Ом (јединица) — разлика између измена
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{{Short description|СИ изведена јединица за електрични отпор}}{{рут}}
{{Infobox Unit
| name = Ом
| image = [[File:Leeds and Northrup one ohm standard resistance.jpg|240px]]
| caption = A laboratory one-ohm standard resistor, circa 1917.
| standard = [[SI derived unit]]
| quantity = [[Electrical resistance]]
| symbol= Ω
| namedafter = [[Georg Ohm]]
| extralabel = Derivation
| extradata = Ω = V/A
| units1 = [[SI base unit]]s
| inunits1 = [[kilogram|kg]]⋅[[metre|m]]<sup>2</sup>⋅[[second|s]]<sup>−3</sup>⋅[[ampere|A]]<sup>−2</sup>
}}
'''Ом''' је [[Међународни систем јединица|СИ]] [[сИ изведене јединице|изведена јединица]] [[електрични отпор|електричног отпора]] (изведена од [[ампер]]а и [[ват]]а). Симбол је [[грчко писмо|грчко]] велико слово [[Омега (слово)|омега]] (Ω). Ом је добио име по [[Георг Ом|Георгу Ому]], [[Немачка|немачком]] [[физика|физичару]] који је открио везу између [[Електрични напон|напон]]а и јачине [[Електрична струја|струје]], изражену у [[Омов закон|Омовом закону]].
По дефиницији из Омовог закона, [[справа]] има отпор од једног ома ако напон од једног волта изазива струју од једног ампера да тече (''R = V/I''). Исто тако, справа која троши један ват снаге са једним ампером струје која тече кроз ту справу има отпор од једног ома (''R = P/I''<sup>2</sup>).
Линија 10 ⟶ 25:
'''R<sub>{K-90}</sub>''' = 25812,807 Ω.
== Дефиниција ==
[[File:Electronic multi meter.jpg|thumb|One of the functions of many types of [[multimeter]]s is the measurement of resistance in ohms.]]
The ohm is defined as an electrical resistance between two points of a conductor when a constant potential difference of one [[volt]], applied to these points, produces in the conductor a current of one [[ampere]], the conductor not being the seat of any [[electromotive force]].<ref name="BIPM_SI8"/>
:<math>\Omega = \dfrac{\text{V}}{\text{A}} = \dfrac{1}{\text{S}} = \dfrac{\text{W}}{\text{A}^2} = \dfrac{\text{V}^2}{\text{W}} = \dfrac{\text{s}}{\text{F}} = \dfrac{\text{H}}{\text{s}} = \dfrac{\text{J} {\cdot} \text{s}}{\text{C}^2} = \dfrac{\text{kg} {\cdot} \text{m}^2}{\text{s} {\cdot} \text{C}^2} = \dfrac{\text{J}}{\text{s} {\cdot} \text{A}^2}=\dfrac{\text{kg}{\cdot}\text{m}^2}{\text{s}^3 {\cdot} \text{A}^2}</math>
in which the following units appear: [[volt]] (V), [[ampere]] (A), [[Siemens (unit)|siemens]] (S), [[watt]] (W), [[second]] (s), [[farad]] (F), [[Henry (unit)|henry]] (H), [[joule]] (J), [[coulomb]] (C), [[kilogram]] (kg), [[metre|and metre]] (m).
Following the [[2019 redefinition of the SI base units]], in which the ampere and the kilogram were redefined in terms of [[Physical constant|fundamental constants]], the ohm is affected by a very small scaling in measurement.
In many cases the resistance of a conductor is approximately constant within a certain range of voltages, temperatures, and other parameters. These are called [[linear]] [[resistor]]s. In other cases resistance varies, such as in the case of the [[thermistor]], which exhibits a strong dependence of its resistance with temperature.
A vowel of the prefixed units kiloohm and megaohm is commonly omitted, producing kilohm and megohm.<ref name="SI10-2002"/><ref name="Thompson-Taylor_2008"/><ref name="NIST_2016"/><ref name="Aubrecht-French-Iona_2012"/>
== Историја ==
The rapid rise of electrotechnology in the last half of the 19th century created a demand for a rational, coherent, consistent, and international system of units for electrical quantities. Telegraphers and other early users of electricity in the 19th century needed a practical standard unit of measurement for resistance. Resistance was often expressed as a multiple of the resistance of a standard length of telegraph wires; different agencies used different bases for a standard, so units were not readily interchangeable. Electrical units so defined were not a coherent system with the units for energy, mass, length, and time, requiring conversion factors to be used in calculations relating energy or power to resistance.<ref name="Hunt_1994"/>
Various artifact standards were proposed as the definition of the unit of resistance. In 1860 [[Werner Siemens]] (1816–1892) published a suggestion for a reproducible resistance standard in [[Johann Christian Poggendorff|Poggendorff's]] ''[[Annalen der Physik und Chemie]]''.<ref name="Siemens_1860"/> He proposed a column of pure mercury, of one square millimeter cross section, one metre long: [[Siemens mercury unit]]. However, this unit was not coherent with other units. One proposal was to devise a unit based on a mercury column that would be coherent – in effect, adjusting the length to make the resistance one ohm. Not all users of units had the resources to carry out [[metrology]] experiments to the required precision, so working standards notionally based on the physical definition were required.
