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Ohm

2007 Schools Wikipedia Selection. Related subjects: Electricity and
Electronics

   A multimeter can be used to measure resistance in ohms. It can also be
   used to measure capacitance, voltage, amperage, ect.
   A multimeter can be used to measure resistance in ohms. It can also be
   used to measure capacitance, voltage, amperage, ect.
   Several resistors. Their resistance, in ohms, is marked using a color
   code.
   Several resistors. Their resistance, in ohms, is marked using a colour
   code.

   The ohm (symbol: Ω) is the SI unit of electrical impedance or, in the
   direct current case, electrical resistance, named after Georg Ohm.

Definition

   An ohm is the electrical resistance offered by a current-carrying
   element that produces a voltage drop of one volt when a current of one
   ampere is flowing through it.

          \Omega = \dfrac{\mbox{V}}{\mbox{A}} = \dfrac{\mbox{m}^2 \cdot
          \mbox{kg}}{\mbox{s}^{3} \cdot \mbox{A}^2}

Explanation

   R is one ohm if V = one volt and I = one ampere
   R is one ohm if V = one volt and I = one ampere

   By definition from Ohm's Law, a device has a resistance of one ohm if a
   voltage of one volt causes a current of one ampere to flow (R = V/I).
   Alternatively and equivalently, a device that dissipates one watt of
   power with one ampere of current flowing through it has a resistance of
   one ohm (R = P / I^ 2).

   Since 1990, the ohm has been maintained internationally using the
   quantum Hall effect, where a conventional value is used for the '
   von-Klitzing constant', fixed by the 18th General Conference on Weights
   and Measures as R[{K-90}] = 25812.807 Ω.

   The complex quantity impedance is a generalisation of resistance. Its
   real part is resistance and its imaginary part is reactance. Impedance,
   resistance and reactance all have units of ohms.

   The symbol for the ohm is the Greek capital letter omega (Ω). If the
   Greek letter cannot be used, the word ohm is used instead. The various
   guides for the use of the International System of Units do not
   explicitly forbid the elision of the final "o" of some SI prefixes,
   although there is nothing in them to suggest that it is allowable,
   either. As a result, one is just about as likely to see "kilohm",
   "kiloohm" and even "kilo-ohm", and the same holds true for hecto-,
   micro-, nano-, pico-, femto-, atto-, zepto-, and yocto-. The only other
   SI unit to suffer from this kind of orthographic uncertainty is the
   ampere. In the particular case of the ohm, one even sees the "a"
   prefixes lose that vowel: hence megohm and gigohm. Higher prefixes are
   rarely used with ohm. In the other direction, milliohms (or millohms)
   are seen where the resistance of cables, etc., are measured.

   Units of ohms, kilohms (10^3 Ω) and megohms (10^6 Ω) are used in
   electronic design documentation. On schematic diagrams and parts lists
   kilohms are abbreviated "K" and megohms are abbreviated "M". Thus, 33
   kilohms would be rendered as 33K, and 5.1 megohms would be 5.1M.
   Another commonly used convention is that the multiplier is used to
   replace the decimal point, so that 5.1 megohms can also be represented
   as 5M1. This convention is used because a decimal point can be
   difficult to see in small or cluttered print. Values less than 1K are
   rendered either (a) without any symbol, or (b) with an "R", following
   the number; so 680 ohms can be shown as 680 or 680R. Resistors are
   usually identified by a reference designator, R, and a cardinal number,
   e.g., R12.

Conversions

   A measurement in ohms is the reciprocal of a measurement in siemens,
   the SI unit of electrical conductance. Note that 'siemens' is both
   singular and plural. The non-SI unit, the mho (simply put, ohm written
   backwards), is equivalent to siemens but is mostly obsolete and rarely
   used.

   Retrieved from " http://en.wikipedia.org/wiki/Ohm"
   This reference article is mainly selected from the English Wikipedia
   with only minor checks and changes (see www.wikipedia.org for details
   of authors and sources) and is available under the GNU Free
   Documentation License. See also our Disclaimer.
