Definition of Transformer

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Definition of Transformer

A 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 Transformer

Thehistory of transformer was commenced in the year 1880. In the year 1950, 400KVelectrical

power transformer was introduced in highvoltage electrical power system. 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 Transformer

Generation of electrical power in lowvoltage level is very much cost effective. Henceelectrical

power is generated in lowvoltage level. Theoretically, this lowvoltage level power can be transmitted

to the receiving end. But if thevoltage level of a power is increased, theelectric current of 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

regulation of 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 highvoltage power 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 transformer in the electrical power system.

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Types of Transformer

Transformers can be categorized in different ways, depending upon their purpose, use, construction

etc. Thetypes of transformer are as follows,

1.Step Up Transformer & Step Down Transformer - Generally used for stepping up and

down the voltage level 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.

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3.Electrical Power Transformer, Distribution Transformer & Instrument Transformer-

Transformer is generally used in transmission network which is normally known aspowertransformer, distribution transformer is used in distribution network and this is lower rating

transformer andcurrent transformer &potential transformer, we use for relay and protection

purpose in electrical power system and in different instruments in industries are called

instrument transformer.

4.Two Winding Transformer &Auto Transformer - Former is generally used where ratio

between highvoltageand lowvoltage is greater than 2. It is cost effective to use later where

the ratio between highvoltage and low voltageis 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 breaker is 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

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given during designing ofcircuit breaker to safe interruption of arc produced during theoperation

of circuit breaker. This was the basicdefinition of circuit breaker.

Introduction to Circuit Breaker

The modern power system deals with huge power network and huge numbers of associatedelectrical 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 current in them which may damage

the equipment and networks permanently. For saving these equipment and the power networks the

faultelectric current should 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 these arcs in circuit breakerin safe manner. Thecircuit

breaker is the special device which does all the required switching operations duringelectric

current carrying condition. This was the basicintroduction to circuit breaker.

What is Circuit Breaker?

Definition of circuit breaker : -Electrical circuit breaker is 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 breaker to safe interruption of arc produced during theoperation

of circuit breaker. This was the basicdefinition of circuit breaker.

Introduction to Circuit Breaker

The 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 equipmentas well as the power network suffer a high stress of faultelectric current in them which may damage

the equipment and networks permanently. For saving these equipment and the power networks the

faultelectric current should 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,

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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 these arcs in circuit breakerin safe manner. Thecircuit

breaker is the special device which does all the required switching operations duringelectric

current carrying condition. This was the basicintroduction to circuit breaker.

Earth Leakage Circuit Breaker (ELCB)

An Earth Leakage Circuit 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 Leakage Circuit 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

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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.

Top

Voltage Base ELCB

•

Voltage-ELCB is a voltage operated circuit breaker. The device will function when theCurrent 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 currentto 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 ELCB’s 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.

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• These ELCBs monitored the voltage on the earth wire, and disconnected the supply if theearth 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 RCD’s 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


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to cause it to trip.

.

• If the installation’s 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 tohave 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 don’t 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


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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.

Diode

Semiconductor diodes

Electronic symbols

Main article: Electronic symbol

The symbol used for a semiconductor diode in a circuit diagram secifies the tye of diode! There

are alternative symbols for some tyes of diodes" though the differences are minor!

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•

Diode

•

Light Emitting Diode(LED)

•

#hotodiode

•

$chottky diode

•

Transient Voltage $uression (TV$)

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•

Tunnel diode

•

Varica

•

%ener diode

•

Tyical diode ackages in same alignment as diode symbol! Thin bar deicts the cathode!

Point-contact diodes

A point-contact diode &orks the same as the 'unction diodes described belo&" but their

construction is simler! A block of ntye semiconductor is built" and a conducting sharoint

contact made &ith some grou metal is laced in contact &ith the semiconductor! $ome metal

migrates into the semiconductor to make a small region of tye semiconductor near the contact!The longoular *+, germanium version is still used in radio receivers as a detector and

occasionally in seciali-ed analog electronics!

Junction diodes

p–n junction diode

Main article: p–n diode

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A .n 'unction diode is made of a crystal of semiconductor " usually silicon"

but germanium and gallium arsenide are also used! /murities are added to it to create a region on

one side that contains negative charge carriers (electrons)" called ntye semiconductor " and a

region on the other side that contains ositive charge carriers (holes)" called tye semiconductor !

0hen t&o materials i!e! ntye and tye are attached together" a momentary flo& of electronsoccur from n to side resulting in a third region &here no charge carriers are resent! This region is

called the deletion region due to the absence of charge carriers (electrons and holes in this case)!

The diode1s terminals are attached to the ntye and tye regions! The boundary bet&een these

t&o regions" called a .n 'unction" is &here the action of the diode takes lace! The crystal allo&s

electrons to flo& from the +tye side (called the cathode) to the #tye side (called the anode)" but

not in the oosite direction!

Electrical resistance and conductance

"Resistive" redirects here. For the term used when referring to touchscreens, see resistive

touchscreen.

Electromagnetism

• Electricity

• 2agnetism

The electrical resistance of an electrical conductor is the oosition to the assage of an electric

current through that conductor! The inverse 3uantity is electrical conductance" the ease &ith &hich

an electric current asses! Electrical resistance shares some concetual arallels &ith the

mechanical notion of friction! The $/ unit of electrical resistance is the ohm (4)" &hile electrical

conductance is measured insiemens ($)!

