any integer number of cycles. Therefore, AC voltage is often expressed as a root mean square RMS value, written as displaystyle V_rm rms, because displaystyle P_rm time~averaged fracV^_rm rmsRFor a sinusoidal voltage V_mathrmrmsfracV_mathrmpeaksqrthe factor sqrt is called the crest factor, which varies for different waveforms. For a triangle wave form V_mathrmrmsfracV_mathrmpeaksqrt For a square wave form displaystyle V_mathrmrmsV_mathrmpeakedit ExampleTo illustrate these concepts, consider a V AC mains supply. It is so called because its root mean square value is V. This means that the timeaveraged power delivered is equivalent to the power delivered by a DC voltage of volts. To determine the peak voltage amplitude, we can modify the above equation to
V_mathrmpeaksqrt V_mathrmrmsFor our V AC, the peak voltage Vpeak is therefore displaystyle V timessqrt, which is about V. The peaktopeak value displaystyle V_PP of the V AC is double that, at about V.edit Further reading Willam A. Meyers, History and Reflections on the Way Things Were Mill Creek Power Plant Making History with AC, IEEE Power Engineering Review, February , Pagesdit See alsoAncient developments Further information Baghdad batteryThales of Miletus, writing at around BC, described a form of static electricity, noting that rubbing fur on various substances, such as amber, would cause a particular attraction between the two. He noted that the amber buttons could attract light objects such as hair and that if they rubbed the amber for long enough they could even get a spark to jump.An object found in Iraq in , dated to about BC and called the Baghdad Battery, resembles a galvanic cell and is believed by some to have been used for electroplating in Mesopotamia, although this has not yet been proven.edit Early developmentsElectricity has been a subject of scientific interest since at least the th century. A friction machine was constructed at about by Otto von Guericke, using a rotating sulphur globe rubbed by hand. Isaac Newton suggested the use of a glass globe instead of a sulphur one Optics, th Query. In the latter part of the th Century, Benjamin Franklin, Ewald Jürgen Georg von Kleist, and Pieter van Musschenbroek the last two the inventors of the Leyden jar made several important discoveries concerning electrostatic machines. The first suggestion of an influence machine appears to have grown out of the invention of Alessandro Voltas electrophorus. Doublers were the first rotating influence machines. Abraham Bennet, the inventor of the gold leaf electroscope, described a doubler or machine for multiplying electric charges Phil. Trans., .edit th century developmentsMichael Faraday, detail from portrait by Thomas Phillips cMichael Faraday, detail from portrait by Thomas Phillips c In the th century, the subject of electrical engineering, with the tools of modern research techniques, started to intensify. Notable developments in this century include the work of Georg Ohm, who in quantified the relationship between the electric current and potential difference in a conductor, Michael Faraday, the discoverer of electromagnetic induction in , and James Clerk Maxwell, who in published a unified theory of electricity and magnetism in his treatise on Electricity and Magnetism. In the s, Georg Ohm also constructed an early electrostatic machine. The homopolar generator was developed first by Michael Faraday during his memorable experiments in . It was the beginning of modern dynamos — that is, electrical generators which operate using a magnetic field. The invention of the industrial generator, which didnt need external magnetic power in by Werner von Siemens made a large series of other inventions in the wake possible. In , the British inventor James Wimshurst developed an apparatus that had two glass disks mounted on two shafts ed. it was not till that the Wimshurst machine was more fully reported to the scientific community.