In 1861, [[Josiah Latimer Clark|Latimer Clark]] (1822–1898) and [[Charles Tilston Bright|Sir Charles Bright]] (1832–1888) presented a paper at the [[British Association for the Advancement of Science]] meeting <ref>{{cite journal
|first1=Latimer|last1=Clark|author1-link=Josiah Latimer Clark
|first2=Sir Charles|last2=Bright |author2-link=Charles Tilston Bright
|title=Measurement of Electrical Quantities and Resistance|journal=[[The Electrician]]|date=9 November 1861|volume=1|issue=1|pages=3–4|url=https://books.google.com/books?id=7BdbAAAAYAAJ&q=ohma&pg=PA3|access-date=27 February 2014}}</ref> suggesting that standards for electrical units be established and suggesting names for these units derived from eminent philosophers, 'Ohma', 'Farad' and 'Volt'. The [[British Association for the Advancement of Science|BAAS]] in 1861 appointed a committee including [[James Clerk Maxwell|Maxwell]] and [[William Thomson, 1st Baron Kelvin|Thomson]] to report upon standards of electrical resistance.<ref>{{cite conference
| title = Report of the Thirty-First Meeting of the British Association for the Advancement of Science; held at Manchester in September 1861
| date = September 1861
| pages=xxxix–xl
| url = https://www.biodiversitylibrary.org/item/93052#page/44/mode/2up }}</ref> Their objectives were to devise a unit that was of convenient size, part of a complete system for electrical measurements, coherent with the units for energy, stable, reproducible and based on the French metrical system.<ref>{{cite conference
|title=Provisional Report of the Committee appointed by the British Association on Standards of Electrical Resistance
|date=September 1862
|conference= Thirty-second Meeting of the British Association for the Advancement of Science
|first1 =A. |last1 = Williamson |author1-link = Alexander William Williamson
|first2 =C. |last2 = Wheatstone |author2-link = Charles Wheatstone
|first3 =W. |last3 = Thomson |author3-link = William Thomson, 1st Baron Kelvin
|first4 =W. H. |last4 = Miller |author4-link = William Hallowes Miller
|first5 =A. |last5 = Matthiessen |author5-link = Augustus Matthiessen
|first6 =Fleeming |last6 = Jenkin |author6-link = Fleeming Jenkin
|publisher= John Murray
|location= London
|pages = 125–163
|url= https://www.biodiversitylibrary.org/page/29361871#page/192/mode/2up
|access-date= 27 February 2014}}</ref> In the third report of the committee, 1864, the resistance unit is referred to as "B.A. unit, or Ohmad".<ref>{{cite conference
|title=Report of the Committee on Standards of Electrical Resistance
|date=September 1864
|conference= Thirty-fourth Meeting of the British Association for the Advancement of Science
|first1 =A. |last1 = Williamson |author1-link = Alexander William Williamson
|first2 =C. |last2 = Wheatstone |author2-link = Charles Wheatstone
|first3 =W. |last3 = Thomson |author3-link = William Thomson, 1st Baron Kelvin
|first4 =W. H. |last4 = Miller |author4-link = William Hallowes Miller
|first5 =A. |last5 = Matthiessen |author5-link = Augustus Matthiessen
|first6 =Fleeming |last6 = Jenkin |author6-link = Fleeming Jenkin
|first7 =Charles |last7 = Bright |author7-link =Charles Tilston Bright
|first8 =James Clerk |last8 = Maxwell |author8-link = James Clerk Maxwell
|first9 =Carl Wilhelm |last9 = Siemens |author9-link = Carl Wilhelm Siemens
|first10 = Balfour |last10 = Stewart |author10-link = Balfour Stewart
|first11 = James Prescott |last11 = Joule |author11-link = James Prescott Joule
|first12 = C. F. |last12 = Varley|author12-link = C. F. Varley
|publisher= John Murray
|location= London
|page = Foldout facing page 349
|url= https://www.biodiversitylibrary.org/item/93072#page/434/mode/1up
|access-date= 27 February 2014}}</ref> By 1867 the unit is referred to as simply ''ohm''.<ref>{{cite conference
|title=Report of the Committee on Standards of Electrical Resistance
|date=September 1867
|conference= Thirty-seventh Meeting of the British Association for the Advancement of Science
|first1 = A. |last1 = Williamson |author1-link = Alexander William Williamson
|first2 = C. |last2 = Wheatstone |author2-link = Charles Wheatstone
|first3 = W. |last3 = Thomson |author3-link = William Thomson, 1st Baron Kelvin
|first4 = W. H. |last4 = Miller |author4-link = William Hallowes Miller
|first5 = A. |last5 = Matthiessen |author5-link = Augustus Matthiessen
|first6 = Fleeming |last6 = Jenkin |author6-link = Fleeming Jenkin
|first7 = Charles |last7 = Bright |author7-link =Charles Tilston Bright
|first8 = James Clerk |last8 = Maxwell |author8-link = James Clerk Maxwell
|first9 = Carl Wilhelm |last9 = Siemens |author9-link = Carl Wilhelm Siemens
|first10 = Balfour |last10 = Stewart |author10-link = Balfour Stewart
|first11 = C. F. |last11 = Varley|author11-link = C. F. Varley
|first12 = G. C. |last12 = Foster
|first13 = Latimer |last13 = Clark |author13-link = Josiah Latimer Clark
|first14 = D. |last14 = Forbes
|first15 = Charles |last15 = Hockin
|first16 = James Prescott |last16 = Joule |author16-link = James Prescott Joule
|publisher= John Murray
|location= London
|page = 488
|url= https://www.biodiversitylibrary.org/item/93115#page/578/mode/1up
|access-date= 27 February 2014}}</ref>
The B.A. ohm was intended to be 10<sup>9</sup> CGS units but owing to an error in calculations the definition was 1.3% too small. The error was significant for preparation of working standards.
On 21 September 1881 the ''[[International Electrical Congress|Congrès internationale des électriciens]]'' (international conference of electricians) defined a ''practical'' unit of ohm for the resistance, based on [[CGS]] units, using a mercury column 1 sq. mm. in cross-section, approximately 104.9 cm in length at 0 °C,<ref>{{cite web |url=http://ethw.org/System_of_Measurement_Units#Evolution_of_Electromagnetic_System_of_Units|title=System of measurement units|website=Engineering and Technology History Wiki|access-date=13 April 2018}}</ref> similar to the apparatus suggested by Siemens.
A ''legal'' ohm, a reproducible standard, was defined by the international conference of electricians at Paris in 1884 as the resistance of a mercury column of specified weight and 106 cm long; this was a compromise value between the B. A. unit (equivalent to 104.7 cm), the Siemens unit (100 cm by definition), and the CGS unit. Although called "legal", this standard was not adopted by any national legislation. The "international" ohm was recommended by unanimous resolution at the [[International Electrical Congress]] 1893 in Chicago.<ref name=eb11-p742/> The unit was based upon the ohm equal to 10<sup>9</sup> units of resistance of the [[CGS|C.G.S. system of electromagnetic units]]. The international ohm is represented by the resistance offered to an unvarying electric current in a mercury column of constant cross-sectional area 106.3 cm long of mass 14.4521 grams and 0 °C. This definition became the basis for the legal definition of the ohm in several countries. In 1908, this definition was adopted by scientific representatives from several countries at the International Conference on Electric Units and Standards in London.<ref name=eb11-p742>{{cite EB1911|wstitle= Units, Physical |volume= 27 | pages = 738–745; see page 742| quote= An Electrical Congress was held in Chicago, U.S.A. in August 1893, to consider......and at the last one held in London in October 1908 were finally adopted|last1= Fleming |first1= John Ambrose |author-link= John Ambrose Fleming }}</ref> The mercury column standard was maintained until the 1948 [[General Conference on Weights and Measures]], at which the ohm was redefined in absolute terms instead of as an artifact standard.