An ob'ect of uniform cross section has a resistance roortional to its resistivity and length andinversely roortional to its crosssectional area! All materials sho& some resistance" e5cet

for suerconductors" &hich have a resistance of -ero!

The resistance (6) of an ob'ect is defined as the ratio of voltage across it ( ) to currentthrough it (! )"

&hile the conductance (7) is the inverse8

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9or a &ide variety of materials and conditions" and ! are directly roortional to each other"

and therefore R and are constant (although they can deend on other factors like temerature

or strain)! This roortionality is called :hm1s la&" and materials that satisfy it are called ;:hmic;

materials!

/n other cases" such as a diode or battery" and ! are not directly roortional" or in other &ords

the !– curve is not a straight line through the origin" and :hm1s la& does not hold! /n this case"

resistance and conductance are less useful concets" and more difficult to define! The ratio V=?> as it

corresonds to the inverse sloe of a chord bet&een the origin and an !– curve! /n other

situations" the derivative may be most useful@ this is called the ;differential resistance;!

http://en.wikipedia.org/wiki/Constant_(mathematics)http://en.wikipedia.org/wiki/Constant_(mathematics)http://en.wikipedia.org/wiki/Ohm's_lawhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Battery_(electricity)http://en.wikipedia.org/wiki/I%E2%80%93V_curvehttp://en.wikipedia.org/wiki/I%E2%80%93V_curvehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-brown-1http://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-kaiser-2http://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-kaiser-2http://en.wikipedia.org/wiki/I%E2%80%93V_curvehttp://en.wikipedia.org/wiki/I%E2%80%93V_curvehttp://en.wikipedia.org/wiki/Derivativehttp://en.wikipedia.org/wiki/Constant_(mathematics)http://en.wikipedia.org/wiki/Ohm's_lawhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Battery_(electricity)http://en.wikipedia.org/wiki/I%E2%80%93V_curvehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-brown-1http://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-kaiser-2http://en.wikipedia.org/wiki/I%E2%80%93V_curvehttp://en.wikipedia.org/wiki/Derivative


14/34


15/34

1 Introduction

• ? Conductors and resistors

• :hm1s la&

• , 6elation to resistivity and conductivity

o ,!* 0hat determines resistivity

• 2easuring resistance

• Tyical resistances

• $tatic and differential resistance

• AC circuits

o !* /medance and admittance

o !? 9re3uency deendence of resistance

• F Energy dissiation and Goule heating

• *H Deendence of resistance on other conditions

o *H!* Temerature deendence

o

*H!? $train deendence

o *H! Light illumination deendence

• ** $uerconductivity

• *? $ee also

• * 6eferences

• *, E5ternal links

Introduction

http://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Introductionhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Conductors_and_resistorshttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Ohm.27s_lawhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Relation_to_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#What_determines_resistivity.3Fhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Measuring_resistancehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Typical_resistanceshttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Static_and_differential_resistancehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#AC_circuitshttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Impedance_and_admittancehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Frequency_dependence_of_resistancehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Energy_dissipation_and_Joule_heatinghttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Dependence_of_resistance_on_other_conditionshttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Temperature_dependencehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Strain_dependencehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Light_illumination_dependencehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Superconductivityhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#See_alsohttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Referenceshttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#External_linkshttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Introductionhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Conductors_and_resistorshttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Ohm.27s_lawhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Relation_to_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#What_determines_resistivity.3Fhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Measuring_resistancehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Typical_resistanceshttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Static_and_differential_resistancehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#AC_circuitshttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Impedance_and_admittancehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Frequency_dependence_of_resistancehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Energy_dissipation_and_Joule_heatinghttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Dependence_of_resistance_on_other_conditionshttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Temperature_dependencehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Strain_dependencehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Light_illumination_dependencehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Superconductivityhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#See_alsohttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#Referenceshttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#External_links


16/34

The hydraulic analogy comares electric current flo&ing through circuits to &ater flo&ing through ies! 0hen a

ie (left) is filled &ith hair (right)" it takes a larger ressure to achieve the same flo& of &ater! #ushing electric

current through a large resistance is like ushing &ater through a ie clogged &ith hair8 /t re3uires a larger ush

(electromotive force) to drive the same flo& (electric current)!

/n the hydraulic analogy" current flo&ing through a &ire (or resistor ) is like &ater flo&ing through

a ie" and the voltage dro across the &ire is like the ressure dro that ushes &ater throughthe ie! Conductance is roortional to ho& much flo& occurs for a given ressure" and

resistance is roortional to ho& much ressure is re3uired to achieve a given flo&!

(Conductance and resistance arerecirocals!)

The voltage drop (i!e!" difference in voltage bet&een one side of the resistor and the other)" not

the voltage itself" rovides the driving force ushing current through a resistor! /n hydraulics" it is

similar8 The ressure difference bet&een t&o sides of a ie" not the ressure itself" determines

the flo& through it! 9or e5amle" there may be a large &ater ressure above the ie" &hich

tries to ush &ater do&n through the ie! But there may be an e3ually large &ater ressure

belo& the ie" &hich tries to ush &ater back u through the ie! /f these ressures are e3ual"

no &ater flo&s! (/n the image at right" the &ater ressure belo& the ie is -ero!)

