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The angle between electric field lines and an equipotential surface is ______.

Asked in GSEB Board July 2018 · Equipotential surfaces and V-r graphs

Answer: (4) π/2

Step-by-step solution

Idea: on an equipotential surface V is the same everywhere, so moving a charge along it takes no work.

Work for a small step Δ s along the surface: W = qE Δ s cos θ, where θ is the angle between ⃗E and the step.

W = 0 for every step along the surface only if cos θ = 0, i.e. θ = 90°.

So field lines always cross equipotential surfaces at right angles: the angle is π/2.

Why the other options are wrong

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