Marcelo Besch said: ↑
@studiot I'm sorry! About what you said: I simply don't understand what difference would exist if the metal body was grounded or not. Look at the GIF sent by @nsaspook: what difference would there be if that faraday cage was grounded?Click to expand...

NSASpook
The difference would be that (most of) the electrostatic field energy would be shunted into ground instead of charging the enclosure and the inside would still be neutral.

Funny how I referred to spook's gif in the immediately following post.

Following on from the thought experiment I suggested earlier I also suggested another one.

Spook has shown how opposite sides of a Faraday enclosure can be at different voltages in order to neutralise, within the enclosure, an externally applied field.

So in free air inside the exposure there is no field.
But what about a conductor passing through the middle and connected to the opposite faces at different voltages?

More to the point,
In post I suggested a thought experiment (or even are real one) to examine the situation of two conductors, with and without ground comprising a signal conductor and a shield.

Why are we doing all the work here?

 

So in free air inside the exposure there is no field.
But what about a conductor passing through the middle and connected to the opposite faces at different voltages?

More to the point,
In post I suggested a thought experiment (or even are real one) to examine the situation of two conductors, with and without ground comprising a signal conductor and a shield.

Why are we doing all the work here?

You were doing all the work because I couldn't really understand the thought experiments. I was thinking about charges and electrical fields, while the experiments refered to potentials. I just wasn't understanding how to relate what I was thinking to what was being proposed in this case.

This weekend I stopped writing and took to thinking about the things you said, and I think I can understand now. That last phrase was what really made my head click:

"So in free air inside the exposure there is no field.
But what about a conductor passing through the middle and connected to the opposite faces at different voltages?"

So now I think I can understand the first thought experiment: a body is interposed between a signal carrying conductor and an electric field. If the body isn't grounded, its charges will move because of the external electric field. The electrons that moved up the field will couple with the external field, but will leave positive charges on the other side which will couple with the wires inside. If inside was only air, the field inside the shield would be zero, but since we placed a signal carrying conductor in there, it will be influenced by the charges disbalance and the noise will get to the conductors.

In the image not_grounded_shield.png I tried to draw what I thought. The first gaussian surface would be neutral inside, not allowing fields to escape. The second would try to become neutral too, by aquiring electrons from the signal carrying conductors. In my drawing I assumed it wouldn't be able to couple completely, so two field lines escape the surface on the right.

If we ground the shield, the electrons can come from the ground, leaving the other side of the shield neutral. This would be seen as a current going to the ground, the equivalent of saying the shield captured the noise and sent it to the ground. That way, nothing would couple to the conductor inside the shield. I illustrated that in the image grounded_shield.png.

I hope I got this right. @studiot about the second thought experiment, I think the ideal place to connect the ground would be as near as possible to where the noise is acting, since it would lessen the path for the current between shield and ground necessary to cancel the noise. That way, you diminish current induced by the shield on the conductors and also reduce the resistance between ground and shield at the point of interest.

PS.: I was never trying to ignore you, I simply couldn't understand the questions that were being asked. On the second experiment, I thought you were talking to spook, since you were asking where would the ideal grounding be placed while I hadn't even understood why should we ground the shield in the first place. I apologize if I was rude at some point in this thread, english is not my first language and sometimes I don't know how to form a phrase without being too direct.

 

Your pictures are taking you there.

Bravo!