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Thread: why is charge not a fundamental unit while current is

  1. #1 why is charge not a fundamental unit while current is 
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    umm hello im new to this group and new to the world of science but today in class we were told that current (ampere) is a base unit andnot charge i was wondering why as ampere can be derived charge/time but charge cannot can someone help me please.


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    Bullshit Intolerant PhDemon's Avatar
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    The Ampere is not defined in terms of charge over time (although it is equal to this as a consequence of how the Coulomb is defined -- see below), it is:

    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.
    i.e. it is defined in terms of force (itself derived from mass, length and time -- the three "really fundamental things") and length, no mention of charge at all, in fact the Coulomb (charge) is defined in terms of the Amp (it is the quantity of charge that flows if 1A of current is flowing for 1 second).


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    Quote Originally Posted by PhDemon View Post
    The Ampere is not defined in terms of charge over time (although it is equal to this as a consequence of how the Coulomb is defined -- see below), it is:

    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.
    i.e. it is defined in terms of force (itself derived from mass, length and time -- the three "really fundamental things") and length, no mention of charge at all, in fact the Coulomb (charge) is defined in terms of the Amp (it is the quantity of charge that flows if 1A of current is flowing for 1 second).
    but then why isnt it a fundamental unit n why is current a fundamental unit
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    Bullshit Intolerant PhDemon's Avatar
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    Probably historical reasons and scientific unwillingness to change definitions just for the sake of it, see here (Base unit definitions: Ampere) for a bit of historical context, electric current was being experimented with before what it actually was was known (the electron was only discovered in 1897, 4 years after the Amp was defined to try and standardise the knowledge at the time), if the electron was known about before current and the original definitions it is possible things would be defined in a way that made charge, not current fundamental (this wouldn't really change anything except some numerical values of constants -- such as the permeability of free space and other things involved in electromagnetism --and the definitions) but it didn't work out like that...

    Please note this is just my take on it, someone who knows more of the historical context of the SI system in general (and electricity and magnetism in particular) may give you a better answer...
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    It says here that there is a proposal to define it in terms of the fundamental charge, but it doesn't say why it wasn't done that way originally. My guess is that it would be rather difficult to determine experimentally how many electrons or protons have been accumulated somewhere.
    Coulomb - Wikipedia, the free encyclopedia
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