Starting & Terminating Angle

Description:

Electric field line always make right angle with both the starting and terminating surfaces.

Proof

Let  the electric field is not perpendicular and it is making certain angle say(θ) with the surface. As shown in figure

Then,

Force F→ = qE→ can be resolved as qEcos θ along the surface and qEsin θ perpendicular to the surface. We find that qEcos θ component does not move the charge.

Therefore we conclude that,

qE cos θ = 0

but(q≠0 and E due to charge ≠0)

implies that, cos θ=

⇒ θ = 90

Hence we conclude that the electric field line makes 90^o with the surface.