Electromagnetic shielding prevents interference in electronic devices. Metal walls block electric fields, while magnetic fields must be deflected around an object. Faraday cages can block both external and internal electric fields. Magnetic shielding redirects magnetic field lines using materials that can be magnetized.
Electromagnetic shielding is the attempt to prevent electromagnetic interference in an electronic device. Electromagnetic waves, which have both an electrical and a magnetic component, can cause some devices to malfunction in a variety of ways. Metal walls can often be used to block the static waves of an electric field. Magnetic waves, on the other hand, cannot be blocked and must be deflected around an object. Some materials can redirect magnetic field lines around a device for this aspect of electromagnetic protection.
The first part of electromagnetic protection is electrostatic protection. Protection against external electric fields can be achieved with a metal box, often known as a Faraday cage. A Faraday cage, named after the English physicist Michael Faraday, is a solid or mesh enclosure made from a conductive material. When an external electric field reaches the walls of a Faraday cage, the electrons that serve as charge carriers will redistribute themselves to compensate for the field. If the walls of the cage are grounded or connected to an external conductive path, the electrical currents in the walls will dissipate.
A Faraday cage can also block electric fields originating within it. Indeed, a typical microwave oven is an example of such an application. The structure of a microwave oven is made of a conductive material, while the door is usually a metal mesh. Since the holes in the shield are smaller than the wavelength of the microwaves, usually defined as between one millimeter and one meter, the microwaves do not escape from the enclosure. For this reason, it is not recommended to remove the metal screen from the microwave door.
While a Faraday cage can provide a lot of electromagnetic protection, it can’t block static magnetic fields, the other component of an electromagnetic wave. While electrostatic waves are generated by stationary charges, magnetic fields are produced by moving charges. An electric current is a collection of moving charges, so magnetic waves are often caused by nearby electric currents. Both constant and slowly changing magnetic fields can be a problem for some electronic devices.
One solution for electromagnetic shielding is to use a shield that redirects magnetic field lines. Unlike electric field lines, magnetic field lines must always return to their point of origin. Therefore, magnetic shielding does not attempt to stop magnetic field lines; rather, it attempts to deflect them around an object. Materials that can be easily magnetized make good magnetic shields. Mumetal, an alloy made primarily of nickel, is a very effective but expensive material.
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