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At a very large distance from a collection of charges whose net charge is not zero, the equipotential surfaces are approximately:

1. spheres
2. planes
3. paraboloids
4. ellipsoids

Correct answer: spheres

Solution

When the total charge is not zero, the dominant (monopole) term of the potential at large distances behaves like that of a point charge, V proportional to 1/r. Surfaces of constant V correspond to constant r, which are spheres centred on the charge distribution. Hence the equipotentials are approximately spheres.

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