📌 Justification:

Assume a charge is moved a small displacement <b>dl</b> along the equipotential surface.

Step 1 — Work Done on Equipotential Surface:
Since potential difference ΔV = 0 on equipotential surface:
W = q × ΔV = 0

Step 2 — Work Done by Electric Force:
W = q × E × dl × cosθ = 0

Step 3 — Since q ≠ 0, E ≠ 0, dl ≠ 0:
cosθ = 0
θ = 90°

∴ <b>Electric field E is always perpendicular (normal)</b> to the equipotential surface.