Definition of TransformerA transformer is a static machine used for transforming power from one circuit to another without changing frequency. This is a very basicdefinition of transformer.History of TransformerThehistory of transformerwas commenced in the year 1880. In the year 1950, 400KVelectrical power transformerwas introduced in highvoltageelectrical powersystem. In the early 1970s, unit rating as large as 1100MVA was produced and 800KV and even higher KV class transformers were manufactured in year of 1980.Use of Power TransformerGeneration of electrical powerin lowvoltagelevel is very much cost effective. Henceelectrical poweris generated in lowvoltagelevel. Theoretically, this lowvoltagelevel power can be transmitted to the receiving end. But if thevoltagelevel of a power is increased, theelectric currentof the power is reduced which causes reduction in ohmic or I2R losses in the system, reduction in cross sectional area of the conductor i.e. reduction in capital cost of the system and it also improves thevoltage regulationof the system. Because of these, low level power must be stepped up for efficientelectrical power transmission. This is done by step up transformer at the sending side of the power system network. As this highvoltagepower may not be distributed to the consumers directly, this must be stepped down to the desired level at the receiving end with the help of step down transformer. These are the uses ofelectrical power transformerin theelectrical powersystem.

Types of TransformerTransformers can be categorized in different ways, depending upon their purpose, use, construction etc. Thetypes of transformerare as follows,1. Step Up Transformer & Step Down Transformer- Generally used for stepping up and down thevoltagelevel of power in transmission and distribution power network.2. Three Phase Transformer & Single Phase Transformer- Former is generally used in three phase power system as it is cost effective than later but when size matters, it is preferable to use bank of three single phase transformer as it is easier to transport three single phase unit separately than one single three phase unit.

3. Electrical Power Transformer, Distribution Transformer & Instrument Transformer- Transformer is generally used in transmission network which is normally known aspower transformer, distribution transformer is used in distribution network and this is lower rating transformer andcurrent transformer&potential transformer, we use for relay and protection purpose inelectrical powersystem and in different instruments in industries are called instrument transformer.4. Two Winding Transformer &Auto Transformer- Former is generally used where ratio between highvoltageand lowvoltageis greater than 2. It is cost effective to use later where the ratio between highvoltageand lowvoltageis less than 2.5. Outdoor Transformer & Indoor Transformer- Transformers that are designed for installing at outdoor are outdoor transformers and transformers designed for installing at indoor are indoor transformers

What is Circuit Breaker?Definition of circuit breaker: -Electrical circuit breakeris a switching device which can be operated manually as well as automatically for controlling and protection ofelectrical powersystem respectively. As the modern power system deals with huge currents, the spacial attention should be given during designing ofcircuit breakerto safe interruption of arc produced during theoperation of circuit breaker. This was the basicdefinition of circuit breaker.Introduction to Circuit BreakerThe modern power system deals with huge power network and huge numbers of associated electrical equipment. During short circuit fault or any other types of electrical fault these equipment as well as the power network suffer a high stress of faultelectric currentin them which may damage the equipment and networks permanently. For saving these equipment and the power networks the faultelectric currentshould be cleared from the system as quickly as possible. Again after the fault is cleared, the system must come to its normal working condition as soon as possible for supplying reliable quality power to the receiving ends. In addition to that for proper controlling of power system, different switching operations are required to be performed. So for timely disconnecting and reconnecting different parts of power system network for protection and control, there must be some special type of switching devices which can be operated safely under hugeelectric currentcarrying condition. During interruption of huge current, there would be large arcing in between switching contacts, so care should be taken to quench thesearcs in circuit breakerin safe manner. Thecircuit breakeris the special device which does all the required switching operations duringelectric currentcarrying condition. This was the basicintroduction to circuit breaker.

What is Circuit Breaker?Definition of circuit breaker: -Electrical circuit breakeris a switching device which can be operated manually as well as automatically for controlling and protection ofelectrical powersystem respectively. As the modern power system deals with huge currents, the spacial attention should be given during designing ofcircuit breakerto safe interruption of arc produced during theoperation of circuit breaker. This was the basicdefinition of circuit breaker.Introduction to Circuit BreakerThe modern power system deals with huge power network and huge numbers of associated electrical equipment. During short circuit fault or any other types of electrical fault these equipment as well as the power network suffer a high stress of faultelectric currentin them which may damage the equipment and networks permanently. For saving these equipment and the power networks the faultelectric currentshould be cleared from the system as quickly as possible. Again after the fault is cleared, the system must come to its normal working condition as soon as possible for supplying reliable quality power to the receiving ends. In addition to that for proper controlling of power system, different switching operations are required to be performed. So for timely disconnecting and reconnecting different parts of power system network for protection and control, there must be some special type of switching devices which can be operated safely under hugeelectric currentcarrying condition. During interruption of huge current, there would be large arcing in between switching contacts, so care should be taken to quench thesearcs in circuit breakerin safe manner. Thecircuit breakeris the special device which does all the required switching operations duringelectric currentcarrying condition. This was the basicintroduction to circuit breaker.

