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Gauss, charge density

  1. Aug 18, 2010 #1


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    Hi all,

    A closed cylinder of Length L, with a radius r and a thickness d is filled with m1q1+m2q2 charges. Their respective volume charge densities are [tex]\rho[/tex]1 and [tex]\rho[/tex]2.
    The volume is surrounded with a neutral solute (n1q1+n2q2=0) with a volume charge density of [tex]\rho[/tex]3.
    (see picture).

    Case 1/
    m2>m1 => m1q1+m2q2<0
    Is there any attraction from the "core" to the outer charges?
    Does it depend of the charges densities and volume thickness?

    Attached Files:

  2. jcsd
  3. Aug 18, 2010 #2
    It isn't at all clear what you mean.
    'Filled with charges' could mean the annular region or the central core.
    m1q1 +m2q2... what does that signify? two sets of charges? do you mean to imply they are of different sign?

    Whatever these things mean, you appear to be describing some sort of initial condition for a collection of charges and asking about their interaction from that position.

    Put simply, each charge experiences a net force due to the summation of all the fields of each other charge in the configuration. You can work that out in terms of an integration of the charge densities but that would be an instantaneous solution for an unstable configuration, which would tell you nothing about the subsequent behaviour of the system.
  4. Aug 18, 2010 #3


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    The interaction between the cylinder and its surroundings depends both on the total charge density inside the material [itex]\rho_1(\textbf{r})+\rho_2(\textbf{r})[/itex] and the charge density outside the material [itex]\rho_3(\textbf{r})[/itex]. The reason for this should be fairly clear; the field of a point charge falls off with distance from the charge. Hence, the force a test charge feels from a source charge depnds on the distance between the two charges. So, if you want to calculate the force on each one of the +/- charges in your cylinder due to the surrounding solution, you need to account for each infinitesimal piece of charge, since each will be at a different position relative to your test charge and hence exert a different force on it.

    As a simple example, consider a single point charge [itex]q[/itex] at the origin and a physical dipole consisting of a charge [itex]q[/itex] at [itex]x=a[/itex] and a charge [itex]-q[/itex] at [itex]x=a+d[/itex] (assume [itex]a\gg d>0[/itex] if you like) connected by a rigid neutral rod. The charge at the origin will experience a net repulsive force since it is closer to the positive end of the dipole than it is to the negative end, even though the net charge on the dipole is zero.
  5. Aug 19, 2010 #4


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    The picture tells us that q1 is positive when q2 is negative.
    It is initial conditions of a pure hypothesis. And these charges are homogeneously mixed within the core. They are moving charges (a fluid).
    That's what I meant but failed to explain it clearly.
    If we know the charge density of a fluid (concentration of charges per volume?), we know the distance that separate these charges.
    I just want to know when we add another distance (the cylinder thickness) if the action of the core may be unable to attract any moving outside charge?
    Is there a way to say that inner cylinder action (on outside charges) is dependent of cylinder thickness?
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