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Volt

2007 Schools Wikipedia Selection. Related subjects: Electricity and
Electronics

   Josephson junction array chip developed by NIST as a standard volt.
   Josephson junction array chip developed by NIST as a standard volt.

   The volt (symbol: V) is the SI derived unit of electric potential
   difference or electromotive force . It is named in honour of the
   Italian physicist Alessandro Volta (1745–1827), who invented the
   voltaic pile, the first modern chemical battery.

Definition

   The volt is defined as the potential difference across a conductor when
   a current of one ampere dissipates one watt of power. Hence, it is the
   base SI representation m^2 · kg · s^-3 · A^-1, which can be equally
   represented as one joule of energy per coulomb of charge, J/C.

          \mbox{V} = \dfrac{\mbox{W}}{\mbox{A}} =
          \dfrac{\mbox{J}}{\mbox{C}} = \dfrac{\mbox{m}^2 \cdot
          \mbox{kg}}{\mbox{s}^{3} \cdot \mbox{A}}

Josephson junction definition

   Since 1990 the volt is maintained internationally for practical
   measurement using the Josephson effect, where a conventional value is
   used for the Josephson constant, fixed by the 18th General Conference
   on Weights and Measures as

          K[{J-90}] = 0.4835979 GHz/µV.

Hydraulic analogy

   In the hydraulic analogy sometimes used to explain electric circuits by
   comparing them to water-filled pipes, voltage is likened to water
   pressure - it determines how fast the electrons will travel through the
   circuit. Current (in amperes), in the same analogy, is a measure of the
   volume of water that flows past a given point, the rate of which is
   determined by the voltage, and the total output measured in watts. The
   equation that brings all three components together is: volts × amperes
   = watts

Common voltages

   A multimeter can be used to measure the voltage between two points
   A multimeter can be used to measure the voltage between two points
   1.5 V C-cell batteries
   1.5 V C-cell batteries

   Nominal voltages of familiar sources:
     * Nerve cell action potential: around 30 mV
     * Single-cell, rechargeable NiMH or NiCd battery: 1.2 V
     * Mercury battery 1.355 V
     * Single-cell, non-rechargeable alkaline battery (e.g. AAA, AA, C and
       D cells): 1.5 V
     * Lithium polymer rechargeable battery: 3.75 V
     * Transistor-transistor logic (TTL) power supply: 5 V
     * PP3 battery: 9 V
     * Automobile electrical system: "12 V", about 11.8 V discharged, 12.8
       V charged, and 13.8-14.4 V while charging (vehicle running).
     * Household mains electricity: 240 V RMS in Australia, 230 V RMS in
       Europe, Asia and Africa, 120 V RMS in North America, 100 V RMS in
       Japan (see List of countries with mains power plugs, voltages and
       frequencies)
     * Rapid transit third rail: 600 to 700 V (see List of current systems
       for electric rail traction)
     * High speed train overhead power lines: 25 kV RMS at 50 Hz, but see
       List of current systems for electric rail traction for exceptions.
     * High voltage electric power transmission lines: 110 kV RMS and up
       (1150 kV RMS is the record as of 2005)
     * Lightning: Varies greatly, often around 100 MV.

   Note: Where 'RMS' ( root mean square) is stated above, the peak voltage
   is \sqrt{2} times greater than the RMS voltage for a sinusoidal signal.

History of the volt

   In 1800, as the result of a professional disagreement over the galvanic
   response advocated by Luigi Galvani, Alessandro Volta developed the
   so-called Voltaic pile, a forerunner of the battery, which produced a
   steady electric current. Volta had determined that the most effective
   pair of dissimilar metals to produce electricity was zinc and silver.
   In the 1880s, the International Electrical Congress, now the
   International Electrotechnical Commission (IEC), approved the volt for
   electromotive force. The volt was defined as the potential difference
   across a conductor when a current of one ampere dissipates one watt of
   power.

   Prior to the development of the Josephson junction voltage standard,
   the volt was maintained in national laboratories using specially
   constructed batteries called standard cells. The United States used a
   design called the Weston cell from 1905 to 1972.
   This SI unit is named after Alessandro Volta. As for all SI units whose
   names are derived from the proper name of a person, the first letter of
   its symbol is uppercase (V). But when an SI unit is spelled out, it
   should always be written in lowercase (volt), unless it begins a
   sentence or is the name "degree Celsius".
   — Based on The International System of Units, section 5.2.

   Retrieved from " http://en.wikipedia.org/wiki/Volt"
   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.