Thomas Edison built the worlds first largescale electrical supply networkThomas Edison built the worlds first largescale electrical supply networkuring the latter part of the s, the study of electricity was largely considered to be a subfield of physics. It was not until the late th century that universities started to offer degrees in electrical engineering. In , Darmstadt University of Technology founded the first chair and the first faculty of electrical engineering worldwide. In , Darmstadt University of Technology and Cornell University introduced the worlds first courses of study in electrical engineering and in the University College London founded the first chair of electrical engineering in the United Kingdom. The University of Missouri subsequently established the first department of electrical engineering in the United States in . During this period work in the area increased dramatically. In Edison switched on the worlds first largescale electrical supply network that provided volts direct current to fiftynine customers in lower Manhattan. In Nikola Tesla filed a number of patents related to a competing form of power distribution known as alternating current. In the following years a bitter rivalry between Tesla and Edison, known as the War of Currents, took place over the preferred method of distribution. AC eventually replaced DC for generation and power distribution, enormously extending the range and improving the safety and efficiency of power distribution.Nikola Tesla made longdistance electrical transmission networks.Nikola Tesla made longdistance electrical transmission networks.The efforts of the two did much to further electrical engineering—Teslas work on induction motors and polyphase systems influenced the field for years to come, while Edisons work on telegraphy and his development of the stock ticker proved lucrative for his company, which ultimately became General Electric.However, by the end of the th century, other key figures in the progress of electrical engineering were beginning to emerge. Charles Proteus Steinmetz help fostered the development of alternating current that made possible the expansion of the electric power industry in the United States, formulating mathematical theories for engineers.Konrad Zuse invented the first electrical computer, the Z. It still is functional and stands in Berlin.During the development of radio, many scientists and inventors contributed to radio technology and electronics. In his classic UHF experiments of , Heinrich Hertz transmitted via a sparkgap transmitter and detected radio waves using electrical equipment. In , Nikola Tesla was able to detect signals from the transmissions of his New York lab at West Point a distance of . km . In , Alexander Popov made wireless transmissions across m and Guglielmo Marconi, around the same time, made a transmission across . km. John Fleming invented the first radio tube, the diode, in .Reginald Fessenden recognized that a continuous wave needed to be generated to make speech transmission possible, and he continued the work of Nikola Tesla, John Stone Stone, and Elihu Thomson on this subject. By the end of , Fessenden sent the first radio broadcast of voice. Also in , Robert von Lieben and Lee De Forest independently developed the amplifier tube, called the triode. Edwin Howard Armstrong enabling technology for electronic television, in .edit Second World War yearsThe second world war saw tremendous advances in the field of electronics especially in RADAR and with the invention of the magnetron by Randle and Boot at the University of Birmingham in . Radio location, radio communication and radio guidance of aircraft were all developed in Britain at this time. An early electronic computing device, Colossus was built by Tommy Flowers of the GPO to decipher the coded messages of the German Lorenz cipher machine. Also developed at this time were advanced clandestine radio transmitters and receivers for use by secret agents. An American invention at the time was a device to scramble the telephone calls between Churchill and Roosevelt. This was called the Green Hornet system and worked by inserting noise into the signal. The noise was then extracted at the receiving end. This system was never broken by the Germans. A great amount of work was undertaken in the United States as part of the War Training Program in the areas of radio direction finding, pulsed linear networks, frequency modulation, vacuum tube circuits, transmission line theory and fundamentals of electromagnetic engineering. These studies were published shortly after the war in what became known as the Radio Communication Series published by McGraw hill . In Konrad Zuse presented the Z, the worlds first fully functional and programmable computer.
Post war developments
Prior to the second world war, the subject was commonly known as radio engineering and basically was restricted to aspects of communications and RADAR, commercial radio and early television. At this time, study of radio engineering at universities could only be undertaken as part of a physics degree.Later, in post war years, as consumer devices began to be developed, the field broadened to include modern TV, audio systems, HiFi and latterly computers and microprocessors. In the ENIAC Electronic Numerical Integrator and Computer of John Presper Eckert and John Mauchly followed, beginning the computing era. The arithmetic performance of these machines allowed engineers to develop completely new technologies and achieve new objectives, including the Apollo missions and the NASA moon landing. The ENIAC Museum Online. Retrieved on January , .The invention of the transistor in by William B. Shockley, John Bardeen and Walter Brattain opened the door for more compact devices and led to the development of the integrated circuit in by Jack Kilby and independently in by Robert Noyce. In the mid to late s, the term radio engineering gradually gave way to the name electronics engineering, which then became a stand alone university degree subject, usually taught alongside electrical engineering with which it had become associated due to some similarities. In Marcian Hoff invented the first microprocessor at Intel and thus ignited the development of the personal computer. The first realization of the microprocessor was the Intel , a bit processor developed in , but only in did the Intel , an bit processor, make the building of the first personal computer, the Altair , possible.Shock effectsedit PsychologicalThe perception of electric shock can be different depending on the voltage, duration, current, path taken, frequency, etc. Current entering the hand has a threshold of perception of about to mA milliampere for DC and about to mA for AC at Hz. Shock perception declines with increasing frequency, ultimately disappearing at frequencies above kHz.edit BurnsHeating due to resistance can cause extensive and deep burns. Voltage levels of > to V shocks tend to cause internal burns due to the large energy which is proportional to the duration multiplied by the square of the voltage available from the source. Damage due to current is through tissue heating. In some cases volts might be fatal to a human being when the electricity passes through organs such as the heart.dit Ventricular fibrillationA lowvoltage to V, or Hz AC current through the chest for a fraction of a second may induce ventricular fibrillation at currents as low as mA. With DC, to mA is required. If the current has a direct pathway to the heart e.g., via a cardiac catheter or other kind of electrode, a much lower current of less than mA, AC or DC can cause fibrillation. Fibrillations are usually lethal because all the heart muscle cells move independently. Above mA, muscle contractions are so strong that the heart muscles cannot move at all.edit Neurological effectsCurrent can cause interference with nervous control, especially over the heart and lungs. Repeated or severe electric shock which does not lead to death has been shown to cause neuropathy.When the current path is through the head, it appears that, with sufficient current, loss of consciousness almost always occurs swiftly. This is borne out by some limited selfexperimentation by early designers of the electric chair and by research from the field of animal husbandry, where electric stunning has been extensively studied .
