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Homework Help: Electric & Gravitational Fields

  1. Feb 1, 2009 #1
    The problem statement, all variables and given/known data

    (Prior to start of questions, it informs me the relative permittivity of free space is 8.9x10^-12)

    2) An alpha particle (charge 3.2x10^-19) approaches to a distance of 1.5x10^-17 from a gold nucleus (charge 1.3x10^-17). Calculate:

    a) the strength of the field created by the gold nucleus at the distance of the alpha particles closest approach.

    2. Relevant equations + Attempt at a solution

    According the back of the book, the answer is 5.2 x 10^22 Vm^-1

    Because it gave me the relative permittivity of free space, I tried using that (even though it could be for other questions, like question 1, where I needed it).

    So I used F= Q1,Q1 / 4xPiex8.9x10^-12xr^2

    Where I used the two charges provided and I used the distance as the radius (r).

    That didn't work.

    So I assumed that due to the units the back of the book gives (Vm^-1), the right equation to use would be V= Ed. But, if I rearrange is to get E .. I don't have the value of V.

    But I do still think that V = Ed is barking up the right tree. But how do I find V? W = VQ? But how do I know W?

    The problem is, I can't think of any other equations that are suitable to use, apart from the above two that I've discussed.

    Any help would be much appreciated.
  2. jcsd
  3. Feb 1, 2009 #2


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    Homework Helper

    It is very inefficient to try finding the right formula by considering the units. Far better to get to work and memorize what each of your formulas is for. For electric field, you just have about 3 of them:
    1) V = Ed which should be thought of as E = V/d
    This one says, "a potential difference causes an electric field between parallel plates"

    2) F = Eq which says "an electric field causes a force on a charge"

    3) E = kq/r^2 which says "a charge causes an electric field around itself"
    (here the k may be replaced with 1/(4*pi*epsilon) in your book)

    Study all three! Then when you read your problem again and think about how you are asked what E field the charge on a nucleus causes, you will INSTANTLY know the right formula to use!
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