1. Electrostatic shielding: in the state of electrostatic balance, whether it is a hollow conductor or a solid conductor; No matter how much charge the conductor itself carries, and whether the conductor is in an external electric field or not, it must be an equipotential body, and its internal field strength is zero. (1) The internal electric field of a closed conductor shell is not affected by external charges or electric fields. (2) The external electric field of the grounded closed conductor shell is not affected by the charge in the shell.
2. Static magnetic shielding: Static magnetic field is a magnetic field generated by a constant current or a permanent magnet. Magnetostatic shield is a shield made of ferromagnetic material with high permeability μ, which is used to shield the external magnetic field. It is similar to electrostatic shielding, but different. There will always be some magnetic leakage in the magnetostatic shield. In order to achieve better shielding effect, multi-layer shielding can be used to shield the residual magnetic flux that leaks into the cavity again and again. Therefore, the magnetic shielding with good effect is generally bulky. However, if you want to create an absolute "magnetostatic vacuum", you can use the Meissner effect of superconductors. That is to say, if the superconductor is placed in an external magnetic field, the magnetic induction intensity B in its body will always be zero. Superconductors are completely diamagnetic and have the best magnetostatic shielding effect, but they are not widely used at present.
3. Electromagnetic shielding: When an electromagnetic field propagates in a conductive medium, its amplitude (e and h) decreases exponentially with the increase of distance. From the point of view of energy, there is energy loss when electromagnetic wave propagates in conductive medium, so it shows the decrease of field amplitude. The electric field on the surface of the conductor is the largest, and the deeper the conductor, the smaller the electric field. This phenomenon is also called skin effect. Skin effect can prevent high-frequency electromagnetic waves from penetrating good conductors to make electromagnetic shielding devices.
Electromagnetic shielding is an effective means to suppress interference, enhance equipment reliability and improve product quality. Reasonable use of electromagnetic shielding can suppress the interference of external high-frequency electromagnetic waves and avoid affecting other equipment as an interference source. For example, in a radio, a hollow aluminum shell is used to wrap the coil so that it is not disturbed by the external time-varying field, thus avoiding noise. So is the shielded wire of audio feeder. The oscilloscope tube is wrapped with iron sheet, which is also to make the stray electromagnetic field not affect the scanning of electron rays. High-frequency electromagnetic waves generated by components or equipment inside the metal shielding shell cannot penetrate the metal shell and will not affect external equipment.
What material is used for electromagnetic shielding? Good shielding effect can be obtained by using thinner copper or aluminum materials in the radio. At high frequency, the magnetic shield with high permeability is generally not used, but the material with high conductivity is used as the electromagnetic shield.
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