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Homework Statement
I am trying to directly calculate the electric field (using Coulomb) at some arbitrary point P(0,0,z). The charge is evenly distributed over the surface of a sphere (radius R, charge density σ). Here I use θ for the polar angle and p for the azimuthal angle.
I will leave out the messy details, but I know by symmetry only projection onto z-axis is relevant. I also determined the angle ψ (that between separation vector π and the z-axis) in terms of z,R,θ, and π.
Homework Equations
E(alongz) = (4∏ε0) ∫02∏∫0∏ [σR^2 sinθ (z - Rcosθ)] / (R^2 + z^2 - 2Rzcosθ)^(3/2) dθ dp
The Attempt at a Solution
∫dp → 2∏
removing 2∏R^2σ constants out from integrand
∫0∏ [(z - Rcosθ)sinθ] / (R^2 + z^2 - 2Rzcosθ)^(3/2) dθ
using u-substitution:
u = cosθ du= -sinθ dθ
θ = 0 → u = 1
θ = ∏ → u =-1
and reversing the limits of integration gives (ignoring constants out front):
∫-11 (z - Ru) / (R^2 + z^2 - 2Rzu)^(3/2) du
(#1)→according to solutions manual→ this works out to:
z^-2 [(z-R) / |z-R| - (-z-R) / |z+R|]
(#2)The manual says:
Integral can be done by partial fractions -- or look it up.
Does anyone have any idea how you would use partial fractions to go from (1) to (2)??
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