Arcflash hazardsThis section needs additional citations for verification.Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. January Approximately % of all injuries and fatalities caused by electrical incidents are not caused by electric shock, but by the intense heat, light, and pressure wave blast caused by electrical faults. The arc flash in an electrical fault produces the same type of light radiation from which electric welders protect themselves using face shields with dark glass, heavy leather gloves, and fullcoverage clothing. The heat produced may cause severe burns, especially on unprotected flesh. The blast produced by vaporizing metallic components can break bones and irreparably damage internal organs. The degree of hazard present at a particular location can be determined by a detailed analysis of the electrical system, and appropriate protection worn if the electrical work must be performed with the electricity on.edit Issues affecting lethalityOther issues affecting lethality are frequency, which is an issue in causing cardiac arrest or muscular spasms, and pathway—if the current passes through the chest or head there is an increased chance of death. From a main circuit or power distribution panel the damage is more likely to be internal, leading to cardiac arrest.The comparison between the dangers of alternating current and direct current has been a subject of debate ever since the War of Currents in the s. DC tends to cause continuous muscular contractions that make the victim hold on to a live conductor, thereby increasing the risk of deep tissue burns. On the other hand, mainsfrequency AC tends to interfere more with the hearts electrical pacemaker, leading to an increased risk of fibrillation. AC at higher frequencies holds a different mixture of hazards, such as RF burns and the possibility of tissue damage with no immediate sensation of pain. Generally, higher frequency AC current tends to run along the skin rather than penetrating and touching vital organs such as the heart. While there will be severe burn damage at higher voltages, it is normally not fatal.It is sometimes suggested that human lethality is most common with alternating current at – volts, however death has occurred outside this range, with supplies as low as volts and supplies at over volts frequently causing fatalities.Electrical discharge from lightning tends to travel over the surface of the body causing burns and may cause respiratory arrest.edit Lethality of a shockAvoiding danger of shockThis article or section contains instructions, advice, or howto content.The purpose of Wikipedia is to present facts, not to teach subject matter. Please help improve this article by removing or rewriting the howto content, which may qualify for a move to Wikiversity, httpwww.wikihow.com, or httphowto.wikia.com. August It is strongly recommended that people should not work on exposed live conductors if at all possible. If this is not possible then insulated gloves and tools should be used. If both hands make contact with surfaces or objects at different voltages, current can exist through the body from one hand to the other. This can lead the current through the heart. Similarly, if the current is from one hand to the feet, significant current will probably exist through the heart. An alternative to using insulated tools is to isolate the operator from ground, so that there is no conductive path from the live conductor, through the operators body, to ground. This method is used for working on live highvoltage overhead power lines. It is possible to have a voltage potential between neutral wires and the ground in the event of an improperly wired disconnected neutral, or if it is part of certain obsolete and now illegalcitation needed switch circuits. The electrical appliance or lighting equipment might provide some voltage drop, but not nearly enough to avoid a shock. Live neutral wires should be treated with the same respect as live wires. Also, the neutral wire must be insulated to the same degree as the live wire to avoid a short circuit.
