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How can ampere be considered as base quantity?

  1. Aug 14, 2013 #1
    Base Quantities are those quantities on the basics of which other quantities can be expressed.
    Ampere is also a base quantity.But the unit of ampere is C/s.This shows that ampere depends on time and coulomb.It make sense to consider ampere as a derived unit and coulomb as base unit.But we don't. can anyone explain it please ?
  2. jcsd
  3. Aug 14, 2013 #2


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    As a base unit, the "unit of the ampere" is:

    that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per meter of length.

    The unit of the Coulomb is one ampere-second. The coulomb depends on the second and the ampere. Not the other way around.
  4. Aug 14, 2013 #3


    Staff: Mentor

    There is an equation


    Both ways are equally valid ways of writing the same equation. So in principle you can equally consider the Ampere to be derived from or the Coulomb derived from the Ampere. In practice you can make a more accurate current prototype than a charge prototype, so the Coulomb is the derived unit.

    In CGS units you don't even have separate units for EM, they are all derived from mass, length, and time units.
  5. Aug 15, 2013 #4


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    It is perhaps worth mentioning that it is very likely that we in a few years time (maybe 2018) will redefine the ampere by first making the electron charge a constant and they use charges/second as the definition (and realization).
    This agrees with the general trend of the SI: giving constants an exact value and then somehow connect the realization/definition to time (i.e as was done for the meter). This automatically gives us higher accuracy since we can measure time much more accurately (seveal orders of magnitude) than anything else.

    Also, no one actually measures the ampere anymore. All calibrations are in reality done by using the Volt (using the CODATA value for the Josephon constant) and the Ohm (Hall effect), and then Ohm's law.
    The reason is -again- that voltage calbrations are extremely accurate (since they depend on time) and so is the Hall effect. The defition of the Ampere is way too awkward to be practical,
  6. Aug 15, 2013 #5


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    Especially the bits where it requires infinite conductors, absolute vacuum and wires without cross-section, I guess :)
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