The Principle of Electrostatic Shielding
If a conductor is placed in an external electric field with an electric field strength of E<sub>out</sub>, the free electrons inside the conductor will move against the direction of the electric field under the influence of the electric field force. Thus, the negative charge is distributed on one side of the conductor, and the positive charge on the other side; this is the phenomenon of electrostatic induction. Due to the redistribution of charges within the conductor, these charges form another electric field in the opposite direction to the external electric field, with an electric field strength. According to the principle of superposition of electric fields, the electric field strength inside the conductor is equal to the superposition of E<sub>out</sub> and E<sub>in</sub>. When the total electric field strength inside the conductor is zero, the free electrons inside the conductor no longer move. In physics, the state in which no charge moves in a conductor is called electrostatic equilibrium. In a conductor in electrostatic equilibrium, the electric field strength inside is zero everywhere. Therefore, it can be deduced that in a conductor in electrostatic equilibrium, the charge is only distributed on the outer surface of the conductor. If this conductor is hollow, when it reaches electrostatic equilibrium, there will also be no electric field inside. Thus, the outer shell of the conductor will "protect" its interior, making it unaffected by the external electric field; this phenomenon is called electrostatic shielding.
The practical significance of electrostatic shielding: Shielding protects instruments or working environments within a metal conductor shell from external electric fields, preventing them from being affected by them. Some electronic devices or measuring equipment require electrostatic shielding to avoid interference. For example, indoor high-voltage equipment is covered with a grounded metal cover or a dense metal mesh, and electron tubes are encased in metal shells. Similarly, power transformers used for full-wave or bridge rectification have a thin metal sheet or a layer of enameled wire wrapped between the primary and secondary windings and grounded to achieve shielding. In high-voltage live-line work, workers wear equalizing suits made of metal wire or conductive fibers, which provide shielding protection. In electrostatic experiments, a vertical electric field of approximately 100 V/m exists near the Earth. To eliminate the effect of this field on electrons and study their motion under gravity alone, eE < meg, which calculates to E < 10⁻¹⁰ V/m. This is an almost non-electrostatic vacuum, achievable only through electrostatic shielding of the evacuated cavity. In fact, electrostatic shielding achieved by a closed conductive cavity is very effective.