Direct current - wikipedia Direct current Unidirectional flow of electric charge `` LVDC '' redirects here. For the computer, see Saturn Launch Vehicle Digital Computer. This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. ( June 2009 ) Direct Current ( red line ). The vertical axis shows current or voltage and the horizontal ' t ' axis measures time and shows the zero value. Direct current ( DC ) is the unidirectional flow of electric charge. A battery is a good example of a DC power supply. Direct current may flow in a conductor such as a wire, but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams. The electric current flows in a constant direction, distinguishing it from alternating current ( AC ). A term formerly used for this type of current was galvanic current. The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage. Direct current may be obtained from an alternating current supply by use of a rectifier, which contains electronic elements ( usually ) or electromechanical elements ( historically ) that allow current to flow only in one direction. Direct current may be converted into alternating current with an inverter or a motor - generator set. Direct current is used to charge batteries and as power supply for electronic systems. Very large quantities of direct - current power are used in production of aluminum and other electrochemical processes. It is also used for some railways, especially in urban areas. High - voltage direct current is used to transmit large amounts of power from remote generation sites or to interconnect alternating current power grids. Part of a series of articles about Electromagnetism Electricity Magnetism Electrostatics Electric charge Static electricity Electric field Conductor Insulator Triboelectricity Electrostatic discharge Induction Coulomb 's law Gauss 's law Electric flux / potential energy Electric dipole moment Polarization density Magnetostatics Ampère 's law Magnetic field Magnetization Magnetic flux Biot -- Savart law Magnetic dipole moment Gauss 's law for magnetism Electrodynamics Lorentz force law Electromagnetic induction Faraday 's law Lenz 's law Displacement current Magnetic potential Maxwell 's equations Electromagnetic field Electromagnetic pulse Electromagnetic radiation Maxwell tensor Poynting vector Liénard -- Wiechert potential Jefimenko 's equations Eddy current London equations Mathematical descriptions of the electromagnetic field Electrical network Electric current Electric potential Voltage Resistance Ohm 's law Series circuit Parallel circuit Direct current Alternating current Electromotive force Capacitance Inductance Impedance Resonant cavities Waveguides Covariant formulation Electromagnetic tensor ( stress -- energy tensor ) Four - current Electromagnetic four - potential Scientists Ampère Coulomb Faraday Gauss Heaviside Henry Hertz Lorentz Maxwell Tesla Volta Weber Ørsted Contents 1 History 2 Various definitions 3 Circuits 4 Applications 4.1 Domestic 4.2 Automotive 4.3 Telecommunication 4.4 High - voltage power transmission 4.5 Other 5 See also 6 References 7 External links History Further information : History of electric power transmission Brush Electric Company 's central power plant with dynamos generating direct current to power arc lamps for public lighting in New York. Beginning operation in December 1880 at 133 West Twenty - Fifth Street, the high voltages it operated at allowed it to power a 2 - mile ( 3.2 km ) long circuit. Direct current was produced in 1800 by Italian physicist Alessandro Volta 's battery, his Voltaic pile. The nature of how current flowed was not understood. French physicist André - Marie Ampère conjectured that current travelled in one direction from positive to negative. When French instrument maker Hippolyte Pixii built the first dynamo electric generator in 1832, he found that as the magnet used passed the loops of wire each half turn, it caused the flow of electricity to reverse, generating an alternating current. At Ampère 's suggestion, Pixii later added a commutator, a type of `` switch '' where contacts on the shaft work with `` brush '' contacts to produce direct current. The late 1870s and early 1880s saw electricity starting to be generated at power stations. These were initially set up to power arc lighting ( a popular type of street lighting ) running on very high voltage ( usually higher than 3000 volt ) direct current or alternating current. This was followed by the wide spread use of low voltage direct current for indoor electric lighting in business and homes after inventor Thomas Edison launched his incandescent