It should be mentioned
It should be mentioned that much care needs to be taken with electrical systems on ships and boats, especially steel or aluminum ones. Anyone standing on a metal deck or leaning against a bulkhead is automatically grounded, so great care must be taken that all live electrical wires are well insulated. As an example of the danger, during WW II, the battleship USS Washington had not one casualty due to enemy action. However, there were some sailors killed by electrocution while doing such things as using electric drills that had defects in them. For the details, see the official histories of this distinguished USN warship.Electrical codes in many parts of the world call for installing a residualcurrent device RCD or GFI, ground fault interrupter in electrical circuits thought to pose a particular hazard to reduce the risk of electrocution. In the USA, for example, a new or remodeled residential dwelling must have them installed in all kitchens, bathrooms, laundry rooms, garages, and also any other room with an unfinished concrete floor such as a workshop. These devices work by detecting an imbalance between the live and neutral wires. In other words, if more current exists through the live wire than is returning though its neutral wire presumably via ground, it assumes something is wrong and breaks the circuit in a tiny fraction of a second. There is some concern that these devices might not be fast enough to protect infants and small children in rare instances.Concrete contains a residual amount of water that makes it somewhat electrically conductive. Also, concrete in contact with any source of water or moisture will absorb some, and the water in concrete always contains dissolved minerals that makes the water significantly conductive.The plumbing system in a home or other small building has historically used metal pipes and thus been connected to ground through the pipes. This is no longer always true because of the extensive use of plastic piping in recent years, but a plastic system cannot be relied upon for safety purposes. Contrary to popular belief, pure water is not a good conductor of electricity. However, most water is not pure and contains enough dissolved particles salts to greatly enhance its conductivity. When the human skin becomes wet, it allows much more current than the dry human body would. Thus, being in the bath or shower will not only ground oneself to return path of the power mains, but lower the bodys resistance as well. Under these circumstances, touching any metal switch or appliance that is connected to the power mains could result in severe electric shock or electrocution. While such an appliance is not supposed to be live on its outer metal switch or frame, it may have become so if a defective live bare wire is accidentally touching it either directly or indirectly via internal metal parts. It is for this reason that mains electrical sockets are prohibited in bathrooms in the United Kingdom. However, the widespread use of plastic cases for everyday appliances, grounding of these appliances, and mandatory installation of Residual Current Devices R.C.D.s have greatly reduced this type of electrocution over the recent past decades. Connecting electrical neutrals to plumbing is against the electrical codes, at least in the United States of America. This is for several reasons. One of these is that connecting any electrical lines to plumbing presents a danger to plumbers or anyone else working on or around plumbing. Also, with metallic plumbing, even small amounts of electric current through them over a significant length of time can cause corrosion to the pipes, the removal of their zinc linings if they have any, and the breakdown of the solder in their joints. The ground wire grounding conductor of the system is allowed to be connected to plumbing. However as previously stated, the neutral grounded conductor is not allowed to be connected. NEC . Grounding Electrodes AElectrodes Permitted for Grounding Metal Underground Water Pipe. This requires a metal underground water pipe in direct contact with the earth for .m ft or more and electrically continuous to the points of connection of the grounding electrode conductor and the bonding conductor.
A properlygrounded appliance greatly reduces the electric shock potential by causing a short circuit if any portion of the metal frame chassis is accidentally touching the live wire. This will cause the circuit breaker to turn off or the fuse to blow resulting in a power outage in that area of the home or building. Often there will be a large bang and possibly smoke which could easily scare anyone nearby. However, this is still much safer than risking electric shock, since the chance of an outofcontrol fire is remote.Where live circuits must be frequently worked on e.g. television repair, an isolation transformer is sometimes used. Unlike ordinary transformers which raise or lower voltage, the coil windings of an isolation transformer are at a ratio which keeps the voltage unchanged. The purpose is to isolate the neutral wire so that it has no connection to ground. Thus, if a technician accidentally touches the live chassis and ground at the same time, nothing would happen.Neither ground fault interrupters RCDGFI nor isolation transformers can prevent electrocution between the live and neutral wires. This is the same path used by functional electrical appliances, so protection is not possible. However, most accidental electrocutions, especially those not involving electrical work and repair, are via ground not the neutral wire.edit Electrocution statisticsThere were electrocutions in the US in , which translates to . deaths per million inhabitants. At that time, the incidence of electrocutions was decreasing. Electrocutions in the workplace make up the majority of these fatalities. From –, an average of workers were killed each year by electrocution. edit Deliberate usesedit Electroconvulsive Therapy
Electric shock is also used as a medical therapy, under carefully controlled conditions Electroconvulsive therapy or ECT is a psychiatric therapy for mental illness. The objective of the therapy is to induce a seizure for theraputic effect. There is no sensation of shock because the patient is anesthetized. The therapy was originally conceived of after it was observed that depressed patients who also suffered from epilepsy experienced some remission after a spontaneous seizure.citation needed The first attempts at deliberately inducing seizure as therapy used not electricity but chemicals however electricity provided finer control for delivering the minimum stimulus needed. Ideally some other method of inducing seizure would be used, as the electricity may be associated with some of the negative side effects of ECT including amnesia. ECT is generally administered three times a week for about treatments. As a treatment for fibrillation or irregular heart rhythms see defibrillator and cardioversion. As a method of pain relief see Transcutaneous Electrical Nerve Stimulator more commonly referred to as a TENS unit. As an aversive punishment for conditioning of mentally handicapped patients with severe behavioral issues. This method is highly controversial and is employed at only one institution in the United States, the Judge Rotenberg Educational Center. The institute also uses electric shock punishments on nonhandicapped children with behavioral problems. Whether this constitutes legitimate medical treatment versus abusive discipline is the subject of ongoing litigation.
Torture
This section needs additional citations for verification.Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. January Main article tortureElectric shocks have been used as a method of torture, since the received voltage and amperage can be controlled with precision and used to cause pain while avoiding obvious evidence on the victims body. Such torture usually uses electrodes attached to parts of the victims body. Another method of electrical torture is stunning with an electroshock gun such as a cattle prod or a taser provided a sufficiently high voltage and nonlethal current is used in the former case.The Nazis are known to have used electrical torture during World War II. An extensive fictional depiction of such torture is included in the book The Secret of Santa Vittoria by Robert Crichton. During the Vietnam War, electric shock torture is said to have been used by both sides.citation needed A scene of electrical torture in the American Deep South is included in the Robert Redford film Brubaker. Amnesty International published an official statement that Russian military forces in Chechnya tortured local women with electric shocks by connecting electric wires to their bra straps. Examples in popular modern culture are the electric torture of Martin Riggs in Lethal Weapon and John Rambo in Rambo First Blood Part II. Japanese serial killer Futoshi Matsunaga used electric shocks for controlling his victims.Advocates for the mentally ill and some psychiatrists such as Thomas Szasz have asserted that electroconvulsive therapy is torture when used without a bona fide medical benefit against recalcitrant or nonresponsive patients. See above for ECT as medical therapy. These same arguments and oppositions apply to the use of extremely painful shocks as punishment for behavior modification, a practice that is openly used only at the Judge Rotenberg Institutecitation needed.Low to moderately highvoltage electric shocks do not result in the type of pain felt at death or organ failure, nor have been proven to result in significant psychological harm of significant duration, e.g., lasting for months or even years,.edit Capital punishment Main article Electric chairThis article or section has been nominated to be checked for its neutrality.Discussion of this nomination can be found on the talk page. April Electric shock delivered by an electric chair is sometimes used as an official means of capital punishment in the United States, although its use has become rare in recent times. Although the electric chair was at one timeopinion needs balancing considered a more humane and modern execution method than hanging, shooting, poison gassing, the guillotine, etc., it has now been replaced in countries which practice capital punishment by lethal injections. Modern reporting has claimed that it sometimes takes several shocks to be lethal, and that the condemned person may actually catch fire before the process is complete. The brain is always severely damaged and inactivated.Other than in parts of the United States, only the Philippines reportedly has used this method, and only for a few years. It remains a legal means of execution in a few states of the USA..History Main article: History of electromagnetismThe natural phenomenon of static electricity was known at least as early as the 6th century BC, as attested by Thales of Miletus. Scientific research into the subject began when machines were built to create it artificially, such as the friction generator developed by Otto von Guericke in the 17th century. The connection between static electricity and storm clouds was famously demonstrated by Benjamin Franklin in 1750 1 2. In 1832, Michael Faraday published the results of his experiment on the identity of electricities, which proved that the electricity induced using a magnet, voltaic electricity produced by a battery, and static electricity were all the same. Since Faradays result, the history of static electricity merged with the study of electricity in general.
