Why Is the Equipotential Line Tangent to the Thick Black Line?

[yitriana]

Homework Statement

Find the points where the equipotential line is tangent to the thick black line

Homework Equations

The Attempt at a Solution

I thought that it was A, G and K, because there, the electric field is horizontal and thus perpendicular to the black line and electric field is always perpendicular to equipotential line. However, I am getting the answer wrong.

How do I approach this problem?

 

[thaer_dude]

A part of the problem is missing. A diagram most likely.