bulb based electric `` utility '' in 1882. Because of the significant advantages of alternating current over direct current in using transformers to raise and lower voltages to allow much longer transmission distances, direct current was replaced over the next few decades by alternating current in power delivery. In the mid-1950s, high - voltage direct current transmission was developed, and is now an option instead of long - distance high voltage alternating current systems. For long distance underseas cables ( e.g. between countries, such as NorNed ), this DC option is the only technically feasible option. For applications requiring direct current, such as third rail power systems, alternating current is distributed to a substation, which utilizes a rectifier to convert the power to direct current. Various definitions Types of direct current The term DC is used to refer to power systems that use only one polarity of voltage or current, and to refer to the constant, zero - frequency, or slowly varying local mean value of a voltage or current. For example, the voltage across a DC voltage source is constant as is the current through a DC current source. The DC solution of an electric circuit is the solution where all voltages and currents are constant. It can be shown that any stationary voltage or current waveform can be decomposed into a sum of a DC component and a zero - mean time - varying component ; the DC component is defined to be the expected value, or the average value of the voltage or current over all time. Although DC stands for `` direct current '', DC often refers to `` constant polarity ''. Under this definition, DC voltages can vary in time, as seen in the raw output of a rectifier or the fluctuating voice signal on a telephone line. Some forms of DC ( such as that produced by a voltage regulator ) have almost no variations in voltage, but may still have variations in output power and current. Circuits A direct current circuit is an electrical circuit that consists of any combination of constant voltage sources, constant current sources, and resistors. In this case, the circuit voltages and currents are independent of time. A particular circuit voltage or current does not depend on the past value of any circuit voltage or current. This implies that the system of equations that represent a DC circuit do not involve integrals or derivatives with respect to time. If a capacitor or inductor is added to a DC circuit, the resulting circuit is not, strictly speaking, a DC circuit. However, most such circuits have a DC solution. This solution gives the circuit voltages and currents when the circuit is in DC steady state. Such a circuit is represented by a system of differential equations. The solution to these equations usually contain a time varying or transient part as well as constant or steady state part. It is this steady state part that is the DC solution. There are some circuits that do not have a DC solution. Two simple examples are a constant current source connected to a capacitor and a constant voltage source connected to an inductor. In electronics, it is common to refer to a circuit that is powered by a DC voltage source such as a battery or the output of a DC power supply as a DC circuit even though what is meant is that the circuit is DC powered. Applications This section needs expansion with : more examples and additional citations ( see also introduction ). You can help by adding to it. ( October 2015 ) Domestic This symbol which can be represented with Unicode character U+ 2393 ( ​ ⎓ ​ ) is found on many electronic devices that either require or produce direct current. DC is commonly found in many extra-low voltage applications and some low - voltage applications, especially where these are powered by batteries or solar power systems ( since both can produce only DC ). Most electronic circuits require a DC power supply. Domestic DC installations usually have different types of sockets, connectors, switches, and fixtures from those suitable for alternating current. This is mostly due to the lower voltages used, resulting in higher currents to produce the same amount of power. It is usually important with a DC appliance to observe polarity, unless the device has a diode bridge to correct for this. Automotive Most automotive applications use DC. The alternator is an AC device which uses a rectifier to produce DC. Usually 12 V DC are used, but a few have a 6 V ( e.g. classic VW Beetle ) or a 42 V electrical system. Telecommunication Through the use of a DC - DC converter, higher DC voltages such as 48 V to 72 V DC can