Electrostatic force with and without a conductor in between

[iVenky]

The figure shows a charge q1 exerting a force on a test charge qu. What happens to the electric force when a conductor is placed between q1 and qu (cases 1 and 2)? Does the force still remains the same? I am asking this because I am actually interested in finding what happens to the flux in general (whether it's electric or magnetic) when a conductor is suddenly inserted in the path of the flux (case 1-4) (assume that there is a static flux).

 

[tech99]

The figure shows a charge q1 exerting a force on a test charge qu. What happens to the electric force when a conductor is placed between q1 and qu (cases 1 and 2)? Does the force still remains the same? I am asking this because I am actually interested in finding what happens to the flux in general (whether it's electric or magnetic) when a conductor is suddenly inserted in the path of the flux (case 1-4) (assume that there is a static flux).

For the static charges case, the lines of force of q1 terminate on the conducting sheet and do not penetrate it.
For a magnetic case, a non magnetic material is transparent to magnetism, except at high frequencies.
A high permeability material, if sufficiently thick, can short circuit the lines of force and provide magnetic screening. For instance, MuMetal.
In the geometry/configuration you show, the material is simply being used as a magnetic conductor and not as a screen. Both sides of the sheet can see a magnetic pole.

 

[iVenky]

For the static charges case, the lines of force of q1 terminate on the conducting sheet and do not penetrate it.
For a magnetic case, a non magnetic material is transparent to magnetism, except at high frequencies.
A high permeability material, if sufficiently thick, can short circuit the lines of force and provide magnetic screening. For instance, MuMetal.
In the geometry/configuration you show, the material is simply being used as a magnetic conductor and not as a screen. Both sides of the sheet can see a magnetic pole.

Thanks for the reply. I would be really grateful if you can explain to me why the forces terminate on the conducting sheet instead of penetrating and how this changes in the case of magnetic field.

 

[gneill]

For the static charges case, the lines of force of q1 terminate on the conducting sheet and do not penetrate it.

That's true. But will they not induce charge separation on the sheet? So the "backside" of the sheet will have a charge distribution that mimics the effect of the field lines from the external charge on the other side. In that case the conducting sheet should be effectively transparent. But then that would raise the question of why a Faraday cage is effective... I think I need to think more and type less

 

[iVenky]

That's true. But will they not induce charge separation on the sheet? So the "backside" of the sheet will have a charge distribution that mimics the effect of the field lines from the external charge on the other side. In that case the conducting sheet should be effectively transparent. But then that would raise the question of why a Faraday cage is effective... I think I need to think more and type less

So the case I am considering is similar to Faraday cage? I read about Faraday cage and if I apply the same logic, there is an electric field induced inside the conductor due to separation of +ve and -ve ions that acts in the opposite direction of the applied E field and cancels it thereby preventing it from penetrating the metal?

I wonder how it shields magnetic field though..not sure about that

 

[gneill]

So the case I am considering is similar to Faraday cage? I read about Faraday cage and if I apply the same logic, there is an electric field induced inside the conductor due to separation of +ve and -ve ions that acts in the opposite direction of the applied E field and cancels it thereby preventing it from penetrating the metal?

Well, the electric field inside the conductor would be zero thanks to the charge separation. In static conditions there would be no current flow across the conductor interior; The separated charges counteract the external field. The question then is what field is presented on the other side of the conducting sheet? Perhaps we need to think of the sheet as an infinite source/sink for charges that will not change it's potential (a virtual ground, if you will). Then the field presented on the other side would, I believe, be zero. The (infinite) conductive sheet should act as a barrier.