Electric power 1

Electric power

Electric power is transmitted on overhead lineslike these, and also on underground high voltage

cables.

Electric power is the rate at which electric energy is transferred by anelectric circuit. The SI unit of power is the watt, one joule per second.

Electric power is usually produced by electric generators, but can alsobe supplied by sources such as electric batteries. It is generallysupplied to businesses and homes by the electric power industrythrough an electric power grid. Electric power is usually sold by thekilowatt hour (3.6 MJ) which is the product of power in kilowattsmultiplied by running time in hours. Electric utilities measure powerusing an electricity meter, which keeps a running total of the electricenergy delivered to a customer.

Definition

Electric power, like mechanical power, is the rate of doing work,measured in watts, and represented by the letter P. The term wattage isused colloquially to mean "electric power in watts." The electric powerin watts produced by an electric current I consisting of a charge of Qcoulombs every t seconds passing through an electric potential(voltage) difference of V is

whereQ is electric charge in coulombst is time in secondsI is electric current in amperesV is electric potential or voltage in volts

An alternate way to derive this formula is to note that voltage is defined as , the amount of work that aunit charge (one coulomb) does when it moves between the two terminals, and the current is defined as ,the number of coulombs flowing per second, so

Electric power 2

Explanation

Animation showing electricload

Electric power is transformed to other forms of power when electric charges movethrough an electric potential (voltage) difference, which occurs in electricalcomponents in electric circuits. From the standpoint of electric power, components inan electric circuit can be divided into two categories:

• Passive devices or loads: When electric charges move through a potentialdifference from a high voltage to a low voltage, that is conventional current(positive charge) moves from the positive terminal to the negative, the potentialenergy of the charges is converted to kinetic energy, which performs work on thedevice. Devices in which this occurs are called passive devices or loads; theyconsume electric power from the circuit, converting it to other forms such asmechanical work, heat, light, etc. Examples are electrical appliances, such as lightbulbs, electric motors, and electric heaters.

Animation showing powersource

• Active devices or power sources: If the charges are forced by an outside force tomove through the device in the direction from a lower electric potential to a higher,so positive charge moves from the negative to the positive terminal, work is beingdone on the charges, so energy is being converted to electric potential energy fromsome other type of energy, such as mechanical energy or chemical energy. Devicesin which this occurs are called active devices or power sources; sources of electriccurrent, such as electric generators and batteries.

Some devices can be either a source or a load, depending on the voltage or currentthrough them. For example, a rechargeable battery acts as a source when it providespower to a circuit, but as a load when it is connected to a battery charger and is beingrecharged.

Passive sign conventionMain article: Passive sign conventionSince electric power can flow in either direction, either into or out of a component, a convention is needed for whichdirection represents positive power flow. Electric power flowing out of a circuit into a component is arbitrarilydefined to have a positive sign, while power flowing into a circuit from a component is defined to have a negativesign. Thus passive components have positive power consumption. This is called the passive sign convention.

Resistive circuitsIn the case of resistive (Ohmic, or linear) loads, Joule's law can be combined with Ohm's law (V = I·R) to producealternative expressions for the dissipated power:

where R is the electrical resistance.

Electric power 3

Alternating currentMain article: AC powerIn alternating current circuits, energy storage elements such as inductance and capacitance may result in periodicreversals of the direction of energy flow. The portion of power flow that, averaged over a complete cycle of the ACwaveform, results in net transfer of energy in one direction is known as real power (also referred to as active power).That portion of power flow due to stored energy, that returns to the source in each cycle, is known as reactive power.The real power P in watts consumed by a device is given by

whereVp is the peak voltage in voltsIp is the peak current in amperesVrms is the root-mean-square voltage in voltsIrms is the root-mean-square current in amperesθ is the phase angle between the current and voltage sine waves

Power triangle: The components of AC power

The relationship between real power, reactive power and apparentpower can be expressed by representing the quantities as vectors. Realpower is represented as a horizontal vector and reactive power isrepresented as a vertical vector. The apparent power vector is thehypotenuse of a right triangle formed by connecting the real andreactive power vectors. This representation is often called the powertriangle. Using the Pythagorean Theorem, the relationship among real,reactive and apparent power is:

Real and reactive powers can also be calculated directly from the apparent power, when the current and voltage areboth sinusoids with a known phase angle θ between them:

