# Analysis of various electrostatic shielding techniques in electromagnetic fields

The shielding of electric fields, magnetic fields and electromagnetic fields is actually different!

The shielding problem of magnetic field is a problem with practical significance and theoretical significance. According to different conditions, the shielding of electromagnetic field can be divided into three cases: electrostatic shielding, hydromagnetic shielding and electromagnetic shielding. The difference is intrinsic and cannot be confused.

Electrostatic shielding

In the state of electrostatic equilibrium, whether it is a hollow conductor or a solid conductor; no matter how much the conductor itself is charged, or whether the conductor is in an external electric field, it must be an equipotential body whose internal field strength is zero, which is the theoretical basis of electrostatic shielding. The electric field in the closed conductor shell has typical and practical significance. We discuss the electrostatic shielding by taking the electric field in the closed conductor shell as an example.

(1) The electric field inside the closed conductor shell is not affected by the external charge or electric field.

If there is no charged body in the shell and there is a charge q outside the shell, the electrostatic induction causes the outer wall of the shell to be charged. There is no electric field in the shell during electrostatic equilibrium. This is not to say that the external charge does not generate an electric field in the shell, and the root field is generated. The electric charge, they and the excitation field of q at any point in the shell space are zero. Therefore, the inner shell of the conductor is not affected by the external charge q or other electric field. The induced charge of the outer wall of the shell plays an automatic adjustment role. When the cavity conductor shell is grounded, the positive charge on the shell will flow into the ground along the grounding wire. After the electrostatic balance, the cavity conductor and the earth are equal, and the field strength in the cavity is still zero. If there is electric charge in the cavity, the cavity The conductor is still equipotential with the ground, and there is no electric field in the conductor. At this time, there is an electric charge in the cavity due to the strange electric charge in the inner wall of the cavity. This electric field is generated by the electric charge in the shell, and the electric charge outside the shell still has no effect on the electric field in the shell.

It can be seen from the above discussion that the internal electric field is not affected by the external charge of the closed conductor shell whether it is grounded or not.

(2) The external electric field of the grounded closed conductor shell is not affected by the charge in the shell.

If the cavity in the shell has a charge q, because of the electrostatic induction, the inner wall of the shell has an equal amount of electric charge, the outer wall of the shell has the same amount of charge, and the electric field exists in the outer space of the shell. This electric field can be said to be indirectly charged by the electric charge in the shell. Produced. It can also be said that it is directly generated by the induced charge outside the shell. However, if the shell is grounded, the charge outside the shell will disappear, and the charge in the shell and the induced charge on the inner wall will generate an electric field outside the shell. The charge has no effect on the electric field outside the shell, and the case must be grounded. This is different from the first case.

Also note here:

1 We say that grounding will eliminate the charge outside the shell, but it does not mean that the outer wall of the shell must be uncharged in any case. If there is a charged body outside the shell, the outer wall of the shell may still be charged, regardless of whether there is charge in the shell.

2 In practical applications, the metal casing does not have to be completely and completely closed, and a metal mesh cover can be used instead of the metal casing to achieve a similar electrostatic shielding effect, although the shielding is not completely and completely.

3 In the case of electrostatic equilibrium, there is no charge flow in the grounding wire, but if the charge in the shielded shell changes with time, or the charge of the charged body near the outer shell changes with time, there will be current in the grounding wire. The shield may also have residual charge, and the shielding effect will be incomplete and incomplete.

In short, whether the closed conductor shell is grounded or not, the internal electric field is not affected by the external charge and electric field; the electric field outside the shell of the closed conductor is not affected by the charge inside the shell. This phenomenon is called electrostatic shielding. Electrostatic shielding has two meanings. :

The first is the practical significance: shielding makes the instrument or working environment inside the metal conductor shell unaffected by the external electric field, and does not affect the external electric field. Some electronic devices or measuring equipment must be electrostatically shielded in order to avoid interference, such as indoor high-voltage equipment. Covered with a grounded metal cover or a dense metal mesh cover, a metal tube for the tube. Also as a power transformer for full-wave rectification or bridge rectification, wrapping metal foil or winding between the primary winding and the secondary winding A layer of enameled wire is grounded to achieve shielding. In high-voltage live working, workers wear a pressure-equalized garment woven with wire or conductive fiber, which can shield the human body. In the electrostatic experiment, due to the presence of the earth A vertical electric field of about 100 V/m. To rule out the effect of this electric field on electrons, to study the movement of electrons only by gravity, there must be eE.

The second is the theoretical significance: indirect verification of Coulomb's law. Gauss's theorem can be derived from Coulomb's law. If the inverse squareness index in Coulomb's law is not equal to 2, the Gauss theorem can not be obtained. Conversely, if the Gauss theorem is proved, the Coulomb is proved. According to Gauss's theorem, the field strength inside the insulated metal spherical shell should be zero, which is also the conclusion of electrostatic shielding. If the instrument is used to detect the electrification in the shielding shell, the Gauss can be determined according to the measurement results. The correctness of the theorem also verifies the correctness of Coulomb's law. The recent experimental results were completed by Williams et al. in 1971, pointing out that

In F=q1q2/r2±δ, δ<(2.7±3.1)×10-16,

It can be seen that the inverse square relationship of Coulomb's law is strictly established within the experimental precision that can be achieved at this stage. From the practical application point of view, we can think that it is correct.

Magnetostatic shielding

The static magnetic field is a constant current or a magnetic field generated by a permanent magnet. The magnetostatic shielding is made of a ferromagnetic material with a high magnetic permeability μ to shield the external magnetic field. It is similar to and different from electrostatic shielding.

The principle of magnetostatic shielding can be explained by the concept of magnetic circuit. If the ferromagnetic material is made into a circuit with a cross section as shown in Fig. 7, in the external magnetic field, most of the magnetic field is concentrated in the ferromagnetic circuit. The material and the air in the cavity are analyzed as a parallel magnetic circuit. Because the magnetic permeability of the ferromagnetic material is several thousand times greater than the magnetic permeability of the air, the magnetic reluctance of the cavity is much larger than the magnetic reluctance of the ferromagnetic material. The vast majority of the magnetic induction line of the external magnetic field will pass along the wall of the ferromagnetic material, and the magnetic flux entering the cavity is extremely small. Thus, the cavity shielded by the ferromagnetic material has substantially no external magnetic field, thereby achieving static The purpose of magnetic shielding. The higher the magnetic permeability of the material, the thicker the wall, the more obvious the shielding effect. Because of the common magnetic permeability of ferromagnetic materials such as soft iron, silicon steel, permalloy as a shielding layer, so the magnetostatic Shielding is also called ferromagnetic shielding.

Magnetostatic shielding has a wide range of applications in electronic devices. For example, leakage flux generated by transformers or other coils can affect the motion of electrons and affect the focusing of electron beams in the oscilloscope or picture tube. In order to improve the quality of the instrument or product, The part that generates the leakage flux must be magnetostatically shielded. In the watch, the core of the movement can be made magnetically protected by a thin shell of soft iron.

It is pointed out earlier that the effect of electrostatic shielding is very good. This is because the electrical conductivity of metal conductors is more than a dozen orders of magnitude higher than that of air, and the difference between the magnetic permeability of ferromagnetic materials and air is only a few orders of magnitude. It is about several thousand times larger. Therefore, the magnetostatic shielding always has some magnetic leakage. In order to achieve better shielding effect, multiple layers of shielding can be used to shield the residual magnetic flux leaking into the cavity again and again. Therefore, the magnetic shielding is effective. Generally, it is cumbersome. However, if you want to create an absolute "static magnetic vacuum", you can use the Meissner effect of the superconductor. A superconductor is placed in an external magnetic field, and the magnetic induction B in the body is always zero. The superconductor is completely resistant. Magnets have the most ideal magnetostatic shielding effect, but they are not universally applicable at present.

Electromagnetic shielding

When the electromagnetic field propagates in the conductive medium, the amplitude of the field (E and H) decays exponentially with increasing distance. From the energy point of view, the electromagnetic wave has energy loss when it propagates in the conductive medium, so it appears as a field. The amplitude of the conductor is reduced. The field of the conductor is the largest, and the deeper the conductor is, the smaller the field is. This phenomenon is also called the skin effect. The skin effect can prevent the high-frequency electromagnetic wave from penetrating into the good conductor and making electromagnetic shielding. Device. It is more universal than static and magnetostatic shielding.

Electromagnetic shielding is an effective means to suppress interference, enhance the reliability of equipment and improve product quality. Reasonable use of electromagnetic shielding can suppress the interference of external high-frequency electromagnetic waves, and can also be used as an interference source to affect other equipment. For example, in radios, The hollow aluminum shell is placed on the outside of the coil so that it is not disturbed by the external time-varying field to avoid noise. The shielded wire for the audio feeder is also the same. The oscilloscope is wrapped with iron, so that the stray electromagnetic field does not affect the electron beam. Scanning. High-frequency electromagnetic waves generated by components or devices inside the metal shield can not penetrate the metal shell without affecting the external equipment.

What material is used for electromagnetic shielding? Because the electromagnetic wave attenuates rapidly in a good conductor, the thickness decayed from the surface of the conductor to 1/e (about 36.8%) of the surface value is called the skin thickness (also known as the penetration depth). With d, there is electromagnetic shielding. When the electromagnetic field propagates in the conductive medium, the amplitude of the field quantity (E and H) decays exponentially with the increase of the distance. From the energy point of view, when the electromagnetic wave propagates in the conductive medium There is energy loss, therefore, it shows the decrease of the amplitude of the field. The field of the conductor surface is the largest, and the deeper the conductor is, the smaller the field is. This phenomenon is also called the skin effect. The skin effect can prevent the high frequency. Electromagnetic waves penetrate into good conductors to form electromagnetic shielding devices. It is more universal than electrostatic and magnetostatic shielding.

Electromagnetic shielding is an effective means to suppress interference, enhance the reliability of equipment and improve product quality. Reasonable use of electromagnetic shielding can suppress the interference of external high-frequency electromagnetic waves, and can also be used as an interference source to affect other equipment. For example, in radios, The hollow aluminum shell is placed on the outside of the coil so that it is not disturbed by the external time-varying field to avoid noise. The shielded wire for the audio feeder is also the same. The oscilloscope is wrapped with iron, so that the stray electromagnetic field does not affect the electron beam. Scanning. High-frequency electromagnetic waves generated by components or devices inside the metal shield can not penetrate the metal shell without affecting the external equipment.

What material is used for electromagnetic shielding? Because the electromagnetic wave attenuates rapidly in a good conductor, the thickness decayed from the surface of the conductor to 1/e (about 36.8%) of the surface value is called the skin thickness (also known as the penetration depth). , indicated by d, there is

Where μ and σ are the magnetic permeability and electrical conductivity of the shielding material respectively. If the TV frequency f=100 MHz, the copper conductor (σ=5.8×107/?m, μ≈μo=4π×10-7H/m) Find d=0.00667mm. It can be seen that the electromagnetic shielding effect of good conductor is significant. If it is iron (σ=107/?m) then d=0.016mm. If it is aluminum (σ=3.54×107/?m) then d= 0.0085mm.

In order to obtain effective shielding, the thickness of the shielding layer must be close to the wavelength of the electromagnetic wave inside the shielding material (λ = 2πd). For example, in the radio, if f = 500 kHz, d = 0.094 mm (λ = 0.59 mm) in copper. In the aluminum d = 0.12mm (λ = 0.75mm). So in the radio with a thin copper or aluminum material can get a good shielding effect. Because the TV frequency is higher, the penetration depth is smaller, the required The thickness of the shielding layer can be thinner. If the mechanical strength is considered, the necessary thickness is required. At high frequencies, the hysteresis loss and eddy current loss of the ferromagnetic material are large, which causes the Q value of the quality factor of the resonant circuit to decrease. Generally, magnetic shielding with high magnetic permeability is not used, and electromagnetic shielding is used for materials with high conductivity. In electromagnetic materials, since the skin current is eddy current, electromagnetic shielding is also called eddy current shielding.

At the power frequency (50Hz), d=9.45mm in copper and d=11.67mm in aluminum. Obviously, it is very unsuitable to use copper and aluminum. If iron is used, d=0.172mm, then it should be used. Ferromagnetic materials. Because the electromagnetic field attenuation in ferromagnetic materials is much larger than that in copper and aluminum. Because of the low frequency, there is no need to consider the Q value. It can be seen that in the low frequency case, the electromagnetic shielding is converted into a magnetostatic shielding. It has the same points and different points as the electrostatic shielding. The same point is made of metal materials with high conductivity; the difference is that the electrostatic shielding can only eliminate the capacitive coupling, prevent static induction, and the shielding must be grounded. The electromagnetic shielding is to make the electromagnetic field only Penetrating into a thin layer of the shield, the eddy current eliminates the interference of the electromagnetic field. The shield can be ungrounded. However, since the conductor used as the electromagnetic shield increases the electrostatic coupling, even if only the electromagnetic shielding is performed, the grounding is good, so that the electromagnetic The shielding also acts as an electrostatic shield.