Electrostatic shielding
Electrostatic shielding: In order to avoid the influence of external electric field on the equipment, or to avoid the influence of the electric field of the electrical equipment on the outside world, a cavity conductor is used to cover the external electric field, so that the internals are not affected, and the electrical equipment is not external to the outside. The effect is called electrostatic shielding.
The shield of the cavity conductor that is not grounded is an outer shield, and the shield of the cavity conductor ground is fully shielded. The cavity conductor is in electrostatic equilibrium in the external electric field, and its internal field strength is always equal to zero. Therefore, it is impossible for the external electric field to have any influence on its internal space. If there is a charged body in the cavity conductor, its internal surface will produce an equal amount of induced charge when it is electrostatically balanced. If the outer casing is not grounded, the outer surface will generate the same amount of induced charge as the inner charged body. At this time, the electric field of the induced electric charge will have an influence on the outside world. At this time, the cavity conductor can only be shielded from the external electric field, but the internal electrification cannot be shielded. The effect of the body on the outside world, so called external shielding. If the case is grounded, even if there is a charged body inside, the algebraic sum of the charge induced by the inner surface and the charge of the charged body is zero, and the induced charge generated by the outer surface flows into the ground through the ground line. The outside world cannot affect the inside of the shell, and the influence of the internal charged body on the outside is also eliminated, so this shielding is called full shielding. In order to prevent interference from external signals, electrostatic shielding is widely used in scientific and technical work. For example, the metal cover on the outside of the electronic equipment, the lead skin on the outside of the communication cable, etc. are all shielding measures for preventing external electric field interference.
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.
Because 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 when the static electricity is balanced. This is not to say that the external charge does not generate an electric field in the shell.
The electric field. Since the outer wall of the shell induces a different electric charge, they are zero with the resultant field excited by q at any point in the inner space of the shell. Therefore, the inside of the conductor shell is not affected by the external charge q or other electric field. The induced charge on the outer wall of the shell acts as an automatic adjustment.
If the cavity conductor casing is grounded, a positive charge on the casing will flow into the ground along the ground. 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 a charge in the cavity, the cavity conductor is still equipotential to the ground and there is no electric field in the conductor. At this time, due to the inductive charge of the inner wall of the cavity, there is an electric field in the cavity. This electric field is generated by the charge in the shell, and the 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 inside 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. produce. It can also be said that it is directly generated by the induced charge outside the shell.
However, if the case is grounded, the charge outside the case will disappear, and the electric charge in the case and the induced charge on the inner wall will generate an electric field outside the case (Fig. 5). It can be seen that if the charge in the shell is not affected by the electric field outside the shell, the casing 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 in any case the outer wall of the shell must be uncharged. 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 (Figure 6).
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. One is the practical meaning: 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. In order to avoid interference, some electronic devices or measuring devices must be electrostatically shielded, such as a metal cover with a grounded high-voltage device cover or a dense metal mesh cover, and a metal tube for the electron tube. Another example is a full-wave rectification or bridge rectification power transformer. A metal foil is wrapped between the primary winding and the secondary winding or an enameled wire is wound and grounded to achieve shielding. In high-voltage live working, workers wear a pressure equalizing suit woven with wire or conductive fiber to shield the human body. In the electrostatic experiment, there is a vertical electric field of about 100 V/m near the earth. To rule out the effect of this electric field on electrons, and to study the movement of electrons only under the action of gravity, it must have eE<meg, which can be calculated as E<10-10V/m, which is an "electrostatic vacuum" with almost no electrostatic field. Only electrostatic shielding of the vacuumed cavity can be achieved. In fact, electrostatic shielding by a closed conductor cavity is very effective.
The second is theoretical: indirect verification of Coulomb's law. The Gauss' theorem can be derived from Coulomb's law. If the inverse squareness index in Coulomb's law is not equal to 2, the Gauss's theorem cannot be obtained. On the contrary, if the Gauss's theorem is proved, the correctness of Coulomb's law 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 shield case, the correctness of the Gauss theorem can be determined by analyzing the measurement results, and the correctness of Coulomb's law is verified. The recent experimental results were completed by Williams et al. in 1971, pointing out
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 a practical point of view, we can think of it as correct.
In a statically balanced conductor, the internal field strength is zero. The hollow conductor is hollowed out into a conductor shell, and the field strength in the shell is still zero everywhere. In this way, the conductor shell can protect the area it surrounds, so that this area is not affected by the external electric field. This phenomenon is called electrostatic shielding.