Earth Leakage Circuit Breaker (ELCB)AnEarth LeakageCircuit Breaker (ELCB) is a device used to directly detect currents leaking to earth from an installation and cut the power and mainly used in TT earthing systems.There are two types of ELCBs:1. Voltage Earth LeakageCircuit Breaker(voltage-ELCB)2. Current Earth Leakage Current Earth Leakage Circuit Breaker (Current-ELCB).Voltage-ELCBs were first introduced about sixty years ago and Current-ELCB was first introduced about forty years ago. For many years, the voltage operated ELCB and the differential current operated ELCB were both referred to as ELCBs because it was a simpler name to remember. But the use of a common name for two different devices gave rise to considerable confusion in the electrical industry.If the wrong type was used on an installation, the level of protection given could be substantially less than that intended.To ignore this confusion, IEC decided to apply the term Residual Current Device (RCD) to differential current operated ELCBs. Residual current refers to any current over and above the load current.TopVoltage Base ELCB Voltage-ELCB is a voltage operated circuit breaker. The device will function when the Current passes through the ELCB. Voltage-ELCB contains relay Coil which it being connected to the metallic load body at one end and it is connected to ground wire at the other end.. If the voltage of the Equipment body is rise (by touching phase to metal part or failure ofinsulation of equipment) which could cause the difference between earth and load body voltage, the danger of electric shock will occur. This voltage difference will produce an electric current from the load metallic body passes the relay loop and to earth. When voltage on the equipment metallic body rose to the danger level which exceed to 50Volt, the flowing current through relay loop could move the relay contact by disconnecting the supply current to avoid from any danger electric shock.. The ELCB detects fault currents from live to the earth (ground) wire within the installation it protects. If sufficient voltage appears across the ELCBs sense coil, it will switch off the power, and remain off until manually reset. A voltage-sensing ELCB does not sense fault currents from live to any other earthed body.

These ELCBs monitored the voltage on the earth wire, and disconnected the supply if the earth wire voltage was over 50 volts.. These devices are no longer used due to its drawbacks like if the fault is between live and a circuit earth, they will disconnect the supply. However, if the fault is between live and some other earth (such as a person or a metal water pipe), they will NOT disconnect, as the voltage on the circuit earth will not change. Even if the fault is between live and a circuit earth, parallel earth paths created via gas or water pipes can result in the ELCB being bypassed. Most of the fault current will flow via the gas or water pipes, since a single earth stake will inevitably have a much higher impedance than hundreds of meters of metal service pipes buried in the ground. The way to identify an ELCB is by looking for green or green and yellow earth wires entering the device.They rely on voltage returning to the trip via the earth wire during a fault and afford only limited protection to the installation and no personal protection at all. You should use plug in 30mA RCDs for any appliances and extension leads that may be used outside as a minimum.Advantages ELCBs have one advantage over RCDs: they are less sensitive to fault conditions, and therefore have fewer nuisance trips.. While voltage and current on the earth line is usually fault current from a live wire, this is not always the case, thus there are situations in which an ELCB can nuisance trip.. When an installation has two connections to earth, a nearby high current lightning strike will cause a voltage gradient in the soil, presenting the ELCB sense coil with enough voltage to cause it to trip.. If the installations earth rod is placed close to the earth rod of a neighboring building, a high earth leakage current in the other building can raise the local ground potential and cause a voltage difference across the two earths, again tripping the ELCB.. If there is an accumulated or burden of currents caused by items with lowered insulation resistance due to older equipment, or with heating elements, or rain conditions can cause the insulation resistance to lower due to moisture tracking. If there is a some mA which is equal to ELCB rating than ELCB may give nuisance Tripping.. If either of the earth wires become disconnected from the ELCB, it will no longer trip or the installation will often no longer be properly earthed.. Some ELCBs do not respond to rectified fault current. This issue is common for ELCBs and RCDs, but ELCBs are on average much older than RCB so an old ELCB is more likely to have some uncommon fault current waveform that it will not respond to.. Voltage-operated ELCB are the requirement for a second connection, and the possibility that any additional connection to earth on the protected system can disable the detector.. Nuisance tripping especially during thunderstorms.Disadvantages They do not detect faults that dont pass current through the CPC to the earth rod. They do not allow a single building system to be easily split into multiple sections with independent fault protection, because earthing systems are usually use common earth Rod. They may be tripped by external voltages from something connected to the earthing system such as metal pipes, a TN-S earth or a TN-C-S combined neutral and earth. As electrically leaky appliances such as some water heaters, washing machines and cookers may cause the ELCB to trip. ELCBs introduce additional resistance and an additional point of failure into the earthing system.Can we assume whether Our Electrical System is protected against Earth Protection or not by only Pressing ELCB Test Switch? Checking the health of the ELCB is simple and you can do it easily by pressing TEST Push Button Switch of ELCB. The test push-button will test whether the ELCB unit is working properly or not. Can we assume that If ELCB is Trip after Pressing TEST Switch of ELCB than your system is protected against earth protection? Then you are wrong.. The test facility provided on the home ELCB will only confirm the health of the ELCB unit, but that test does not confirm that the ELCB will trip when an electric shock hazard does occur. It is a really sad fact that all the while this misunderstanding has left many homes totally unprotected from the risk of electric shocks.. This brings us or alarming us to think over second basic requirement for earth protection. The second requirement for the proper operation of a home shock protection system is electrical grounding.. We can assume that the ELCB is thebrain for the shock protection, and the grounding as the backbone. Therefore, without a functional grounding (Proper Earthing of Electrical System) there is totally no protection against electrical shocks in your house even if You have installed ELCB and its TEST switch show proper result. Looking after the ELCB alone is not enough. The electrical Earthing system must also be in good working order for the shock protection system to work. In addition to routine inspections that should be done by the qualified electrician, this grounding should preferably be inspected regularly at shorter intervals by the homeowner and need to pour Water in Earthing Pit at Regular interval of Time to minimize Earth Resistance.