T&o roertiesIgeometry (shae) and materialImostly determine the resistance and

conductance of a &ire" resistor" or other element!

7eometry is imortant because it is more difficult to ush &ater through a long" narro& ie than

a &ide" short ie! /n the same &ay" a long" thin coer &ire has higher resistance (lo&er

conductance) than a short" thick coer &ire!

2aterials are imortant as &ell! A ie filled &ith hair restricts the flo& of &ater more than a

clean ie of the same shae and si-e! /n a similar &ay" electrons can flo& freely and easily

through a coer &ire" but cannot as easily flo& through asteel &ire of the same shae and si-e"

and they essentially cannot flo& at all through an insulator like rubber " regardless of its shae!

The difference bet&een" coer" steel" and rubber is related to their microscoic structure

and electron configuration" and is 3uantified by a roerty called resistivity!

http://en.wikipedia.org/wiki/Hydraulic_analogyhttp://en.wikipedia.org/wiki/Hydraulic_analogyhttp://en.wikipedia.org/wiki/Hydraulic_analogyhttp://en.wikipedia.org/wiki/Electromotive_forcehttp://en.wikipedia.org/wiki/Electromotive_forcehttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Hydraulic_analogyhttp://en.wikipedia.org/wiki/Hydraulic_analogyhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Voltage_drophttp://en.wikipedia.org/wiki/Voltage_drophttp://en.wikipedia.org/wiki/Voltage_drophttp://en.wikipedia.org/wiki/Pressure_drophttp://en.wikipedia.org/wiki/Pressure_drophttp://en.wikipedia.org/wiki/Pressure_drophttp://en.wikipedia.org/wiki/Multiplicative_inversehttp://en.wikipedia.org/wiki/Voltage_drophttp://en.wikipedia.org/wiki/Voltage_drophttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Steelhttp://en.wikipedia.org/wiki/Insulator_(electrical)http://en.wikipedia.org/wiki/Insulator_(electrical)http://en.wikipedia.org/wiki/Rubberhttp://en.wikipedia.org/wiki/Rubberhttp://en.wikipedia.org/wiki/Electron_configurationhttp://en.wikipedia.org/wiki/Resistivityhttp://en.wikipedia.org/wiki/Hydraulic_analogyhttp://en.wikipedia.org/wiki/Electromotive_forcehttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Hydraulic_analogyhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Voltage_drophttp://en.wikipedia.org/wiki/Pressure_drophttp://en.wikipedia.org/wiki/Multiplicative_inversehttp://en.wikipedia.org/wiki/Voltage_drophttp://en.wikipedia.org/wiki/Voltagehttp://en.wikipedia.org/wiki/Electronhttp://en.wikipedia.org/wiki/Copperhttp://en.wikipedia.org/wiki/Steelhttp://en.wikipedia.org/wiki/Insulator_(electrical)http://en.wikipedia.org/wiki/Rubberhttp://en.wikipedia.org/wiki/Electron_configurationhttp://en.wikipedia.org/wiki/Resistivity


17/34

Conductors and resistors

A 4 resistor " as identified by itselectronic color code (blue.green.blackgold)! An ohmmeter could be used to

verify this value!

$ubstances in &hich electricity can flo& are called conductors! A iece of conducting material of

a articular resistance meant for use in a circuit is called a resistor ! Conductors are made of

highconductivity materials such as metals" in articular coer and aluminium! 6esistors" on the

other hand" are made of a &ide variety of materials deending on factors such as the desired

resistance" amount of energy that it needs to dissiate" recision" and costs!

Ohm's law

The currentvoltage characteristics of four devices8 T&o resistors" a diode" and a battery! The hori-ontal a5is

is voltage dro" the vertical a5is is current! :hm1s la& is satisfied &hen the grah is a straight line through the

origin! Therefore" the t&o resistors are ;ohmic;" but the diode and battery are not!

Main article: #hm$s law

:hm1s la& is an emirical la& relating the voltage across an element to the current ! through it8

( is directly roortional to ! )! This la& is not al&ays true8 9or e5amle" it is false

for diodes" batteries" etc! Jo&ever" it is true to a very good aro5imation for &ires

and resistors (assuming that other conditions" including temerature" are held fi5ed)!

2aterials or ob'ects &here :hm1s la& is true are called ohmic " &hereas ob'ects that do not

obey :hm1s la& are non%ohmic !

Relation to resistivity and conductivity

http://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Electronic_color_codehttp://en.wikipedia.org/wiki/Electronic_color_codehttp://en.wikipedia.org/wiki/Ohmmeterhttp://en.wikipedia.org/wiki/Ohmmeterhttp://en.wikipedia.org/wiki/Electrical_conductorhttp://en.wikipedia.org/wiki/Electrical_conductorhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Current-voltage_characteristichttp://en.wikipedia.org/wiki/Current-voltage_characteristichttp://en.wikipedia.org/wiki/Current-voltage_characteristichttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Battery_(electricity)http://en.wikipedia.org/wiki/Battery_(electricity)http://en.wikipedia.org/wiki/Voltage_drophttp://en.wikipedia.org/wiki/Voltage_drophttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Ohm's_lawhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Battery_(electrical)http://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Electronic_color_codehttp://en.wikipedia.org/wiki/Ohmmeterhttp://en.wikipedia.org/wiki/Electrical_conductorhttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Current-voltage_characteristichttp://en.wikipedia.org/wiki/Resistorhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Battery_(electricity)http://en.wikipedia.org/wiki/Voltage_drophttp://en.wikipedia.org/wiki/Electric_currenthttp://en.wikipedia.org/wiki/Ohm's_lawhttp://en.wikipedia.org/wiki/Diodehttp://en.wikipedia.org/wiki/Battery_(electrical)http://en.wikipedia.org/wiki/Resistor


18/34

A iece of resistive material &ith electrical contacts on both ends!

