Set the normal electric field to zero and cancel its nonzero overall factor:Taking the power gives , henceThe numerical value is independent of the nonzero magnitude of . Beyond this radius the signed normal electric field is positive. Thus its magnitude first falls to zero, rises again and eventually decays; it is not globally monotone.
For , expanding each denominator givesThis is a dipole rather than a monopole tail. The real-plus-image charge distribution has zero total charge and dipole moment ; in its equatorial plane a dipole produces , confirming the coefficient and sign.
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