DiodeSemiconductor diodes Electronic symbolsMain article:Electronic symbolThe symbol used for a semiconductor diode in acircuit diagramspecifies the type of diode. There are alternative symbols for some types of diodes, though the differences are minor. Diode Light Emitting Diode(LED) Photodiode Schottky diode Transient Voltage Suppression(TVS) Tunnel diode Varicap Zener diode Typical diode packages in same alignment as diode symbol. Thin bar depicts thecathode.Point-contact diodesApoint-contact diodeworks the same as the junction diodes described below, but their construction is simpler. A block of n-type semiconductor is built, and a conducting sharp-point contact made with some group-3 metal is placed in contact with the semiconductor. Some metal migrates into the semiconductor to make a small region of p-type semiconductor near the contact. The long-popular 1N34 germanium version is still used in radio receivers as a detector and occasionally in specialized analog electronics.Junction diodespn junction diodeMain article:pn diodeA pn junction diode is made of a crystal ofsemiconductor, usually silicon, butgermaniumandgallium arsenideare also used. Impurities are added to it to create a region on one side that contains negativecharge carriers(electrons), calledn-type semiconductor, and a region on the other side that contains positive charge carriers (holes), calledp-type semiconductor. When two materials i.e. n-type and p-type are attached together, a momentary flow of electrons occur from n to p side resulting in a third region where no charge carriers are present. This region is called thedepletion regiondue to the absence of charge carriers (electrons and holes in this case). The diode's terminals are attached to the n-type and p-type regions. The boundary between these two regions, called apn junction, is where the action of the diode takes place. The crystal allows electrons to flow from the N-type side (called thecathode) to the P-type side (called theanode), but not in the opposite direction.Electrical resistance and conductance"Resistive" redirects here. For the term used when referring to touchscreens, seeresistive touchscreen.Electromagnetism

Electricity Magnetism

Theelectrical resistanceof anelectrical conductoris the opposition to the passage of anelectric currentthrough that conductor. The inverse quantity iselectrical conductance, the ease with which an electric current passes. Electrical resistance shares some conceptual parallels with the mechanical notion offriction. TheSIunit of electrical resistance is theohm(), while electrical conductance is measured insiemens(S).An object of uniform cross section has a resistance proportional to itsresistivityand length and inversely proportional to its cross-sectional area. All materials show some resistance, except forsuperconductors, which have a resistance of zero.The resistance (R) of an object is defined as the ratio ofvoltageacross it (V) tocurrentthrough it (I), while the conductance (G) is the inverse:

For a wide variety of materials and conditions,VandIare directly proportional to each other, and thereforeRandGareconstant(although they can depend on other factors like temperature or strain). This proportionality is calledOhm's law, and materials that satisfy it are called "Ohmic" materials.In other cases, such as adiodeorbattery,VandIarenotdirectly proportional, or in other words theIVcurveis not a straight line through the origin, and Ohm's law does not hold. In this case, resistance and conductance are less useful concepts, and more difficult to define. The ratio V/I is sometimes still useful, and is referred to as a "chordal resistance" or "static resistance",[1][2]as it corresponds to the inverse slope of a chord between the origin and anIVcurve. In other situations, thederivativemay be most useful; this is called the "differential resistance".

