The figure shows equipotential surfaces in a region of uniform electric field. The correct interpretation is: 
AEquipotentials cross each other in regions of uniform field.
BThe equipotential surfaces are equally spaced parallel planes perpendicular to $\vec E$.
CThe spacing between equipotentials increases along the direction of $\vec E$.
DThe equipotential surfaces are spheres centred on a fixed point.
Answer & Solution
Correct answer: B. The equipotential surfaces are equally spaced parallel planes perpendicular to $\vec E$.
A uniform field has constant magnitude, so $dV = -E\,dx$ gives uniformly spaced equipotentials perpendicular to $\vec E$. Equipotentials never cross — at a crossing point $\vec E$ would have two directions, which is impossible.
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