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Thread: Question on definition of ampere

  1. #1 Question on definition of ampere 
    New Member
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    May 2013
    I've found this on my Phy textbook:
    'One ampere is defined as the constant current which, flowing in each of the two infinitely long parallel straight conductors of negligibly small cross-sectional area, seperated by a distance of 1m in vacuum, would produce a force between the wires of 2x10-7Nm-1?'

    My question is, why 2x10-7Nm-1? If ampere is a new unit, there shouldn't be any problem in putting a value as simple as 1Nm-1 as the force for the definition of ampere, would there be any practical or historical reason behind this?

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  3. #2  
    Brassica oleracea Strange's Avatar
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    Oct 2011
    I guess just because that would give an inconveniently large number.

    This is confrimed by Wikipedia which says of the (original CGS ampere): "The size of the unit was chosen so that the units derived from it in the MKSA system would be conveniently sized."

    ei incumbit probatio qui dicit, non qui negat
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  4. #3  
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    May 2013
    That's interesting, thanks a lot, I'm often confused by the mathematical definition of Physics laws and units
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