1Introduction 2Conductors and resistors 3Ohm's law 4Relation to resistivity and conductivity 4.1What determines resistivity? 5Measuring resistance 6Typical resistances 7Static and differential resistance 8AC circuits 8.1Impedance and admittance 8.2Frequency dependence of resistance 9Energy dissipation and Joule heating 10Dependence of resistance on other conditions 10.1Temperature dependence 10.2Strain dependence 10.3Light illumination dependence 11Superconductivity 12See also 13References 14External linksIntroduction

Thehydraulic analogycompares electric current flowing through circuits to water flowing through pipes. When a pipe (left) is filled with hair (right), it takes a larger pressure to achieve the same flow of water. Pushing electric current through a large resistance is like pushing water through a pipe clogged with hair: It requires a larger push (electromotive force) to drive the same flow (electric current).In thehydraulic analogy, current flowing through a wire (orresistor) is like water flowing through a pipe, and thevoltage dropacross the wire is like thepressure dropthat pushes water through the pipe. Conductance is proportional to how much flow occurs for a given pressure, and resistance is proportional to how much pressure is required to achieve a given flow. (Conductance and resistance arereciprocals.)Thevoltagedrop(i.e., difference in voltage between one side of the resistor and the other), not thevoltageitself, provides the driving force pushing current through a resistor. In hydraulics, it is similar: The pressuredifferencebetween two sides of a pipe, not the pressure itself, determines the flow through it. For example, there may be a large water pressure above the pipe, which tries to push water down through the pipe. But there may be an equally large water pressure below the pipe, which tries to push water back up through the pipe. If these pressures are equal, no water flows. (In the image at right, the water pressure below the pipe is zero.)Two propertiesgeometry (shape) and materialmostly determine the resistance and conductance of a wire, resistor, or other element.Geometry is important because it is more difficult to push water through a long, narrow pipe than a wide, short pipe. In the same way, a long, thin copper wire has higher resistance (lower conductance) than a short, thick copper wire.Materials are important as well. A pipe filled with hair restricts the flow of water more than a clean pipe of the same shape and size. In a similar way,electronscan flow freely and easily through acopperwire, but cannot as easily flow through asteelwire of the same shape and size, and they essentially cannot flow at all through aninsulatorlikerubber, regardless of its shape. The difference between, copper, steel, and rubber is related to their microscopic structure andelectron configuration, and is quantified by a property calledresistivity.Conductors and resistors

A 65 resistor, as identified by itselectronic color code(bluegreenblack-gold). Anohmmetercould be used to verify this value.Substances in which electricity can flow are calledconductors. A piece of conducting material of a particular resistance meant for use in a circuit is called aresistor. Conductors are made of high-conductivitymaterials such as metals, in particular copper and aluminium. Resistors, on the other hand, are made of a wide variety of materials depending on factors such as the desired resistance, amount of energy that it needs to dissipate, precision, and costs.Ohm's law

Thecurrent-voltage characteristicsof four devices: Tworesistors, adiode, and abattery. The horizontal axis isvoltage drop, the vertical axis iscurrent. Ohm's law is satisfied when the graph is a straight line through the origin. Therefore, the two resistors are "ohmic", but the diode and battery are not.Main article:Ohm's lawOhm's law is an empirical law relating the voltageVacross an element to the currentIthrough it:

(Vis directly proportional toI). This law is not always true: For example, it is false fordiodes,batteries, etc. However, itistrue to a very good approximation for wires andresistors(assuming that other conditions, including temperature, are held fixed). Materials or objects where Ohm's law is true are calledohmic, whereas objects that do not obey Ohm's law arenon-ohmic.Relation to resistivity and conductivity

