Understanding Electric Field Mapping for Oppositely Charged Point Charges

[t!m]

If two oppositely charged point charges are separated, with a fairly large circular (spherical) conductor between them, then the equipotential surfaces will kind of wrap around the contour of the conductor, correct? And the electric field would look like it does for a normal dipole, with the Efield lines crossing through the conductor, but none actually inside?

And vice versa for an insulator? Perpendicular equipotentials and electric field lines which avoid the insulator?

I feel like this is right, but my (slightly distracted) TA told me the opposite. Thanks a lot.

 

[berkeman]

When the sphere in the middle is conducting, what do you know about how the E-field vectors hit it? What is special about the angle of the E-field vector as it hits a conductor's surface? And why is that true? What appears on the surface of the conductor to influence this angle? And what is different about what appears on one side of this sphere (the side toward the + point charge) versus the other?

Also, one mind experiment that you can do that will help you visualize the field in this problem is to simplify it first to an infinite metal sheet that is placed directly between the point charges. How is the dipole E-field distribution affected by the sheet? Now shrink the sheet down to a more finite size (and some thickness) between the point charges. What happens to the E-field now?

In the case of the "insulator", none of the above things matter. Why? What is the one physical parameter of the insulator that affects the E-field vectors as they flow from the + to the - point charge? How will that physical parameter affect the E-field vectors?

 

[t!m]

Well the Electric field will always be perpendicular to the conductor. If it weren't then there would be some component of the vector along the the surface. And induction would take place, right? The negative charges would migrate towards the positive charge and vice versa.

With the insulator, charges can't move freely inside of it, but I still don't know how that affects the field around it, really.

I'd like to note that neither sphere (probably a bad word, in hindsight) is hollow. The way the experiment was done was with circular electrodes painted onto a resistive boards and completely filled in with the paint.

 

[t!m]

Ok, I think I've got it - how's this sound?

Conductor:
The electric field will come out of each charge as normal, but then curve towards the conductor, hitting it at a perpendicular, assuming they pass by near enough. There will be no field inside.

Insulator:
The electric field will pass through the insulator as normal, only slightly skewed, coming out the other side. There will be a field inside.

 

[berkeman]

That sounds pretty good. Way to go!

 

[t!m]

Awesome, thanks a lot. I appreciate it.