1. The problem statement, all variables and given/known data rod -a-------|---------a on x-axis, for x>-a has -Q charge, for X<a has +Q charge, find the electric field at point (x,0) x is positive 2. Relevant equations lambda = Q/a linear charge density dQ = (lambda)(ds) E=KQ/r^2 3. The attempt at a solution i make dE=kdQ/(x-s)^2 integrate kdQ/(x-s)^2 between -a and a ( i can make it 0 to a and multiply by two, symmetry) i get the result 2KQ/a[1/x^2 - 1/(x-a)^2] for E sub-x now if i want to find x>>a i need o somehow change [1/x^2 - 1/(x-a)^2] to the 2nd taylor expansion term but how? And is my integration correct?