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Explain coulombs and ampres

  1. Feb 23, 2012 #1
    What is exactly one Coulomb? Wikipedia says "One coulomb is the magnitude (absolute value) of electrical charge in 6.24150965(16)×10^18 protons or electrons." It also says " It is defined as the charge transported by a steady current of one ampere in one second"

    But also 1 proton has a charge of 1.602176565(35)×10^−19 C.
    How is that?

    Also what about the Ampere?
    "the ampere is a measure of the amount of electric charge passing a point in an electric circuit per unit time with 6.241 × 10^18 electrons, or one coulomb per second constituting one ampere."

    I'm not very clear on what a Coloumb is, can anybody explain?
  2. jcsd
  3. Feb 23, 2012 #2


    Staff: Mentor

    think water: a coulumb is a measurement like a liter of water and

    an ampere is a measurement like water coming out of a hose at N liters per second.

    The reason for the one coulomb magnitude being what it is is because the convention was established long before we knew about electrons and protons as being the fundamental carriers of electric charge.
  4. Feb 23, 2012 #3


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    Staff: Mentor

    Hint: take the reciprocal of either number. What do you get?
  5. Feb 23, 2012 #4


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    Staff Emeritus
    Science Advisor
    Gold Member

    The coulomb is the fundamental unit of electric charge, so in that sense, it has a somewhat arbitrary definition, just like other base units do. (The kilogram is just defined as the mass of some platinum cylinder in a lab in France).

    Actually, however, the coulomb is a derived unit in the SI system, not the base unit. The ampere is actually the base unit (along with the metre, kilogram, and the second), and a coulomb is defined as 1 ampere*1 second. In other words, the coulomb is the amount of electric charge that flows past in one second when you have an electric current of 1 ampere flowing.

    So what is the definition of 1 ampere? Since it is a base unit, it should just have an arbitrary definition that is standard and universally adopted (just like for the kilogram). In this case, I think the ampere was chosen as the amount of electric current that is measured to produce an attractive force of 1 newton between two current-carrying wires whose currents are in opposite directions.

    (Anytime you have an electric current, you have a magnetic field. For two parallel, current carrying wires whose currents are moving in opposite directions, the opposing magnetic fields generated for each wire will produce an attractive force between the wires).

    EDIT: I just checked, and the definition is that the two wires have to be separated by 1 metre, and the force produced is supposed to be 2e-7 N per metre of length (not 1 newton as I stated before)
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