1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

S.I Unit

  1. Dec 11, 2009 #1
    Why ampere is not a S.I unit?
     
  2. jcsd
  3. Dec 11, 2009 #2
  4. Dec 11, 2009 #3

    f95toli

    User Avatar
    Science Advisor
    Gold Member

    The Ampere is even a base unit in the SI.
     
  5. Dec 11, 2009 #4
    Sorry i posted the wrong question? I tried to say why Ampere is the basic/fundamental unit although it is derived from charge/time(columb/sec)
     
  6. Dec 11, 2009 #5

    Integral

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Did you try google?

    An amp is defined in terms of the current required to achieve a certain force between parallel conductors. Not in terms of Coulomb per sec.
     
  7. Dec 11, 2009 #6

    D H

    User Avatar
    Staff Emeritus
    Science Advisor

    With the notable exception of the kilogram, the SI base units are defined in a way as to make the units independently observable. In SI terminology, the coulomb is a derived unit. The reason is that measuring an ampere is a considerably easier (and more precise) task than is measuring a coulomb.
     
  8. Dec 11, 2009 #7

    Dale

    Staff: Mentor

    In the SI unit system current is the fundamental quantity and charge is a derived quantity (1 C = 1 A * 1 s).

    The reason is simply that current is easier to measure precisely than charge.

    Edit: I see D H was faster than I was
     
  9. Dec 12, 2009 #8

    f95toli

    User Avatar
    Science Advisor
    Gold Member

    But it is still a very very difficult experiment if the accuracy is to be good enough. No one has -as far as I know- actually realized the Ampere this is way in quite a while (although I am sure there are a few old setups around).
    In the "real world" the Ampere actually a derived unit, calculated from the realization of the Volt (Josephson effect) and the Ohm (quantum hall effect).
    One of the big topics in metrology over the past few years has been to come up with a useful current standard, something that would allow us to define current in term of charges/second OR to realize the Ampere in terms of time, the latter would be useful since we can meaure time with higher accuracy than anything else (this is how the Volt is realzied. V=Kj*f, Kj being the Josephson constant adn f the frequency)
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook