Industry information

Why does the metal mesh cover also have the effect of electrostatic shielding?

The basic principle of electrostatic shielding is as follows: Assume that a cavity conductor (the closed conductor is discussed first) has a charge Q outside, because the surface layer of the cavity conductor will have a magnetically induced charge due to the tip discharge effect. The magnetically induced charge is in the cavity. The electric field caused by the interior and the electric field caused by Q in the cavity cancel each other out, thus making the compressive strength of the electric field inside the cavity zero.

Let's discuss the metal mesh again. Although the dust cover has the hollow part you often say, it does not harm the magnetic induction charge on the metal mesh cover. The electric field outside the metal mesh cover will also be offset by the electric field of the magnetic induction charge. .

Basic principle of electrostatic shielding

(1) If the conductor is placed in an external electric field with an electric field compressive strength E, the free charge in the conductor will move against the direction of the electric field under the action of the electric field force. That way, the negative charge of the conductor is spread on one side and the positive charge is spread on the other side, which is the tip discharge condition. Because of the re-distribution of the charge in the conductor, this charge generates another electric field in the opposite direction to the external electric field, and the compressive strength of the electric field is within E. According to the superposition theorem of magnetic field strength, the compressive strength of the electric field inside the conductor is equivalent to the accumulation outside E and inside E, until the electric field in the opposite direction accumulates and cancels each other out, making the total electric field compressive strength inside the conductor zero. When the compressive strength of the total electric field inside the conductor is zero, the free charge in the conductor will not move. In physics, the absence of charge movement in a conductor is called electrostatic equilibrium. In a conductor in electrostatic equilibrium, the compressive strength of the internal electric field is zero everywhere. Therefore, it can be seen that in a conductor in electrostatic equilibrium, the charge spreads only on the surface of the conductor. If the conductor is hollow, when it is electrostatically balanced, there will be no electric field inside. In this way, the casing of the conductor will have a "protection" effect on its interior, so that the interior will not be harmed by the external electric field. This kind of situation is called electrostatic shielding.

(2) Faraday risked being electrocuted before by doing a world-famous test - the Faraday cage test. Faraday locked himself in a metal cage. When a strong electrostatic induction charge and discharge occurred outside the cage, nothing happened.

(3) Electrostatic shielding: In order to better prevent the harm of the external electric field to the experimental instrument, or to better prevent the electric field of the electrical equipment from harming the outside, a cavity conductor is used to shield the external electric field, so that the The inside is not affected, and the electrical equipment is not harmful to the outside, which is called electrostatic shielding. The shielding of the cavity conductor ungrounded device is the outer shielding, and the shielding of the cavity conductor grounding device is the inner shielding.

(4) In the case of electrostatic equilibrium, whether it is a hollow conductor or a solid conductor; no matter how much the conductor is energized, or whether the conductor is in an external electric field, it must be an equipotential body, and its internal magnetic field strength is Zero, which is the theoretical basis of electrostatic shielding.

(5) Coulomb's Law is indirectly verified. Gauss's law can be calculated from Coulomb's law. If the value of the inverse square meter in Coulomb's law is not equal to 2, Gauss's law cannot be obtained. Conversely, if Gauss's theorem is confirmed, then the accuracy of Coulomb's law is confirmed. According to Gauss's theorem, the magnetic field strength inside the spherical shell of insulating metal material should be zero, which is also the result of electrostatic shielding. If the equipment is used to test whether the shielding shell is energized or not, the accuracy of Gauss's theorem can be judged by analyzing the accurate measurement results, which also verifies the accuracy of Coulomb's law. The most recent experimental results were conducted by Williams et al in 1971, emphasizing that the formula

In F=q1q2/r2±δ, δ<(2.7±3.1)×10-16

(6) It can be seen that within the experimental precision that can be achieved at present, the square inverse proportional relationship of Coulomb's law is strictly established. From the point of view of specific application, we can feel that it is appropriate.

(7) In chemically cross-linked polymer materials, the side gene of the polymer material chain is fixed in the molecular structure for the purpose of chemical cross-linking, and the counter ions corresponding to the functional groups of the side groups will accompany Due to the free diffusion of the organic solvent molecular structure, the concentration value continues to decrease, which leads to the expansion of the relative density of the internal charge of the suspected glue. The molecular structure of the suspected glue causes the volume of the suspected glue to expand due to the electrostatic induction repulsion. The macroeconomics is mainly manifested by the solubilization of the suspected glue. Under high salinity conditions, the concentration of counter ions (such as Na) in aqueous solutions is very large. If the chemically cross-linked polymer is melted with a high-salt aqueous solution, because the free diffusion counter ions of the molecular structure of the organic solvent also diffuse into the interior of the molecular structure of the gum, the counter ions and the side groups on the molecular structure of the polymer Fusion shields the charge repulsion within the molecular structure, resulting in a decrease in the solubility of the polymer molecular structure. This type of situation is called charge screening.

(8) It must be noted that if the external electric field is an alternating electric field, the standard of electrostatic shielding will not be established.

Main purpose

(1) Electrostatic shielding has two practical significances. One is practical significance: shielding the equipment or office environment in the metal conductor shell will not be harmed by the external electric field, nor will it cause harm to the external electric field. Some electronic components or measuring equipment need to implement electrostatic shielding in order to better avoid the influence, such as the metal material cover of the grounding device or the denser metal mesh cover on the high-voltage equipment cover in the room, and the metal hose for the rectifier tube. shell. Another example is a toroidal transformer used for full-wave rectification or bridge rectification. The primary and secondary windings and secondary windings are wrapped with metal sheets or a layer of wire-covered wire is wrapped around it and grounded to achieve the shielding effect. In high-level piezoelectric work, workers wear pressure-equalizing suits woven with iron wire or conductive chemical fibers, which can shield and protect the body. In the electrostatic experiment, there is a vertical electric field of about 100V/m around the earth. In order to clear the effect of this electric field on electronic devices, and scientific research on the fitness of electronic devices only under the effect of force, it must have eE<10-10V/m, which is an "electrostatic induction vacuum pump" with basically no electrostatic field. This can only be accomplished by electrostatic shielding of the evacuated cavity. In practice, electrostatic shielding accomplished by a closed conductor cavity is quite reasonable.