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I've run into this concept of 'screening' several times before, but I admit I've never sat down and really thought about why the process works. My understanding of the effect is that mobile electric charge carriers (say in a fluid) are able to arrange themselves in such a way that reduces or effectively cancels out the charge from a particular particle at a large distance. To quote wikipedia:

"For example, consider a fluid composed of electrons. Each electron possesses an electric field which repels other electrons. As a result, it is surrounded by a region in which the density of electrons is lower than usual. This region can be treated as a positively-charged "screening hole". Viewed from a large distance, this screening hole has the effect of an overlaid positive charge which cancels the electric field produced by the electron. Only at short distances, inside the hole region, can the electron's field be detected."

I really struggle to see what is happening here: how can a region of relative neutrality 'cancel out' the field of an electron?

Any help, insights, or suggestions for further reading would be greatly appreciated.

Thanks

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This description is partly wrong, at least misleading. There is a average density of electrons in that gas, and all that "negative" and "positive" are mere deviations of that average, which is essentially negative charge/volume everywhere. This kind of effect is basic in theories of electrolytes (eg. Debye-Hückel) where one has positive and negative ions, and everything is easy to understand. To choose that exotic electron gas is really silly. But this is Wiki, its "live". –  Georg Nov 27 '11 at 14:22
    
This came up in nuclear physics classes for me. It is a way for there to be an electric force between an ion and a neutral atom. I didn't understand it very well, and I'm curious to hear a better explanation. I think looking at the electrons as a "fluid" to some degree shouldn't be a problem. Obviously the effect comes from orbitals undergoing some rearrangement under the presence of an electric field. –  AlanSE Nov 27 '11 at 15:46
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2 Answers

The description is not of screening, but of the effect that the electric fields of two charge distributions which are laid one on top of the other add up. This means that if you have a negatively charged fluid, and a spherical neutral bubble in the middle, this is the sum of a uniformly negative charged fluid and a positively charged bubble that cancel. This gives no insight into screening.

Screening of electric fields works because when electric fields are present on the surface of a conductor, they pull mobile charge carriers to the surface, and they keep doing this until they are perpendicular to the surface, so that they do no more work. Each step of moving the mobile charges produces heat, so it reduces the energy in the electric field, and the end result is that the field is reduced as much as possible. The reduced field is zero inside the conductor (if it weren't, it would still be moving charges, and dissipating energy).

Also, see this answer for a quick calculation of the jellium screening in a reasonable semiclassical approximation: What is the penetration length of static electric field into conducting metals?

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Yes, giving this detail a name of its own and choosing words already used in electronics is silly. –  Georg Nov 27 '11 at 14:24
    
@Ron Maimon, thankyou very much. Just to make sure I understand, is screening of ions in a fluid (@Georg mentioned Debye-Huckel, this is exactly what I'm looking at) the exact same type of affect as the screening within a conductor, with perhaps the one difference that screening in a conductor is usually far more 'complete' in the sense that there is (to my understanding) effectively no electric field within a conductor, while there may still be some fields in an electrolyte solution, even if they're partially screened. –  tom Nov 28 '11 at 2:40
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@tom: the electrolyte solution is conducting, so it's the same, except the screening length model is accurate, the screening length is much longer than that in a good conductor, and the ions move much slower than electrons, so the response time to zero out the field will be longer. –  Ron Maimon Nov 28 '11 at 4:25
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Experiments with a sensitive electric field sensor clearly show that when a charged object is near the sensor, the sensor indicates that there is a charge object not far away. If I move a neutral sheet of paper between the sensor and the charged object, the sensor indicates that it has less coming from the charged object. The paper can completely shield the field.You can use a racket used to kill bugs to demonstrate that effect. It seems that there is something leaving the charged object that goes to the sensor and even a neutral paper can block it. It seems to travel in strait line to the sensor. There is nothing in the classical physics that suggest that but the facts are there. Someone will have to find the theory behind that fact. Louis Rancourt, physics professor

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Can you expand on why classical physics would be unable to account for this? Polarization of the intervening dielectric would explain it. –  Emilio Pisanty Aug 24 '13 at 1:22
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