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Ampere and charge density

  1. May 7, 2004 #1
    Hi all,

    I cann't figure out the relationship between a current and the charge density. I have a current Io which circulated in a hollow cylindrique, how can I related with the charge density?

    Something like charge density = Io * ....

    Thanks in advance,

  2. jcsd
  3. May 7, 2004 #2


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    Science Advisor

    A conductor has no net charge, current is just the flow
    of electrons inside it. If you do have a net charge then
    you need to devide it by the volume (C/ft^3). If, perhaps,
    you meant current density - then you need to devide the
    value of the current through a cross section area by the
    size of that area (Amperes/square inch).

    Live long and prosper.
  4. May 7, 2004 #3
    Well, I'm trying to find the potentiel of an circular ring. The formula is :

    V = lamda * a /(2*eo*(a2 + d2)1/2)

    all the parametres are known expect one is the lamda. Lamda is the lineaire charge density so, if I know the current and should know the charge density?


  5. May 7, 2004 #4


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    Science Advisor

    Is your ring open or closed, and connected to something
    else - a circuit or something ? If you just have charge
    on a ring then there's NO current, and if there's just current
    in a circuit there's NO charge to create an external potential
    outside the ring. Perhaps if you describe the whole problem
    I'll be able to help you more.

    Live long and prosper.
  6. May 10, 2004 #5

    I have a coil which I will applied a current into. I want to know the voltage which circuled between the conductor rings. So, I think that if I can calculed one ring, it will be the same for other rings. Is it possible?

  7. Jul 8, 2004 #6
    Current= charge density * velocity of the charges
  8. Jul 8, 2004 #7
    Perhaps this will help:
    1.- Voltage = Inductance * d (Current)/dt

    2.- Voltage = d (flux) / dt

    If you calculate for one ring and want to calculate for the whole coil, you'll have to assume that in the coil all the current flows in a circular path, which may be a good aprox. and then integrate the whole thing
  9. Jul 13, 2004 #8
    Thanks for your reply. I have found something in electronic circuit. I have saw the term that you have provided to me which is:
    Voltage = Inductance * d(current)/dt but I have found the whole equation.

    Inductance * d(Current)/dt + Current * d(Inductance)/dt + Resistance * Current = 0

    If I decompose the terms,

    Voltage = Inductance * d(current)/dt for one ring
    Voltage = Resistance * Current for lost in Ohmic
    Voltage = Current * d(Inductance)/dt , what is this term for or means ?


  10. Jul 14, 2004 #9
    The equation above is for time variant Inductances. It comes from noting these:

    [tex]V= \frac{d\phi}{dt} [/tex]

    where V= voltage and [tex]\phi[/tex] is the flux of magnetic field. For linear inductors:

    [tex]\phi= L i[/tex] where L is the inductance and i is the current


    [tex]V= \frac{d\phi}{dt} = L\frac{d i}{dt}+ i \frac{d L}{dt}[/tex]

    But in most situations the inductance is time invariant, therefore the second term vanishes
    Last edited: Jul 14, 2004
  11. Jul 14, 2004 #10
    Thanks for the details. If the inductance is time variant, then how can I calculated the voltage between two rings of the coil?

  12. Jul 14, 2004 #11
    I think you should have some extra information to get the [tex] \frac{dL}{dt}[/tex]. For instance, some info like [tex] L(t)[/tex]
    Last edited: Jul 14, 2004
  13. Jul 15, 2004 #12
    I mean to calculed the voltage between two rings, if the inductance is time variant, I have to add this term [tex] \frac{dL}{dt}[/tex]?

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