Faraday cage

The Faraday cage (also Faraday cage ) is a sleeve closed on all sides of an electrical conductor (e.g., wire mesh or plate) which acts as an electrical shield. For external static or quasi-static electric fields, the inner region due to the induction remains field-free. In time-varying processes such as electromagnetic waves, the shielding is based on the forming in the conductive sheath eddy currents that counteract the external electromagnetic field. Static or slowly varying magnetic fields (such as the earth's magnetic field ) are not shielded by a Faraday cage.

The term goes to the English physicist Michael Faraday back ( 1791-1867 ). The quantity of shielding effect is recognized over the shielding effectiveness (for example, a shield ).

A Faraday cage leads inter alia to the following effects:

• Should a lightning in a Faraday cage, such as a car or a plane, people remain safely in the interior, because the electric field strength in the interior is considerably lower than in the outer space. In the vicinity of openings in the metal shell an external field penetrates a proportionate in the screened room. The extent of penetration depends on the extent of the opening in relation to the wavelength of the spectrum of the external field.
• If an electrical discharge inside a Faraday cage produced, whereas outside observers remain unchallenged. This principle is exploited in the metallic liner in high-voltage laboratories.

Shielding of electrostatic fields

The electrostatic shield or quasi-static electric fields, is due to the effect of electrostatic induction. An electrically conductive sheath, such as a hollow sphere, placed in an applied external electrostatic field E, there is due to the force effect to those in the case free charges Q to spatial redistribution of charges at the surface to the tangential standing on the surface of electrical external field component is zero, and thus a balance is found. Characterized originates or terminates in the static case, the electric flux at the surface of the sleeve, whereby the interior of the sheath is blank. This shielding effect is not restricted to a particular shape of the sheath and occurs with any shape hollow bodies, as long as they are electrically conductive.

The damping is in a fully closed conducting shell in the static case ideal and infinite, for quasi -stationary fields this is satisfied to a good approximation. With the aid of the Gaussian law can be used for the normal component of the electric field in the empty space immediately above the outer cover:

And in the interior to

Determine the surface charge density and the dielectric constant.

The conductive sheath is an equipotential surface, which is referred to in the parlance electric wall. It is essential that the shielding effect is effective only against external electric fields. An electric river which rises by an insulated from the sheath charge accumulation inside the envelope, which the separate charge with the opposite sign is located in the outer region, thus resulting in the outdoor space to an electric field. However, there is an electrical connection between the charge-bearing portions and the inner cover, the electric charges are moved to the surface and the inner region remains field-free. This principle of charge transfer is used in some high-voltage generators, such as the Van de Graaff generator for charge storage and generation of high electric voltages.

At not too high-frequency alternating fields, a Faraday cage instead of a closed conductor wall also from a cage of conductor rods, wires or a plate made ​​with small openings. The shielding attenuation is related to the mesh size should not exceed about 1/10 of the wavelength.

Shield of alternating fields ( electrodynamics)

An ideal Faraday cage shields and high-frequency alternating fields, because on the surface of the cage eddy currents are induced, which counteract the external field according to Lenz's law. The shielding effect is in this case, but not ideal, but characterized by finite screening attenuation and penetration depth into the screen.

Faraday cages made ​​of non - ferromagnetic metal shield due to its finite conductivity then high-frequency alternating fields from when the metal layer is significantly stronger than the penetration depth of the induced currents.

Slots lead to the interruption of the induced currents in the screen. Electromagnetic waves penetrate the shield comparatively good if slots are parallel to the magnetic field component of the wave on the screen. The shield attenuation can be gradually with increasing aperture becomes small as the wavelength of the incoming electromagnetic wave is in the order of the slot dimensions.

Applications

Faraday cages are often used where the influence of external electric or electromagnetic fields can adversely affect the functioning of a device or internal electromagnetic fields which are not intended to reach the outside. For example, it is used for shielding of measuring instruments, electrical wiring, or measurement rooms eg used before transmitters. The Faraday cage is then for example the housing of a conductive material or a thin metallic foil with which the room to be protected is covered.

The shield may include entire rooms, such as electromagnetically shielded rooms calmed test environment in EMC labs ( anechoic chamber ).

The principle of the Faraday shield is also used in the lightning protection for buildings. Here it is approximated by a coarse structure of lightning conductors and earthed parts of the building.

Even cars and airplanes with a conductive sheath act as Faraday cages. Electromagnetic fields, the wavelength of which are small compared to the electrically open joints and columns of the body are shielded, however, not efficient. This explains why in the car mobile reception is possible.

Small, Abschirmkäfige often made ​​of tinplate can be found at the high-frequency components in electronic devices (mobile phones, radio and TV tuner, wireless baby monitors, etc.).

The microwave oven is an example of a Faraday shield, in which a certain extent inside and outside are reversed. The metallic cooking space shields the environment from the high-power microwave radiation within the oven. You can usually find a resonance seal, which is only effective for a very specific wavelength at the door.

The metal housing a magnetron ensures that the high energy electromagnetic field is in the interior of the magnetron. A small part of the field is conducted through the antenna connection to the outside.

The simplified, two-dimensional design of a Faraday cage is called a corona ring and (varistors ) is in the high voltage range used for example in insulators and surge arresters. In the ring inside the field strength is very low, therefore, there is no loss-making field emission occur at corners and tips, such as the mounting harness.