Circuit board manufacturer: electrostatic shielding of shielding magnetic field materials

Circuit board manufacturer: electrostatic shielding of shielding magnetic field materials

Circuit board manufacturer, circuit board design and PCBA processing manufacturer explain to you that circuit board manufacturer: electrostatic shielding of shielding magnetic field materials

The problem of field shielding is of both practical and theoretical significance. According to different conditions, electromagnetic field shielding can be divided into three cases: electrostatic shielding, magnetostatic shielding and electromagnetic shielding. These three cases have both qualitative differences and internal connections, which cannot be confused.

Electrostatic shielding

In the state of static equilibrium, no matter hollow conductor or solid conductor; No matter how much the conductor is charged or whether it is in an external electric field, it must be an equipotential body with zero internal field strength, which is the theoretical basis of electrostatic shielding. Because the electric field in the closed conductor shell has typical and practical significance, we take the electric field in the closed conductor shell as an example to discuss the electrostatic shielding.

Circuit board manufacturer, circuit board design and PCBA processing manufacturer explain to you that circuit board manufacturer: electrostatic shielding of shielding magnetic field materials

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

If there is no charged body inside the shell but there is charge q outside the shell, electrostatic induction will charge the outer wall of the shell. There is no electric field in the shell when static electricity is balanced. This does not mean that the charge outside the shell does not generate an electric field inside the shell. Because the outer wall of the shell induces the charge with different sign, the combined field strength excited by them and q at any point in the space inside the shell is zero. Therefore, the inside of the conductor shell will not be affected by the charge q outside the shell or other electric fields. The induced charge on the outer wall of the shell plays an automatic regulating role. If the above hollow conductor shell is grounded, the induced positive charge on the shell will flow into the ground along the grounding wire. After the electrostatic balance, the potential of the cavity conductor and the earth is equal, and the field strength in the cavity is still zero. If there is charge in the cavity, the cavity conductor is still equipotential with the ground, and there is no electric field in the conductor. At this time, there is an electric field in the cavity because there is a different sign of induced charge on the inner wall of the cavity. This electric field is generated by the charge inside the shell, and the charge outside the shell has no effect on the electric field inside the shell.

From the above discussion, it can be seen that whether the closed conductor shell is grounded or not, the internal electric field is not affected by the charge outside the shell.

（2） The electric field outside the grounded closed conductor shell is not affected by the charge inside the shell.

If there is charge q in the cavity inside the shell, because of electrostatic induction, the inner wall of the shell has an equal amount of charge with different signs, the outer wall of the shell has an equal amount of charge with the same sign, and there is an electric field in the space outside the shell, which can be said to be indirectly generated by the charge q inside the shell. 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 electric field generated by the charge q inside the shell and the induced charge on the inner wall outside the shell will be zero. It can be seen that if the charge inside the shell does not affect the electric field outside the shell, the shell must be grounded. This is different from the first case.

Attention shall also be paid 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 not be charged under any circumstances. If there is a charged body outside the shell, the outer wall of the shell may still be charged, regardless of whether there is a charge inside the shell.

② In practical application, the metal shell does not need to be strictly and completely closed, and the metal mesh cover can also achieve similar electrostatic shielding effect instead of the metal shell, although this shielding is not complete and thorough.

③ In electrostatic balance, 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 shell changes with time, there will be current in the grounding wire. The shielding cover may also have residual charges, and the shielding effect will be incomplete and incomplete.

In a word, 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 grounded closed conductor shell is not affected by the charge inside the shell. This phenomenon is called electrostatic shielding. Electrostatic shielding has two meanings:

One is the practical significance: the shielding makes the instrument or working environment in the metal conductor shell not affected by the external electric field, nor will it affect the external electric field. In order to avoid interference, some electronic devices or measuring equipment should implement electrostatic shielding, such as the grounded metal cover or dense metal mesh cover on the indoor high-voltage equipment cover, and the metal tube shell for electronic tubes. Another example is the power transformer for full wave rectification or bridge rectification, which is wrapped with a metal sheet or a layer of enamelled wire between the primary winding and the secondary winding and grounded to achieve shielding effect. In high-voltage live working, the workers wear uniform woven with metal wire or conductive fiber, which can shield and protect the human body. In electrostatic experiments, there is a vertical electric field of about 100V/m near the earth. In order to exclude the effect of this electric field on electrons and study the movement of electrons only under the action of gravity, it is necessary to have eE<meg, which can be calculated as E<10-10V/m. This is a "electrostatic vacuum" with almost no electrostatic field, which can only be realized by electrostatic shielding of the cavity pumped into vacuum. In fact, the electrostatic shielding realized by a closed conductive cavity is very effective.

The second is the theoretical significance: the indirect verification of Coulomb's law. The Gauss theorem can be derived from Coulomb's law. If the inverse square exponent in Coulomb's law is not equal to 2, the Gauss theorem cannot be obtained. On the contrary, if the Gauss theorem is proved, it will prove the correctness of Coulomb's law. According to the Gauss theorem, the field strength inside the insulated metal spherical shell should be zero, which is also the conclusion of electrostatic shielding. If an instrument is used to detect whether the shielding shell is charged or not, the correctness of Gauss theorem can be determined by analyzing the measurement results, which also verifies the correctness of Coulomb's law. The latest experimental results were completed by Williams et al. in 1971. It was pointed out that

F=q1q2/r2± δ Medium, δ＜ （2.7±3.1） × 10-16，

It can be seen that the inverse square relation of Coulomb's law is strictly valid within the experimental accuracy that can be achieved at this stage. From the point of view of practical application, we can think that it is correct. PCB manufacturers, PCB designers and PCBA processors will explain to you the PCB manufacturers: electrostatic shielding of magnetic shielding materials.