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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

tom
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2 Answers2

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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?

Community
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Ron Maimon
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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

louis rancourt
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