== Види још ==
*[[Омов закон]]
== Референце ==
{{Reflist|refs=
<ref name="BIPM_SI8">[http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf BIPM SI Brochure: Appendix 1, p. 144]</ref>
<ref name="Hunt_1994">{{cite journal |author-last=Hunt |author-first=Bruce J. |title=The Ohm Is Where the Art Is: British Telegraph Engineers and the Development of Electrical Standards |journal=Osiris |date=1994 |volume=9 |series=2 |pages=48–63 |url=https://liberalarts.utexas.edu/hps/publications/index.php |access-date=2014-02-27 |doi=10.1086/368729|s2cid=145557228|url-status=dead |archive-url=https://web.archive.org/web/20140308030456/https://webspace.utexas.edu/huntbj/1994-Ohm-Osiris.pdf |archive-date=2014-03-08}}</ref>
<ref name="NIST_2016">{{cite web |title=NIST Guide to the SI |id=Special Publication 811 |date=2016-08-25 |publisher=[[National Institute of Standards and Technology]] (NIST), Physical Measurement Laboratory |at=Chapter 9: Rules and Style Conventions for Spelling Unit Names, 9.3: Spelling unit names with prefixes |publication-place=Gaithersburg, Maryland, USA |url=https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-9-rules-and-style-conventions-spelling-unit-names |access-date=2021-01-31 |url-status=live |archive-url=https://web.archive.org/web/20210131024623/https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-9-rules-and-style-conventions-spelling-unit-names |archive-date=2021-01-31}} [http://physics.nist.gov/Pubs/SP811/sec09.html]</ref>
<ref name="Thompson-Taylor_2008">{{cite book |title=Guide for the Use of the International System of Units (SI) |chapter=Chapter 9.3 Spelling unit names with prefixes |author-first1=Ambler |author-last1=Thompson |author-first2=Barry N. |author-last2=Taylor |date= 2008 |edition=2nd corrected printing, 2008 |id={{CODEN|NSPUE3}}. NIST Special Publication 811 |publisher=[[National Institute of Standards and Technology]], U.S. Department of Commerce |location=Gaithersburg, Maryland, USA |url=https://physics.nist.gov/cuu/pdf/sp811.pdf |access-date=2021-01-31 |url-status=live |archive-url=https://web.archive.org/web/20210131024615/https://physics.nist.gov/cuu/pdf/sp811.pdf |archive-date=2021-01-31 |quote=Reference [[#SI10-2002|[6]]] points out that there are three cases in which the final vowel of an SI prefix is commonly omitted: megohm (not megaohm), kilohm (not kiloohm), and hectare (not hectoare). In all other cases in which the unit name begins with a vowel, both the final vowel of the prefix and the vowel of the unit name are retained and both are pronounced.}} (85 pages)</ref>
<ref name="SI10-2002">{{cite book |title=IEEE/ASTM SI 10-2002: IEEE/ASTM Standard for Use of the International System of Units (SI): The Modern Metric System. |others=SASB/SCC14 - SCC14 - Quantities, Units, and Letter Symbols |date=2002-12-30 |url=https://standards.ieee.org/standard/SI10-2002.html}}</ref>
<ref name="Aubrecht-French-Iona_2012">{{cite journal |author-first1=Gordon J. |author-last1=Aubrecht II |author-first2=Anthony P. |author-last2=French |author-first3=Mario |author-last3=Iona |date=2012-01-20 |title=About the International System of Units (SI) Part IV. Writing, Spelling, and Mathematics |journal=[[The Physics Teacher]] |volume=50 |issue=2 |doi=10.1119/1.3677278 |bibcode=2012PhTea..50...77A |pages=77–79}}</ref>
<ref name="Siemens_1860">{{cite journal |author-first=Werner |author-last=Siemens |author-link=Werner Siemens |periodical=[[Annalen der Physik und Chemie]] |title=Vorschlag eines reproducirbaren Widerstandsmaaßes |volume=186 |issue=5 |date=1860 |pages=1–20 |language=de |doi=10.1002/andp.18601860502 |bibcode=1860AnP...186....1S |url=https://zenodo.org/record/1423670}}</ref>
}}
== Литература ==
Линија 20 ⟶ 139:
{{refend}}
== Спољашње везе ==
{{Commons category|Ohm}}
* [http://www.fh-nuernberg.de/institutionen/bibliothek/bibsuche/texte_online_aufrufen/historische_buecher_der_gso_fh/werke_von_georg_simon_ohm/ Scanned books of Georg Simon Ohm at the library of the University of Applied Sciences Nuernberg]
* [http://www.bipm.fr/en/si/si_brochure/ Official SI brochure]
* [http://physics.nist.gov/Pubs/SP811/contents.html NIST Special Publication 811]
* [http://www.sizes.com/units/ohm.htm History of the ohm at sizes.com]
* [http://seaus.free.fr/spip.php?article964 History of the electrical units.]
{{СИ јединице}}
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