Is the Electric Field Parallel to the Surface of Conductors and Insulators?

[hunneysaummya]

"In a static situation, the electric field of a conductor can have
no component parallel to the surface coz this would violate the condition
that the charges on the surface are at rest." would this statement be valid
for electric field at the surface of an insulator? Explain your answer and
the reason for any differences b/w the cases of a conductor and an
insulator.

So, first I thought that this statement won't be valid coz charges can't
move in an insulator but then I thought that these charges could be excess
charges and not mobile charge carriers. So, the statement should be valid.

 

[Delphi51]

Kind of a tricky question. The first sentence is incomplete. On a conductor, the charges are free to move and if there was an E along the surface it would cause the charges to move such that that E would be canceled out. In most cases this would be a uniform distribution of charge whose combined E at any point would be perpendicular to the surface.

On an insulator, the charges can't move. You could put a spot of charge in one place only and the resulting E would go out in all directions from that spot.