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Electric Field Lines

1 vote

I have doubt in option b and c . Which one would be correct .

As I know field lines are perpendicular to conductor .

asked Nov 13, 2016 in Physics Problems by koolman (4,286 points)
Consider also the angle the field lines meet the plates.
What should be that , I am not getting that .
Notice in option B the field is perpendicular to the top and bottom plates but this is not the case in C.
So the answer should be B

1 Answer

3 votes
Best answer

As you are aware that the electric field is always perpendicular to the surface of a conductor, and you are doubtful between B and C, I think your doubt must be whether the plates are conducting and if not, whether this condition applies also to non-conductors.

The question does not specify if the plates are conducting, so we should not necessarily assume that they are. It is possible to have charged insulating plates.

The condition applies to conductors because the charges in conductors are freely mobile. If the surface field has a tangential component then charge will flow and this component will be nullified. This is not the case for insulators : charge cannot flow freely. Therefore the condition does not apply for insulators/dielectrics.

So both B and C are possible. If the question-setter intended only one answer to be correct, then the question is flawed.

answered Nov 13, 2016 by sammy gerbil (28,806 points)
selected Nov 13, 2016 by koolman
Is it possible to have a charged insulating plates
Yes. eg in this problem (https://physicstasks.eu/1533/field-of-thick-charged-plate).

Plates are often assumed to be conducting (especially when they are electrodes), and charged insulator layers are more commonly called sheets or planes, but there is no standard definition in physics that this is so. Insulating plates can be charged by contact, or charge can be embedded by some manufacturing process.  

The common meaning is a flat dish. Meaning #3 in the Oxford Dictionary (https://en.oxforddictionaries.com/definition/plate) says "metal or other material".

So the issue is really one of definition, rather than physics.