Solution

ID: past-exam-of-the-physics-b-course-of-the-university-of-cambridge/2011/paper-1/section-a/a1/solution

For an outward radial electric field, Gauss's law gives the enclosed charge
Taking the enclosing sphere to infinity gives . A nonzero local field is compatible with zero net charge. Indeed, for ,
Positive inner charge is balanced by negative outer charge. There is no point charge at the origin, since . The field's magnitude does not specify its inward/outward sign, but reversing a globally radial direction leaves the zero total unchanged. This is net charge from the asymptotic radial electric field.

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