Causes of static electricityhe materials we observe and interact with from day-to-day are formed from atoms and molecules that are electrically neutral, having an equal number of positive charges protons, in the nucleus and negative charges electrons, in shells surrounding the nucleus. The phenomenon of static electricity requires a sustained separation of positive and negative charges.edit Contact induced charge separation Main article: Triboelectric effectElectrons can be exchanged between materials on contact; materials with weakly bound electrons tend to lose them, while materials with sparsely filled outer shells tend to gain them. This is known as the triboelectric effect and results in one material becoming positively charged and the other negatively charged. The polarity and strength of the charge on a material once they are separated depends on their relative positions in the triboelectric series. The tribo electric effect is the main cause of static electricity as observed in everyday life, and in common high-school science demonstrations involving rubbing different materials together e.g. fur and an acrylic rod.edit Pressure induced charge separationMain article: Piezoelectric effectertain types of crystals and ceramics generate a separation of charge in response to applied mechanical stress.edit Heat induced charge separation Main article: Pyroelectric effectCertain materials generate a separation of charge in response to heating. All pyroelectric materials are also piezoelectric, the two properties being closely related.edit Charge induced charge separation Main article: Electrostatic inductionA charged object brought into the vicinity of an electrically neutral object will cause a separation of charge within the conductor as charges of the same polarity are repelled and charges of the opposite polarity are attracted. As the force due to the interaction of electric charges falls off rapidly with increasing distance, the effect of the closer opposite polarity charges is greater and the two objects feel a force of attraction. The effect is most pronounced when the neutral object is an electrical conductor as the charges are more free to move around.Careful grounding of part of an object with a charge induced charge separation can permanently add or remove electrons, leaving the object with a global, permanent charge. This process is integral to the workings of the Van de Graaf Generator, a device commonly used to demonstrate the effects of static electricity.edit Static discharge Main article: Electrostatic dischargeThe spark associated with static electricity is caused by electrostatic discharge, or simply static discharge, as excess charge is neutralized by a flow of charges from or to the surroundings. In general, significant charge accumulations can only persist in regions of low electrical conductivity very few charges free to move in the surroundings, hence the flow of neutralizing charges often results from neutral atoms and molecules in the air being torn apart to form separate positive and negative charges which then travel in opposite directions as an electric current, neutralizing the original accumulation of charge. Air typically breaks down in this way at around 30,000 volts-per-centimetre depending on humidity.3 The discharge superheats the surrounding air causing the bright flash, and produces a shockwave causing the clicking sound.The feeling of a static electric shock is caused by the stimulation of nerves as the neutralizing current flows through the human body. Due to the ubiquitous presence of water in places inhabited by people, the accumulated charge is generally not enough to cause a dangerously high current.edit LightningNatural static dischargeNatural static discharge Main article: LightningLightning is a dramatic natural example of static discharge. While the details are unclear and remain the subject of debate, the initial charge separation is thought to be associated with contact between ice particles within storm clouds. Whatever the cause may be, the resulting lightning bolt is simply a scaled up version of the sparks seen in more domestic occurrences of static discharge. The flash occurs because the air in the discharge channel is heated to such a high temperature that it emits light by incandescence. The clap of thunder is the result of the shockwave created as the superheated air rapidly expands.edit Simple experiments Note: a humid atmosphere provides a conducting path for the rapid neutralization of static charge; hence the following examples work best in dry, winter conditions.Static electricity is notable as a physical phenomenon that can be demonstrated using simple experiments that can convey genuine understanding of the physics involved.4edit Charged adhesive tapeRepulsion between lengths of tape with like chargesRepulsion between lengths of tape with like chargesAttraction between lengths of tape with opposite chargesAttraction between lengths of tape with opposite chargesA simple and illuminating example of the effects of static electricity can be observed using adhesive tape such as Scotch tape, on the negative side of the triboelectric series, hence tends to gain electrons and acquire negative charge charged by peeling.5If a length of tape adhered to a smooth surface is rapidly peeled off, the tape will acquire an excess negative charge generally polypropylene with an acrylic adhesive6. Do this with two lengths of tape and they will repel each other, demonstrating the fact that like charges repel. Each individual length of tape will experience a small attraction to almost any object as the presence of the excess negative charge induces a charge separation in nearby objects. Negative charges are pushed further away, while positive charges are attracted, and the strength of the attractive and repulsive forces falls off quite rapidly with distance. This effect is most pronounced in materials such as metals, that conduct electricity, as the negative charges are free to move within the material.Finally, try attaching two lengths of tape together, exhaling on them along the entire length to neutralize the charge, then rapidly pulling them apart. There will be some imbalance in the distribution of negative charge between the two pieces such that one is more positive and the other more negative; you should now find that the two lengths of tape attract each other, demonstrating the fact that opposite charges attract. Attaching the adhesive side of one length of tape to the non-adhesive side of the other reduces the chance of tearing and increases the charge imbalance, and hence the strength of the attractive force.
