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Homework Help: Electromagnetism - Potentials due to point charge and along line of of charge

  1. Oct 23, 2011 #1
    1. The problem statement, all variables and given/known data
    Consider an infinitely long and thin line of charge, with density 8nC/m. Calculate the electric potential difference ((phi)1-2) between two points in air at radial disances 1mm and 3mm.

    2. Relevant equations
    I am assuming:
    Phi(r) = λ/2(pi)ε0 * ln(rR/r) where rR is a reference point

    3. The attempt at a solution
    Our lecturer told us that we did not have to derive electric potential - just use what we had discussed in the notes, which, as far I can tell, is the above.

    As far as I can see, this is just plugging in 8x10^-9 c for λ, 3x10^-3m for rR, and 1x10^-3m for r.

    The problem: this is a 20 point question! One fifth of the assignment's points.

    I feel like there is something to this that I am missing, but, despite having poured through our notes and researched on the internet (and even found a worked problem that is similar), I cannot find anything else to do other than plug in for λ, rR, and r.

    If I'm wrong and there's a lot more I have to do, could someone please help point me in the right direction?

    1. The problem statement, all variables and given/known data
    The potential at position r due to a point charge q at position r' is

    phi(r) = q/4piε0 * 1/|r-r'|

    a.) Calculate grad phi and hence the electric field E.
    b.) What is the force experienced by a charge q1 at position r?
    c.) What is the potential energy of the charge at q1?

    2. Relevant equations
    Given above; however, also worth knowing is
    E = -grad phi
    U = q2 * phi

    3. The attempt at a solution
    This is like the above question, where, in my mind at least, the points allocated don't seem to match up with the amount of work to be done.

    Part a I have no problems with. I thought of r in terms of x,y, and z (again, according to our lecturer's advice), and then I used partial derivative with respect to x, then stated that, according to symmetry, y and z worked out to be the same, giving an answer of
    grad phi = q/4piε0 * r'-r/|r-r'|^3

    and since E = -grad phi, this reversed the top value (r'-r) to give
    E = = q/4piε0 * r-r'/|r-r'|^3.

    That felt like a solid 10 points worth of work.

    To do part b, it seems that all I have to do is multiply another q into the equation, and multiply that by -1 (since the charge is going from q1 (which I assume is equivalent to "q2") to q (which I assume is equivalent to "q1")). This feels more like 2 or 3 points worth of work.

    Then for part c, I have done a negative integral of what I calculated for F, with respect to r. This gives me

    U = q*q1/4piε0 * 1/|r-r'|

    Not only does this answer not seem correct, but it also doesn't feel like 10 points worth of work.

    Again, if someone could help me see where I went wrong, and point me in the direction of whatever I'm missing, I would be very grateful.

  2. jcsd
  3. Oct 26, 2011 #2
    All of that seems correct....
    Probably the teacher expected you to calculate the potential in the first question. But it is good what you have done.
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