be stepped down to 36 V, 24 V, 18 V, 12 V, or 5 V to supply different loads. In a telecommunications system operating at 48 V DC, it is generally more efficient to step voltage down to 12 V to 24 V DC with a DC - DC converter and power equipment loads directly at their native DC input voltages, versus operating a 48 V DC to 120 V AC inverter to provide power to equipment. Many telephones connect to a twisted pair of wires, and use a bias tee to internally separate the AC component of the voltage between the two wires ( the audio signal ) from the DC component of the voltage between the two wires ( used to power the phone ). Telephone exchange communication equipment, such as DSLAMs, uses standard − 48 V DC power supply. The negative polarity is achieved by grounding the positive terminal of power supply system and the battery bank. This is done to prevent electrolysis depositions. High - voltage power transmission Main article : High - voltage direct current High - voltage direct current ( HVDC ) electric power transmission systems use DC for the bulk transmission of electrical power, in contrast with the more common alternating current systems. For long - distance transmission, HVDC systems may be less expensive and suffer lower electrical losses. Other Applications using fuel cells ( mixing hydrogen and oxygen together with a catalyst to produce electricity and water as byproducts ) also produce only DC. Light aircraft electrical systems are typically 12 V or 20 V DC. See also Electronics portal Energy portal Alternating current DC bias Electric current High - voltage direct current power transmission. Neutral direct - current telegraph system References Jump up ^ Andrew J. Robinson, Lynn Snyder - Mackler ( 2007 ). Clinical Electrophysiology : Electrotherapy and Electrophysiologic Testing ( 3rd ed. ). Lippincott Williams & Wilkins. p. 10. ISBN 978 - 0 - 7817 - 4484 - 3. Jump up ^ N.N. Bhargava and D.C. Kulshrishtha ( 1984 ). Basic Electronics & Linear Circuits. Tata McGraw - Hill Education. p. 90. ISBN 978 - 0 - 07 - 451965 - 3. Jump up ^ National Electric Light Association ( 1915 ). Electrical meterman 's handbook. Trow Press. p. 81. Jump up ^ Mel Gorman. `` Charles F. Brush and the First Public Electric Street Lighting System in America ''. Ohio History. Kent State University Press. Ohio Historical Society. 70 : 142. Jump up ^ Alessandro Giuseppe Antonio Anastasio Volta -- grants.hhp.coe.uh.edu Jump up ^ Jim Breithaupt, Physics, Palgrave Macmillan -- 2010, page 175 Jump up ^ `` Pixii Machine invented by Hippolyte Pixii, National High Magnetic Field Laboratory ''. Archived from the original on 2008 - 09 - 07. Retrieved 2008 - 06 - 12. Jump up ^ The First Form of Electric Light History of the Carbon Arc Lamp ( 1800 -- 1980s ) Jump up ^ Roger S. Amos, Geoffrey William Arnold Dummer ( 1999 ). Newnes Dictionary of Electronic ( 4th ed. ). Newnes. p. 83. ISBN 0 - 7506 - 4331 - 5. External links AC / DC : What 's the Difference ? -- PBS Learning Media DC And AC Supplies -- ITACA BNF : cb125728771 ( data ) GND : 4021242 - 7 LCCN : sh85041613 Retrieved from `` https://en.wikipedia.org/w/index.php?title=Direct_current&oldid=854747388 '' Categories : Electrical engineering Electric current Electric power Hidden categories : All articles with dead external links Articles with dead external links from April 2017 Articles with permanently dead external links Articles with short description Articles needing additional references from June 2009 All articles needing additional references Articles to be expanded from October 2015 All articles to be expanded Articles using small message boxes Wikipedia articles with BNF identifiers Wikipedia articles with GND identifiers Wikipedia articles with LCCN identifiers Talk Contents About Wikipedia Afrikaans አማርኛ Asturianu Azərbaycanca বাংলা Беларуская Български Boarisch Bosanski Català Čeština Dansk Deutsch Eesti Ελληνικά Español Esperanto Euskara فارسی Français Galego 한국어 Հայերեն हिन्दी Hrvatski Bahasa Indonesia Íslenska Italiano עברית Kreyòl ayisyen Latviešu Lietuvių Limburgs Magyar Македонски മലയാളം Bahasa Melayu Nederlands नेपाली 日本 語 Nordfriisk Norsk Norsk nynorsk Occitan ਪੰਜਾਬੀ پنجابی Polski Português Română Русский Scots Seeltersk Shqip Simple English Slovenčina Slovenščina Српски / srpski Srpskohrvatski / српскохрватски Basa Sunda Suomi Svenska Tagalog தமிழ் తెలుగు ไทย Türkçe Українська اردو Tiếng Việt Winaray Wolof 吴语 粵語 中文 66 more Edit links This page was last edited on 13 August 2018, at 14 : 14 ( UTC ). 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Question: is a 12 volt system ac or dc
Answer: DC