The ratio of real power to apparent power is called power factor and is a number always between 0 and 1. Where thecurrents and voltages have non-sinusoidal forms, power factor is generalized to include the effects of distortion

Electromagnetic fieldsElectrical energy flows wherever electric and magnetic fields exist together and fluctuate in the same place. Thesimplest example of this is in electrical circuits, as the preceding section showed. In the general case, however, thesimple equation P = IV must be replaced by a more complex calculation, the integral of the cross-product of theelectrical and magnetic field vectors over a specified area, thus:

The result is a scalar since it is the surface integral of the Poynting vector.

Electric power 4

Electric power supplyMain article: power supply

Electricity generationMain article: electricity generationThe fundamental principles of electricity generation were discovered during the 1820s and early 1830s by the Britishscientist Michael Faraday. His basic method is still used today: electricity is generated by the movement of a loop ofwire, or disc of copper between the poles of a magnet.[1]

For electric utilities, it is the first process in the delivery of electricity to consumers. The other processes, electricitytransmission, distribution, and electrical power storage and recovery using pumped-storage methods are normallycarried out by the electric power industry.Electricity is most often generated at a power station by electromechanical generators, primarily driven by heatengines fueled by chemical combustion or nuclear fission but also by other means such as the kinetic energy offlowing water and wind. There are many other technologies that can be and are used to generate electricity such assolar photovoltaics and geothermal power.

Battery powerMain article: battery (electricity)A battery is a device consisting of one or more electrochemical cells that convert stored chemical energy intoelectrical energy.[2] Since the invention of the first battery (or "voltaic pile") in 1800 by Alessandro Volta andespecially since the technically improved Daniell cell in 1836, batteries have become a common power source formany household and industrial applications. According to a 2005 estimate, the worldwide battery industry generatesUS$48 billion in sales each year,[3] with 6% annual growth.There are two types of batteries: primary batteries (disposable batteries), which are designed to be used once anddiscarded, and secondary batteries (rechargeable batteries), which are designed to be recharged and used multipletimes. Batteries come in many sizes, from miniature cells used to power hearing aids and wristwatches to batterybanks the size of rooms that provide standby power for telephone exchanges and computer data centers.

Electric power industryMain article: electric power industryThe electric power industry provides the production and delivery of power, in sufficient quantities to areas that needelectricity, through a grid connection. The grid distributes electrical energy to customers. Electric power is generatedby central power stations or by distributed generation.Many households and businesses need access to electricity, especially in developed nations, the demand beingscarcer in developing nations. Demand for electricity is derived from the requirement for electricity in order tooperate domestic appliances, office equipment, industrial machinery and provide sufficient energy for both domesticand commercial lighting, heating, cooking and industrial processes. Because of this aspect of the industry, it isviewed as a public utility as infrastructure.

Electric power 5

Notes[1] 'The Institution of Engineering & Technology: Michael Faraday' (http:/ / www. theiet. org/ about/ libarc/ archives/ biographies/ faraday. cfm)[2] "battery" (def. 4b) (http:/ / www. merriam-webster. com/ dictionary/ battery), Merriam-Webster Online Dictionary (2009). Retrieved 25 May

2009.[3] Power Shift: DFJ on the lookout for more power source investments (http:/ / www. dfj. com/ cgi-bin/ artman/ publish/ article_141. shtml).

Draper Fisher Jurvetson. Retrieved 20 November 2005.

References• Reports on August 2003 Blackout, North American Electric Reliability Council website (http:/ / www. nerc. com/

~filez/ blackout. html)• Croft, Terrell; Summers, Wilford I. (1987). American Electricians' Handbook (Eleventh ed.). New York:

McGraw Hill. ISBN 0-07-013932-6.• Fink, Donald G.; Beaty, H. Wayne (1978). Standard Handbook for Electrical Engineers (Eleventh ed.). New

York: McGraw Hill. ISBN 0-07-020974-X.

External links• U.S. Department of Energy: Electric Power (http:/ / www. energy. gov/ energysources/ electricpower. htm)• GlobTek, Inc. Glossary of Electric power Power Supply Terms (http:/ / en. globtek. com/ glossary. php#.

UvZfsRBdWE8)

Article Sources and Contributors 6

Article Sources and ContributorsElectric power  Source: http://en.wikipedia.org/w/index.php?oldid=623080736  Contributors: Achraf52, Aellio26, Alansohn, Alberto Orlandini, Alcartur, Almeria.raul, Andyjsmith,Arcturus4669, Atlant, Ayyoubzadeh, B3Vmax, BD2412, BenB4, Bert Hickman, Biscuittin, Bjankuloski06en, Borgx, Brufydsy, Burakbe, C J Cowie, Canol, Cbdorsett, Ceyockey,CharlesHBennett, Chasingsol, Chetvorno, Cometstyles, Connormah, Cookie4869, Couposanto, Courcelles, Cvismeg, Cwobeel, DA3N, DMacks, DV8 2XL, DVdm, Da best editor, Dark Prince92, Deli nk, Demonuk, Dicklyon, Dirac1933, Donner60, Dpsypher, Dual Freq, El C, EncMstr, Epbr123, Erebus555, Essicajay, Everyking, Excirial, Fat&Happy, Favonian, Fgnievinski, Frosty,Fæ, Giftlite, GlassCobra, GliderMaven, Godefroy, Gralo, Hadrianheugh, Hazmat2, HeKeRnd, Headbomb, Heron, Hold6448, Huch393011, Iadrian yu, Icairns, Icep, Indefatigable,Itaintnothinbutathang, J.delanoy, Ja 62, JaconaFrere, Jadlerstein, JavaTenor, JayJay, Jeff G., Jmh649, Jo3sampl, John of Reading, Johnathanboy89, Johnymac619, Joshuatitsworth,Karlawilloughby, Kenyon, Kreidos, L'Aquatique, Lambtron, Lampak, Legion fi, Lenko, Levineps, Light current, Linas, LiuyuanChen, LogX, Luna Santin, M jurrens, MER-C, MGTom, Mac,Manop, Mdd4696, Meisterkoch, Meters, Michael Anon, Micru, Mierlo, MischiefMaker42, Mni9791, Msadaghd, Musant, Mwtoews, Nachoman-au, Narayansg, Neopentrix, Nilfanion, Nneonneo,Nposs, NuclearEnergy, Nukeless, Old Moonraker, Omegatron, Oxymoron83, P1ayer, Paolo.dL, Paste, Paverider, Paweł Ziemian, Pete mech eng, Pharaoh of the Wizards, Philip Trueman,Pinethicket, Piotrus, Primasz, Purgatory Fubar, QrTTf7fH, Quaisaq, Quibik, RL0919, Randolph02, Reatlas, Recognizance, Rgoodermote, Rhorn, Rodhullandemu, Rogerbrent, RossPatterson,RottweilerCS, RoyBoy, SEWilco, SchreyP, Shaon24, Shiftchange, Sigmundg, Soliloquial, Spamsara, Spinningspark, Srleffler, Stephenchou0722, Switchercat, Talkstosocks, Teapeat, The ThingThat Should Not Be, Tholme, Tommy2010, Trigga556557, Twoborg, Ugog Nizdast, Vegaswikian, Vipinhari, Voltageman, Walor, Wasami007, Werieth, Whquaint, William Avery, Wow098,Wtshymanski, Xcalizorz, Xxglennxx, Zanimum, Ziiiziii, Владимир Пацюк, 不 寐 听 江, 321 anonymous edits

Image Sources, Licenses and ContributorsFile:Electricalgrid.jpg  Source: http://en.wikipedia.org/w/index.php?title=File:Electricalgrid.jpg  License: Public Domain  Contributors: ShiftChangeFile:Electric load animation 2.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Electric_load_animation_2.gif  License: Creative Commons Zero  Contributors: User:ChetvornoFile:Electric power source animation 2.gif  Source: http://en.wikipedia.org/w/index.php?title=File:Electric_power_source_animation_2.gif  License: Creative Commons Zero  Contributors:User:ChetvornoFile:Power Triangle 01.svg  Source: http://en.wikipedia.org/w/index.php?title=File:Power_Triangle_01.svg  License: Creative Commons Attribution-Sharealike 3.0  Contributors: Hazmat2

LicenseCreative Commons Attribution-Share Alike 3.0//creativecommons.org/licenses/by-sa/3.0/