Main article: Electrical resistivity and conductivity

The resistance of a given ob'ect deends rimarily on t&o factors8 0hat material it is made

of" and its shae! 9or a given material" the resistance is inversely roortional to the cross

sectional area@ for e5amle" a thick coer &ire has lo&er resistance than an other&ise

identical thin coer &ire! Also" for a given material" the resistance is roortional to the

length@ for e5amle" a long coer &ire has higher resistance than an other&iseidentical

short coer &ire! The resistance R and conductance G of a conductor of uniform cross

section" therefore" can be comuted as

&here is the length of the conductor" measured in metres =m>" & is the cross

section area of the conductor measured ins3uare metres =mK>" (sigma) is

the electrical conductivity measured in siemens er meter ($MmN*)" and O (rho) is

theelectrical resistivity (also called specific electrical resistance) of the material"

measured in ohmmetres (4Mm)! The resistivity and conductivity are roortionality

constants" and therefore deend only on the material the &ire is made of" not the

geometry of the &ire! 6esistivity and conductivity are recirocals8 !

6esistivity is a measure of the material1s ability to oose electric current!

This formula is not e5act8 /t assumes the current density is totally uniform in the

conductor" &hich is not al&ays true in ractical situations! Jo&ever" this formula still

rovides a good aro5imation for long thin conductors such as &ires!

http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Metrehttp://en.wikipedia.org/wiki/Metrehttp://en.wikipedia.org/wiki/Square_metrehttp://en.wikipedia.org/wiki/Sigma_(letter)http://en.wikipedia.org/wiki/Electrical_conductivityhttp://en.wikipedia.org/wiki/Siemens_(unit)http://en.wikipedia.org/wiki/Siemens_(unit)http://en.wikipedia.org/wiki/Rho_(letter)http://en.wikipedia.org/wiki/Electrical_resistivityhttp://en.wikipedia.org/wiki/Multiplicative_inversehttp://en.wikipedia.org/wiki/Current_densityhttp://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Metrehttp://en.wikipedia.org/wiki/Square_metrehttp://en.wikipedia.org/wiki/Sigma_(letter)http://en.wikipedia.org/wiki/Electrical_conductivityhttp://en.wikipedia.org/wiki/Siemens_(unit)http://en.wikipedia.org/wiki/Rho_(letter)http://en.wikipedia.org/wiki/Electrical_resistivityhttp://en.wikipedia.org/wiki/Multiplicative_inversehttp://en.wikipedia.org/wiki/Current_density


19/34

Another situation for &hich this formula is not e5act is &ith alternating current (AC)"

because the skin effect inhibits current flo& near the center of the conductor! Then"

the geometrical crosssection is different from the effective crosssection in &hich

current actually flo&s" so resistance is higher than e5ected! $imilarly" if t&o

conductors near each other carry AC current" their resistances increase due tothe ro5imity effect! At commercial o&er fre3uency" these effects are significant for

large conductors carrying large currents" such as busbars in an electrical substation"

=> or large o&er cables carrying more than a fe& hundred ameres!

What determines resistivity?

Main article: Electrical resistivity and conductivity

The resistivity of different materials varies by an enormous amount8 9or e5amle"

the conductivity of teflon is about *HHtimes lo&er than the conductivity of coer!

0hy is there such a difference Loosely seaking" a metal has large numbers of

;delocali-ed; electrons that are not stuck in any one lace" but free to move across

large distances" &hereas in an insulator (like teflon)" each electron is tightly bound

to a single molecule" and a great force is re3uired to ull it a&ay! $emiconductorslie

bet&een these t&o e5tremes! 2ore details can be found in the article8 Electrical

resistivity and conductivity! 9or the case of electrolyte solutions" see the

article8 Conductivity (electrolytic)!

6esistivity varies &ith temerature! /n semiconductors" resistivity also changes

&hen light is shining on it! These are discussed belo&!

Measuring resistanc

Main article: ohmmeter

An instrument for measuring resistance is called an ohmmeter ! $imle ohmmeters

cannot measure lo& resistances accurately because the resistance of their

measuring leads causes a voltage dro that interferes &ith the measurement" so

more accurate devices use fourterminal sensing!

Typical resistances

'ee also: Electrical resistivities of the elements (data page) and Electrical resistivity

and conductivity

http://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Skin_effecthttp://en.wikipedia.org/wiki/Proximity_effect_(electromagnetism)http://en.wikipedia.org/wiki/Utility_frequencyhttp://en.wikipedia.org/wiki/Busbarhttp://en.wikipedia.org/wiki/Electrical_substationhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-3http://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-3http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/PTFEhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Electrolytehttp://en.wikipedia.org/wiki/Conductivity_(electrolytic)http://en.wikipedia.org/wiki/Conductivity_(electrolytic)http://en.wikipedia.org/wiki/Ohmmeterhttp://en.wikipedia.org/wiki/Ohmmeterhttp://en.wikipedia.org/wiki/Ohmmeterhttp://en.wikipedia.org/wiki/Ohmmeterhttp://en.wikipedia.org/wiki/Four-terminal_sensinghttp://en.wikipedia.org/wiki/Four-terminal_sensinghttp://en.wikipedia.org/wiki/Electrical_resistivities_of_the_elements_(data_page)http://en.wikipedia.org/wiki/Electrical_resistivities_of_the_elements_(data_page)http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Alternating_currenthttp://en.wikipedia.org/wiki/Skin_effecthttp://en.wikipedia.org/wiki/Proximity_effect_(electromagnetism)http://en.wikipedia.org/wiki/Utility_frequencyhttp://en.wikipedia.org/wiki/Busbarhttp://en.wikipedia.org/wiki/Electrical_substationhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-3http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/PTFEhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Electrolytehttp://en.wikipedia.org/wiki/Conductivity_(electrolytic)http://en.wikipedia.org/wiki/Ohmmeterhttp://en.wikipedia.org/wiki/Ohmmeterhttp://en.wikipedia.org/wiki/Four-terminal_sensinghttp://en.wikipedia.org/wiki/Electrical_resistivities_of_the_elements_(data_page)http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivityhttp://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity


20/34

Component Resistance (4)

* meter of coer &ire

&ith *mm diameter H!H?=,>

* km overhead o&er line

(typical)H!H=>

AA battery (typical

internal resistance )H!*=>

/ncandescent light bulb

filament (typical)?HH*HHH=>

Juman body *HHH to *HH"HHH=>

Static and differential resistance

http://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-4http://en.wikipedia.org/wiki/Overhead_power_linehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-5http://en.wikipedia.org/wiki/AA_batteryhttp://en.wikipedia.org/wiki/Internal_resistancehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-6http://en.wikipedia.org/wiki/Incandescent_light_bulbhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-7http://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-8http://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-4http://en.wikipedia.org/wiki/Overhead_power_linehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-5http://en.wikipedia.org/wiki/AA_batteryhttp://en.wikipedia.org/wiki/Internal_resistancehttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-6http://en.wikipedia.org/wiki/Incandescent_light_bulbhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-7http://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-8


21/34

The /V curve of a nonohmic device (urle)! The static resistance at oint & is

the inverse sloe of line *through the origin! The differential resistance at & is the inverse sloe

of tangent line + !

The /V curve of a comonent &ithnegative differential resistance" an unusual henomenon &here

the /V curve is nonmonotonic!

'ee also: 'mall%signal model

2any electrical elements" such as diodes and batteriesdo not satisfy :hm1s la&!

These are called non%ohmic or nonlinear " and are characteri-ed by an !– curve"

&hich is not a straight line through the origin!

6esistance and conductance can still be defined for nonohmic elements! Jo&ever"

unlike ohmic resistance" nonlinear resistance is not constant but varies &ith the

voltage or current through the device@ its oerating oint! There are t&o tyes8 =*>=?>

• tatic resistance (also called chordal or + resistance) This corresonds to

the usual definition of resistance@ the voltage divided by the current

!

/t is the sloe of the line (chordP from the origin through the oint on the curve! $tatic

resistance determines the o&er dissiation in an electrical comonent! #oints on

the ! curve located in the ?nd or ,th 3uadrants" for &hich the sloe of the chordal line is

negative" have negative static resistance! #assive devices" &hich have no source of energy"

cannot have negative static resistance! Jo&ever active devices such as transistors or o

ams can synthesi-e negative static resistance &ith feedback" and it is used in some circuitssuch as gyrators!

• !ifferential resistance (also called dynamic " incremental or small

signal resistance) Differential resistance is the derivative of the

voltage &ith resect to the current@ the sloe of the ! curve at a oint

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!

/f the ! curve is nonmonotonic (&ith eaks and troughs)" the curve has a negative sloe in

some regionsIso in these regions the device has negative differential resistance! Devices

&ith negative differential resistance can amlify a signal alied to them" and are used to

make amlifiers and oscillators! These include tunnel diodes" 7unn diodes" /2#ATTdiodes" magnetron tubes" and uni'unction transistors!

Impedance and admittance

The voltage (red) and current (blue) versus time (hori-ontal a5is) for

a caacitor (to) and inductor (bottom)! $ince the amlitude of the current and

voltage sinusoids are the same" the absolute value of imedance is * for both

the caacitor and the inductor (in &hatever units the grah is using)! :n the

other hand" the hase difference bet&een current and voltage is FHQ for the

caacitor@ therefore" the comle5 hase of the imedance of the caacitor is

FHQ! $imilarly" the hase difference bet&een current and voltage is RFHQ for

the inductor@ therefore" the comle5 hase of the imedance of the inductor is

RFHQ!

Main articles: Electrical impedance and &dmittance

0hen an alternating current flo&s through a circuit" the relation

bet&een current and voltage across a circuit element is

characteri-ed not only by the ratio of their magnitudes" but also the

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difference in their hases! 9or e5amle" in an ideal resistor" the

moment &hen the voltage reaches its ma5imum" the current also

reaches its ma5imum (current and voltage are oscillating in hase)!

But for a caacitor or inductor " the ma5imum current flo& occurs as

the voltage asses through -ero and viceversa (current andvoltage are oscillating FHQ out of hase" see image at

right)! Comle5 numbers are used to kee track of both the hase

and magnitude of current and voltage8

&here8

• t is time"

• (t ) and ! (t ) are" resectively" voltage and current as a

function of time"

• - " ! - " " and / are comle5 numbers"

• is called imedance"

• / is called admittance"

• 6e indicates real art"

• is the angular fre3uency of the AC current"

• is the imaginary unit!

The imedance and admittance may be e5ressed as comle5

numbers that can be broken into real and imaginary arts8

&here R and are resistance and conductance

resectively" 0 is reactance" and * is suscetance! 9or

ideal resistors" and / reduce to R and resectively" but

http://en.wikipedia.org/wiki/Phase_(waves)http://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Inductorhttp://en.wikipedia.org/wiki/Inductorhttp://en.wikipedia.org/wiki/Complex_numberhttp://en.wikipedia.org/wiki/Complex_numberhttp://en.wikipedia.org/wiki/Electrical_impedancehttp://en.wikipedia.org/wiki/Admittancehttp://en.wikipedia.org/wiki/Admittancehttp://en.wikipedia.org/wiki/Real_parthttp://en.wikipedia.org/wiki/Real_parthttp://en.wikipedia.org/wiki/Angular_frequencyhttp://en.wikipedia.org/wiki/Imaginary_unithttp://en.wikipedia.org/wiki/Electrical_reactancehttp://en.wikipedia.org/wiki/Susceptancehttp://en.wikipedia.org/wiki/Phase_(waves)http://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Inductorhttp://en.wikipedia.org/wiki/Complex_numberhttp://en.wikipedia.org/wiki/Electrical_impedancehttp://en.wikipedia.org/wiki/Admittancehttp://en.wikipedia.org/wiki/Real_parthttp://en.wikipedia.org/wiki/Angular_frequencyhttp://en.wikipedia.org/wiki/Imaginary_unithttp://en.wikipedia.org/wiki/Electrical_reactancehttp://en.wikipedia.org/wiki/Susceptance


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for AC net&orks

containing caacitors and inductors" 0 and * are non-ero!

for AC circuits" 'ust as for DC

circuits!

"re#uency dependence of

resistance

Another comlication of AC circuits is that the resistance

and conductance can be fre3uencydeendent! :ne

reason" mentioned above is the skin effect (and the

related ro5imity effect)! Another reason is that the

resistivity itself may deend on fre3uency (see Drude

model" deelevel tras" resonant fre3uency" Sramers.

Sronig relations" etc!)

Energy dissipation and oule heating

6unning current through a material &ith high resistance creates heat"

in a henomenon called Goule heating! /n this icture" a cartridge

heater " &armed by Goule heating" is glo&ing red hot!

Main article: 1oule heating

6esistors (and other elements &ith resistance) oose the

flo& of electric current@ therefore" electrical energy is

re3uired to ush current through the resistance! This

electrical energy is dissiated" heating the resistor in the

rocess! This is called1oule heating (after Games #rescott

Goule)" also called ohmic heating or resistive heating !

http://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Inductorhttp://en.wikipedia.org/wiki/Skin_effecthttp://en.wikipedia.org/wiki/Skin_effecthttp://en.wikipedia.org/wiki/Proximity_effect_(electromagnetism)http://en.wikipedia.org/wiki/Drude_modelhttp://en.wikipedia.org/wiki/Drude_modelhttp://en.wikipedia.org/wiki/Drude_modelhttp://en.wikipedia.org/wiki/Deep-level_traphttp://en.wikipedia.org/wiki/Deep-level_traphttp://en.wikipedia.org/wiki/Resonant_frequencyhttp://en.wikipedia.org/wiki/Kramers%E2%80%93Kronig_relationshttp://en.wikipedia.org/wiki/Kramers%E2%80%93Kronig_relationshttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Cartridge_heaterhttp://en.wikipedia.org/wiki/Cartridge_heaterhttp://en.wikipedia.org/wiki/Cartridge_heaterhttp://en.wikipedia.org/wiki/Incandescencehttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/James_Prescott_Joulehttp://en.wikipedia.org/wiki/James_Prescott_Joulehttp://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Inductorhttp://en.wikipedia.org/wiki/Skin_effecthttp://en.wikipedia.org/wiki/Proximity_effect_(electromagnetism)http://en.wikipedia.org/wiki/Drude_modelhttp://en.wikipedia.org/wiki/Drude_modelhttp://en.wikipedia.org/wiki/Deep-level_traphttp://en.wikipedia.org/wiki/Resonant_frequencyhttp://en.wikipedia.org/wiki/Kramers%E2%80%93Kronig_relationshttp://en.wikipedia.org/wiki/Kramers%E2%80%93Kronig_relationshttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Cartridge_heaterhttp://en.wikipedia.org/wiki/Cartridge_heaterhttp://en.wikipedia.org/wiki/Incandescencehttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/James_Prescott_Joulehttp://en.wikipedia.org/wiki/James_Prescott_Joule


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The dissiation of electrical energy is often undesired"

articularly in the case of transmission losses in o&er

lines! Jigh voltage transmission hels reduce the losses by

reducing the current for a given o&er!

:n the other hand" Goule heating is sometimes useful" for

e5amle in electric stovesand other electric heaters (also

called resistive heaters)! As another e5amle"incandescent

lams rely on Goule heating8 the filament is heated to such

a high temerature that it glo&s ;&hite hot; &iththermal

radiation (also called incandescence)!

The formula for Goule heating is8

&here 2 is the o&er (energy er unit time) converted

from electrical energy to thermal energy" R is the

resistance" and ! is the current through the resistor!

Dependence of resistance on other conditions

$emperature dependence

Main article: Electrical resistivity and conductivity 3

4emperature dependence

+ear room temerature" the resistivity of metals

tyically increases as temerature is increased" &hile

the resistivity of semiconductors tyically decreases as

temerature is increased! The resistivity of insulators

and electrolytes may increase or decrease deending

on the system! 9or the detailed behavior and

e5lanation" see Electrical resistivity and conductivity!

As a conse3uence" the resistance of &ires" resistors"

and other comonents often change &ith temerature!

This effect may be undesired" causing an electronic

circuit to malfunction at e5treme temeratures! /n some

cases" ho&ever" the effect is ut to good use! 0hen

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temeraturedeendent resistance of a comonent is

used urosefully" the comonent is called aresistance

thermometer or thermistor ! (A resistance thermometer

is made of metal" usually latinum" &hile a thermistor

is made of ceramic or olymer!)

6esistance thermometers and thermistors are

generally used in t&o &ays! 9irst" they can be used

as thermometers8 By measuring the resistance" the

temerature of the environment can be inferred!

$econd" they can be used in con'unction &ithGoule

heating (also called selfheating)8 /f a large current is

running through the resistor" the resistor1s temerature

rises and therefore its resistance changes! Therefore"

these comonents can be used in a circuitrotection

role similar to fuses" or for feedback in circuits" or for

many other uroses! /n general" selfheating can turn

a resistor into a nonlinear andhysteretic circuit

element! 9or more details see Thermistor$elfheating

effects!

/f the temerature 4 does not vary too much" a linear

aro5imation is tyically used8

&here is called the temperature coefficient of

resistance" is a fi5ed reference temerature

(usually room temerature)" and is the

resistance at temerature ! The arameter is

an emirical arameter fitted from measurement

data! Because the linear aro5imation is only an

aro5imation" is different for different referencetemeratures! 9or this reason it is usual to secify

the temerature that &as measured at &ith a

suffi5" such as " and the relationshi only holds

in a range of temeratures around the reference!=F>

http://en.wikipedia.org/wiki/Resistance_thermometerhttp://en.wikipedia.org/wiki/Resistance_thermometerhttp://en.wikipedia.org/wiki/Thermistorhttp://en.wikipedia.org/wiki/Thermometerhttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Fuse_(electrical)http://en.wikipedia.org/wiki/Fuse_(electrical)http://en.wikipedia.org/wiki/Feedbackhttp://en.wikipedia.org/wiki/Nonlinear_elementhttp://en.wikipedia.org/wiki/Hysteresishttp://en.wikipedia.org/wiki/Thermistor#Self-heating_effectshttp://en.wikipedia.org/wiki/Thermistor#Self-heating_effectshttp://en.wikipedia.org/wiki/Linear_approximationhttp://en.wikipedia.org/wiki/Linear_approximationhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-9http://en.wikipedia.org/wiki/Resistance_thermometerhttp://en.wikipedia.org/wiki/Resistance_thermometerhttp://en.wikipedia.org/wiki/Thermistorhttp://en.wikipedia.org/wiki/Thermometerhttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Fuse_(electrical)http://en.wikipedia.org/wiki/Feedbackhttp://en.wikipedia.org/wiki/Nonlinear_elementhttp://en.wikipedia.org/wiki/Hysteresishttp://en.wikipedia.org/wiki/Thermistor#Self-heating_effectshttp://en.wikipedia.org/wiki/Thermistor#Self-heating_effectshttp://en.wikipedia.org/wiki/Linear_approximationhttp://en.wikipedia.org/wiki/Linear_approximationhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-9


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The temerature coefficient is tyically

RU*HN SN* to RU*HN SN* for metals near room

temerature! /t is usually negative for

semiconductors and insulators" &ith highly variable

magnitude!=*H>

train dependence

Main article: 'train gauge

Gust as the resistance of a conductor deends

uon temerature" the resistance of a conductor

deends uon strain! By lacing a conductor

under tension (a form of stress that leads to strain

in the form of stretching of the conductor)" the

length of the section of conductor under tension

increases and its crosssectional area decreases!

Both these effects contribute to increasing the

resistance of the strained section of conductor!

nder comression (strain in the oosite

direction)" the resistance of the strained section of

conductor decreases! $ee the discussion on strain

gauges for details about devices constructed to

take advantage of this effect!

%ight illumination

dependence

Main articles: 2hotoresistor and 2hotoconductivity

$ome resistors" articularly those made

from semiconductors" e5hibit photoconductivity "

meaning that their resistance changes &hen light

is shining on them! Therefore they arecalled photoresistors (or light dependent resistors)!

These are a common tye of light detector !

Superconductivity

Main article: 'uperconductivity

http://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-10http://en.wikipedia.org/wiki/Strain_gaugehttp://en.wikipedia.org/wiki/Strain_(materials_science)http://en.wikipedia.org/wiki/Tension_(mechanics)http://en.wikipedia.org/wiki/Stress_(physics)http://en.wikipedia.org/wiki/Strain_gaugehttp://en.wikipedia.org/wiki/Strain_gaugehttp://en.wikipedia.org/wiki/Strain_gaugehttp://en.wikipedia.org/wiki/Photoresistorhttp://en.wikipedia.org/wiki/Photoconductivityhttp://en.wikipedia.org/wiki/Photoconductivityhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Photoconductivityhttp://en.wikipedia.org/wiki/Photoresistorhttp://en.wikipedia.org/wiki/Photodetectorhttp://en.wikipedia.org/wiki/Superconductivityhttp://en.wikipedia.org/wiki/Electrical_resistance_and_conductance#cite_note-10http://en.wikipedia.org/wiki/Strain_gaugehttp://en.wikipedia.org/wiki/Strain_(materials_science)http://en.wikipedia.org/wiki/Tension_(mechanics)http://en.wikipedia.org/wiki/Stress_(physics)http://en.wikipedia.org/wiki/Strain_gaugehttp://en.wikipedia.org/wiki/Strain_gaugehttp://en.wikipedia.org/wiki/Photoresistorhttp://en.wikipedia.org/wiki/Photoconductivityhttp://en.wikipedia.org/wiki/Semiconductorhttp://en.wikipedia.org/wiki/Photoconductivityhttp://en.wikipedia.org/wiki/Photoresistorhttp://en.wikipedia.org/wiki/Photodetectorhttp://en.wikipedia.org/wiki/Superconductivity


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$uerconductors are materials that have e5actly

-ero resistance and infinite conductance" because

they can have VWH and /XH! This also means there

is no 'oule heating" or in other &ords

no dissiation of electrical energy! Therefore" ifsuerconductive &ire is made into a closed loo"

current flo&s around the loo forever!

$uerconductors re3uire cooling to temeratures

near , S &ith li3uid helium for most metallic

suerconductors like +b$n alloys" or cooling to

temeratures near S &ith li3uid nitrogen for the

e5ensive" brittle and delicate ceramic high

temerature suerconductors! +evertheless" there

are many technological alications ofsuerconductivity" including suerconducting

magnets!

Diference between Ammeter and Voltmeter

Ammeter Voltmeter

Connection It is to be connected in series

mode

It is to be connected in parallel

mode

Resistance It has comparatively low resistance It has high resistance

Uses It is used to find the amount of

current flowing in the circuit

It is used to find the potential

difference in the circuit

Circuit Circuit must be disconnected in

order to attach the ammeter

Circuit does not need to be

disconnected

Accuracy Considered as less accurate Considered as more accurate

compared to ammeter

Voltage, Current, Resistance, and Ohm's Low

Electricity Basics

When 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

http://en.wikipedia.org/wiki/Superconductorhttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Dissipationhttp://en.wikipedia.org/wiki/Liquid_heliumhttp://en.wikipedia.org/wiki/Liquid_heliumhttp://en.wikipedia.org/wiki/Niobiumhttp://en.wikipedia.org/wiki/Tinhttp://en.wikipedia.org/wiki/Liquid_nitrogenhttp://en.wikipedia.org/wiki/Liquid_nitrogenhttp://en.wikipedia.org/wiki/High_temperature_superconductorshttp://en.wikipedia.org/wiki/High_temperature_superconductorshttp://en.wikipedia.org/wiki/High_temperature_superconductorshttp://en.wikipedia.org/wiki/Technological_applications_of_superconductivityhttp://en.wikipedia.org/wiki/Technological_applications_of_superconductivityhttp://en.wikipedia.org/wiki/Superconducting_magnethttp://en.wikipedia.org/wiki/Superconducting_magnethttp://en.wikipedia.org/wiki/Superconductorhttp://en.wikipedia.org/wiki/Joule_heatinghttp://en.wikipedia.org/wiki/Dissipationhttp://en.wikipedia.org/wiki/Liquid_heliumhttp://en.wikipedia.org/wiki/Niobiumhttp://en.wikipedia.org/wiki/Tinhttp://en.wikipedia.org/wiki/Liquid_nitrogenhttp://en.wikipedia.org/wiki/High_temperature_superconductorshttp://en.wikipedia.org/wiki/High_temperature_superconductorshttp://en.wikipedia.org/wiki/Technological_applications_of_superconductivityhttp://en.wikipedia.org/wiki/Technological_applications_of_superconductivityhttp://en.wikipedia.org/wiki/Superconducting_magnethttp://en.wikipedia.org/wiki/Superconducting_magnet


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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 Ohm

Covered in this Tutorial

• How electrical charge relates to voltage, current, and resistance.

• What voltage, current, and resistance are.

• What Ohm’s 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 Charge

Electricity 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:

• Voltage is the difference in charge between two points.

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• Current is the rate at which charge is flowing.

• Resistance is a material’s tendency to resist the flow of charge (current).

So, when we talk about these values, we’re 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 Ohm was a Bavarian scientist who studied electricity. Ohm starts by describing a unit of

resistance that is defined by current and voltage. So, let’s start with voltage and go from there.

Voltage

We 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 ismeasured 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 (don’t panic if this makes no

sense, all will be explained). The unit “volt” is named after the Italian physicistAlessandro Volta who

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 water pressure, and current

is represented by the waterflow. So for this analogy, remember:

• Water = Charge

• Pressure = Voltage

• Flow = Current

Consider 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.

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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.

Current

We 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”.

Let’s 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 ra

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Definition of Transformer