A piece of resistive material with electrical contacts on both ends.Main article:Electrical resistivity and conductivityThe resistance of a given object depends primarily on two factors: What material it is made of, and its shape. For a given material, the resistance is inversely proportional to the cross-sectional area; for example, a thick copper wire has lower resistance than an otherwise-identical thin copper wire. Also, for a given material, the resistance is proportional to the length; for example, a long copper wire has higher resistance than an otherwise-identical short copper wire. The resistanceRand conductanceGof a conductor of uniform cross section, therefore, can be computed as

whereis the length of the conductor, measured inmetres[m],Ais the cross-section area of the conductor measured insquare metres[m], (sigma) is theelectrical conductivitymeasured insiemensper meter (Sm1), and (rho) is theelectrical resistivity(also calledspecific electrical resistance) of the material, measured in ohm-metres (m). The resistivity and conductivity are proportionality constants, and therefore depend only on the material the wire is made of, not the geometry of the wire. Resistivity and conductivity arereciprocals:. Resistivity is a measure of the material's ability to oppose electric current.This formula is not exact: It assumes thecurrent densityis totally uniform in the conductor, which is not always true in practical situations. However, this formula still provides a good approximation for long thin conductors such as wires.Another situation for which this formula is not exact is withalternating current(AC), because theskin effectinhibits current flow near the center of the conductor. Then, thegeometricalcross-section is different from theeffectivecross-section in which current actually flows, so resistance is higher than expected. Similarly, if two conductors near each other carry AC current, their resistances increase due to theproximity effect. Atcommercial power frequency, these effects are significant for large conductors carrying large currents, such asbusbarsin anelectrical substation,[3]or large power cables carrying more than a few hundred amperes.What determines resistivity?Main article:Electrical resistivity and conductivityThe resistivity of different materials varies by an enormous amount: For example, the conductivity ofteflonis about 1030times lower than the conductivity of copper. Why is there such a difference? Loosely speaking, a metal has large numbers of "delocalized" electrons that are not stuck in any one place, but free to move across large distances, whereas in an insulator (like teflon), each electron is tightly bound to a single molecule, and a great force is required to pull it away.Semiconductorslie between these two extremes. More details can be found in the article:Electrical resistivity and conductivity. For the case ofelectrolytesolutions, see the article:Conductivity (electrolytic).Resistivity varies with temperature. In semiconductors, resistivity also changes when light is shining on it. These are discussed below.

Measuring resistancMain article:ohmmeterAn instrument for measuring resistance is called anohmmeter. Simple ohmmeters cannot measure low resistances accurately because the resistance of their measuring leads causes a voltage drop that interferes with the measurement, so more accurate devices usefour-terminal sensing.Typical resistancesSee also:Electrical resistivities of the elements (data page)andElectrical resistivity and conductivityComponentResistance()

1 meter of copper wirewith 1mm diameter0.02[4]

1kmoverhead power line(typical)0.03[5]

AA battery(typicalinternal resistance)0.1[6]

Incandescent light bulbfilament(typical)200-1000[7]

Human body1000 to 100,000[8]

Static and differential resistance

TheIV curveof a non-ohmic device (purple). Thestatic resistanceat pointAis theinverseslopeof lineBthrough the origin. Thedifferential resistanceatAis the inverse slope oftangent lineC.

TheIV curveof a component withnegative differential resistance, an unusual phenomenon where the IV curve is non-monotonic.See also:Small-signal modelMany electrical elements, such asdiodesandbatteriesdonotsatisfyOhm's law. These are callednon-ohmicornonlinear, and are characterized by anIVcurve, which isnota straight line through the origin.Resistance and conductance can still be defined for non-ohmic elements. However, unlike ohmic resistance, nonlinear resistance is not constant but varies with the voltage or current through the device; itsoperating point. There are two types:[1][2] Static resistance(also calledchordalorDC resistance) - This corresponds to the usual definition of resistance; the voltage divided by the current.It is the slope of the line (chord} from the origin through the point on the curve. Static resistance determines the power dissipation in an electrical component. Points on theIVcurve located in the 2nd or 4th quadrants, for which the slope of the chordal line is negative, havenegative static resistance.Passivedevices, which have no source of energy, cannot have negative static resistance. However active devices such as transistors orop-ampscan synthesize negative static resistance with feedback, and it is used in some circuits such asgyrators. Differential resistance(also calleddynamic,incrementalorsmall signal resistance) -Differential resistanceis the derivative of the voltage with respect to the current; theslopeof theIVcurve at a point.If theIVcurve is nonmonotonic(with peaks and troughs), the curve has a negative slope in some regionsso in these regions the device hasnegative differential resistance. Devices with negative differential resistance can amplify a signal applied to them, and are used to make amplifiers and oscillators. These includetunnel diodes,Gunn diodes,IMPATT diodes,magnetrontubes, andunijunction transistors.

Impedance and admittance

The voltage (red) and current (blue) versus time (horizontal axis) for acapacitor(top) andinductor(bottom). Since theamplitudeof the current and voltagesinusoidsare the same, theabsolute valueofimpedanceis 1 for both the capacitor and the inductor (in whatever units the graph is using). On the other hand, thephase differencebetween current and voltage is -90 for the capacitor; therefore, thecomplex phaseof theimpedanceof the capacitor is -90. Similarly, thephase differencebetween current and voltage is +90 for the inductor; therefore, the complex phase of the impedance of the inductor is +90.Main articles:Electrical impedanceandAdmittanceWhen an alternating current flows through a circuit, the relation between current and voltage across a circuit element is characterized not only by the ratio of their magnitudes, but also the difference in theirphases. For example, in an ideal resistor, the moment when the voltage reaches its maximum, the current also reaches its maximum (current and voltage are oscillating in phase). But for acapacitororinductor, the maximum current flow occurs as the voltage passes through zero and vice-versa (current and voltage are oscillating 90 out of phase, see image at right).Complex numbersare used to keep track of both the phase and magnitude of current and voltage:

where: tis time, V(t) andI(t) are, respectively, voltage and current as a function of time, V0,I0,Z, andYare complex numbers, Zis calledimpedance, Yis calledadmittance, Re indicatesreal part, is theangular frequencyof the AC current, is theimaginary unit.The impedance and admittance may be expressed as complex numbers that can be broken into real and imaginary parts:

whereRandGare resistance and conductance respectively,Xisreactance, andBissusceptance. For ideal resistors,ZandYreduce toRandGrespectively, but for AC networks containingcapacitorsandinductors,XandBare nonzero.for AC circuits, just asfor DC circuits.Frequency dependence of resistanceAnother complication of AC circuits is that the resistance and conductance can be frequency-dependent. One reason, mentioned above is theskin effect(and the relatedproximity effect). Another reason is that the resistivity itself may depend on frequency (seeDrude model,deep-level traps,resonant frequency,KramersKronig relations, etc.)Energy dissipation and Joule heating

Running current through a material with high resistance creates heat, in a phenomenon calledJoule heating. In this picture, acartridge heater, warmed by Joule heating, isglowing red hot.Main article:Joule heatingResistors (and other elements with resistance) oppose the flow of electric current; therefore, electrical energy is required to push current through the resistance. This electrical energy is dissipated, heating the resistor in the process. This is calledJoule heating(afterJames Prescott Joule), also calledohmic heatingorresistive heating.The dissipation of electrical energy is often undesired, particularly in the case oftransmission lossesinpower lines.High voltage transmissionhelps reduce the losses by reducing the current for a given power.On the other hand, Joule heating is sometimes useful, for example inelectric stovesand otherelectric heaters(also calledresistive heaters). As another example,incandescent lampsrely on Joule heating: the filament is heated to such a high temperature that it glows "white hot" withthermal radiation(also calledincandescence).The formula for Joule heating is:

wherePis thepower(energy per unit time) converted from electrical energy to thermal energy,Ris the resistance, andIis the current through the resistor.Dependence of resistance on other conditionsTemperature dependenceMain article:Electrical resistivity and conductivity Temperature dependenceNear room temperature, the resistivity of metals typically increases as temperature is increased, while the resistivity of semiconductors typically decreases as temperature is increased. The resistivity of insulators and electrolytes may increase or decrease depending on the system. For the detailed behavior and explanation, seeElectrical resistivity and conductivity.As a consequence, the resistance of wires, resistors, and other components often change with temperature. This effect may be undesired, causing an electronic circuit to malfunction at extreme temperatures. In some cases, however, the effect is put to good use. When temperature-dependent resistance of a component is used purposefully, the component is called aresistance thermometerorthermistor. (A resistance thermometer is made of metal, usually platinum, while a thermistor is made of ceramic or polymer.)Resistance thermometers and thermistors are generally used in two ways. First, they can be used asthermometers: By measuring the resistance, the temperature of the environment can be inferred. Second, they can be used in conjunction withJoule heating(also called self-heating): If a large current is running through the resistor, the resistor's temperature rises and therefore its resistance changes. Therefore, these components can be used in a circuit-protection role similar tofuses, or forfeedbackin circuits, or for many other purposes. In general, self-heating can turn a resistor into anonlinearandhystereticcircuit element. For more details seeThermistor#Self-heating effects.If the temperatureTdoes not vary too much, alinear approximationis typically used:

whereis called thetemperature coefficient of resistance,is a fixed reference temperature (usually room temperature), andis the resistance at temperature. The parameteris an empirical parameter fitted from measurement data. Because the linear approximation is only an approximation,is different for different reference temperatures. For this reason it is usual to specify the temperature thatwas measured at with a suffix, such as, and the relationship only holds in a range of temperatures around the reference.[9]The temperature coefficientis typically +3103K1to +6103K1for metals near room temperature. It is usually negative for semiconductors and insulators, with highly variable magnitude.[10]Strain dependenceMain article:Strain gaugeJust as the resistance of a conductor depends upon temperature, the resistance of a conductor depends uponstrain. By placing a conductor undertension(a form ofstressthat leads to strain in the form of stretching of the conductor), the length of the section of conductor under tension increases and its cross-sectional area decreases. Both these effects contribute to increasing the resistance of the strained section of conductor. Under compression (strain in the opposite direction), the resistance of the strained section of conductor decreases. See the discussion onstrain gaugesfor details about devices constructed to take advantage of this effect.Light illumination dependenceMain articles:PhotoresistorandPhotoconductivitySome resistors, particularly those made fromsemiconductors, exhibitphotoconductivity, meaning that their resistance changes when light is shining on them. Therefore they are calledphotoresistors(orlight dependent resistors). These are a common type oflight detector.SuperconductivityMain article:SuperconductivitySuperconductorsare materials that have exactly zero resistance and infinite conductance, because they can have V=0 and I0. This also means there is nojoule heating, or in other words nodissipationof electrical energy. Therefore, if superconductive wire is made into a closed loop, current flows around the loop forever. Superconductors require cooling to temperatures near 4 K withliquid heliumfor most metallic superconductors likeNbSnalloys, or cooling to temperatures near 77K withliquid nitrogenfor the expensive, brittle and delicate ceramichigh temperature superconductors. Nevertheless, there are manytechnological applications of superconductivity, includingsuperconducting magnets.Difference between Ammeter and VoltmeterAmmeterVoltmeter

ConnectionIt is to be connected in series modeIt is to be connected in parallel mode

ResistanceIt has comparatively low resistanceIt has high resistance

UsesIt is used to find the amount of current flowing in the circuitIt is used to find the potential difference in the circuit

CircuitCircuit must be disconnected in order to attach the ammeterCircuit does not need to be disconnected

AccuracyConsidered as less accurateConsidered as more accurate compared to ammeter

Voltage, Current, Resistance, and Ohm's LowElectricity BasicsWhen beginning to explore the world of electricity and electronics, it is vital to start by understanding the basics of voltage, current, and resistance. These are the three basic building blocks required to manipulate and utilize electricity. At first, these concepts can be difficult to understand because we cannot see them. One cannot see with the naked eye the energy flowing through a wire or the voltage of a battery sitting on a table. Even the lightning in the sky, while visible, is not truly the energy exchange happening from the clouds to the earth, but a reaction in the air to the energy passing through it. In order to detect this energy transfer, we must use measurement tools such as multimeters, spectrum analyzers, and oscilloscopes to visualize what is happening with the charge in a system. Fear not, however, this tutorial will give you the basic understanding of voltage, current, and resistance and how the three relate to each other.

Georg OhmCovered in this Tutorial How electrical charge relates to voltage, current, and resistance. What voltage, current, and resistance are. What Ohms Law is and how to use it to understand electricity. A simple experiment to demonstrate these concepts.Suggested Reading What is Electricity What is a Circuit?Electrical ChargeElectricity is the movement of electrons. Electrons create charge, which we can harness to do work. Your lightbulb, your stereo, your phone, etc., are all harnessing the movement of the electrons in order to do work. They all operate using the same basic power source: the movement of electrons.The three basic principles for this tutorial can be explained using electrons, or more specifically, the charge they create: Voltageis the difference in charge between two points. Currentis the rate at which charge is flowing. Resistanceis a materials tendency to resist the flow of charge (current).So, when we talk about these values, were really describing the movement of charge, and thus, the behavior of electrons. A circuit is a closed loop that allows charge to move from one place to another. Components in the circuit allow us to control this charge and use it to do work.Georg Ohmwas a Bavarian scientist who studied electricity. Ohm starts by describing a unit of resistance that is defined by current and voltage. So, lets start with voltage and go from there.VoltageWe define voltage as the amount of potential energy between two points on a circuit. One point has more charge than another. This difference in charge between the two points is called voltage. It is measured in volts, which, technically, is the potential energy difference between two points that will impart one joule of energy per coulomb of charge that passes through it (dont panic if this makes no sense, all will be explained). The unit volt is named after the Italian physicistAlessandro Voltawho invented what is considered the first chemical battery. Voltage is represented in equations and schematics by the letter V.When describing voltage, current, and resistance, a common analogy is a water tank. In this analogy, charge is represented by the wateramount, voltage is represented by the waterpressure, and current is represented by the waterflow. So for this analogy, remember: Water = Charge Pressure = Voltage Flow = CurrentConsider a water tank at a certain height above the ground. At the bottom of this tank there is a hose.

The pressure at the end of the hose can represent voltage. The water in the tank represents charge. The more water in the tank, the higher the charge, the more pressure is measured at the end of the hose.We can think of this tank as a battery, a place where we store a certain amount of energy and then release it. If we drain our tank a certain amount, the pressure created at the end of the hose goes down. We can think of this as decreasing voltage, like when a flashlight gets dimmer as the batteries run down. There is also a decrease in the amount of water that will flow through the hose. Less pressure means less water is flowing, which brings us to current.CurrentWe can think of the amount of water flowing through the hose from the tank as current. The higher the pressure, the higher the flow, and vice-versa. With water, we would measure the volume of the water flowing through the hose over a certain period of time. With electricity, we measure the amount of charge flowing through the circuit over a period of time. Current is measured in Amperes (usually just referred to as Amps). An ampere is defined as 6.241*1018electrons (1 Coulomb) per second passing through a point in a circuit. Amps are represented in equations by the letter I.Lets say now that we have two tanks, each with a hose coming from the bottom. Each tank has the exact same amount of water, but the hose on one tank is narrower than the hose on the other.

We measure the same amount of pressure at the end of either hose, but when the water begins to flow, the flow rate of the water in the tank with the narrower hose will be less than the flow rate of the water in the tank with the wider hose. In electrical terms, the current through the narrower hose is less than the current through the wider hose. If we want the flow to be the same through both hoses, we have to increase the amount of water (charge) in the tank with the narrower hose.

This increases the pressure (voltage) at the end of the narrower hose, pushing more water through the tank. This is analogous to an increase in voltage that causes an increase in current.Now were starting to see the relationship between voltage and current. But there is a third factor to be considered here: the width of the hose. In this analogy, the width of the hose is the resistance. This means we need to add another term to our model: Water = Charge (measured in Coulombs) Pressure = Voltage (measured in Volts) Flow = Current (measured in Amperes, or Amps for short) Hose Width = ResistanceResistanceConsider again our two water tanks, one with a narrow pipe and one with a wide pipe.

It stands to reason that we cant fit as much volume through a narrow pipe than a wider one at the same pressure. This is resistance. The narrow pipe resists the flow of water through it even though the water is at the same pressure as the tank with the wider pipe.

In electrical terms, this is represented by two circuits with equal voltages and different resistances. The circuit with the higher resistance will allow less charge to flow, meaning the circuit with higher resistance has less current flowing through it.This brings us back to Georg Ohm. Ohm defines the unit of resistance of 1 Ohm as the resistance between two points in a conductor where the application of 1 volt will push 1 ampere, or 6.2411018electrons. This value is usually represented in schematics with the greek letter , which is called omega, and pronounced ohm.Ohm's LawCombining the elements of voltage, current, and resistance, Ohm developed the formula:

Where V = Voltage in volts I = Current in amps R = Resistance in ohmsThis is called Ohms law. Lets say, for example, that we have a circuit with the potential of 1 volt, a current of 1 amp, and resistance of 1 ohm. Using Ohms Law we can say:

Lets say this represents our tank with a wide hose. The amount of water in the tank is defined as 1 volt and the narrowness (resistance to flow) of the hose is defined as 1 ohm. Using Ohms Law, this gives us a flow (current) of 1 amp.Using this analogy, lets now look at the tank with the narrow hose. Because the hose is narrower, its resistance to flow is higher. Lets define this resistance as 2 ohms. The amount of water in the tank is the same as the other tank, so, using Ohms Law, our equation for the tank with the narrow hose is

But what is the current? Because the resistance is greater, and the voltage is the same, this gives us a current value of 0.5 amps:So, the current is lower in the tank with higher resistance. Now we can see that if we know two of the values for Ohms law, we can solve for the third. Lets demonstrate this with an experiment.An Ohm's Law ExperimentFor this experiment, we want to use a 9 volt battery to power an LED. LEDs are fragile and can only have a certain amount of current flowing through them before they burn out. In the documentation for an LED, there will always be a current rating. This is the maximum amount of current that can flow through the particular LED before it burns out.Materials RequiredIn order to perform the experiments listed at the end of the tutorial, you will need: A multimeter A 9-Volt battery A 560-Ohm resistor(or the next closest value) An LEDDividing by zero gives us infinite current! Well, not infinite in practice, but as much current as the battery can deliver. Since we do NOT want that much current flowing through our LED, were going to need a resistor. Our circuit should look like this:We can use Ohms Law in the exact same way to determine the reistor value that will give us the desired current value: