Operation Faraday cage

1 operation

1.1 continuous

1.1.1 interior charges
1.1.2 exterior fields


1.2 faraday cage

operation

animation showing how faraday cage (box) works. when external electrical field (arrows) applied, electrons (little balls) in metal move left side of cage, giving negative charge, while remaining unbalanced charge of nuclei give right side positive charge. these induced charges create opposing electric field cancels external electric field throughout box.


continuous

a continuous faraday shield hollow conductor. externally or internally applied electromagnetic fields produce forces on charge carriers (usually electrons) within conductor; charges redistributed accordingly due electrostatic induction. redistributed charges reduce voltage within surface, extent depending on capacitance, however, full cancellation not occur.

interior charges

if charge placed inside ungrounded faraday cage, internal face of cage becomes charged (in same manner described external charge) prevent existence of field inside body of cage, however, charging of inner face re-distributes charges in body of cage. charges outer face of cage charge equal in sign , magnitude 1 placed inside cage. since internal charge , inner face cancel each other out, spread of charges on outer face not affected position of internal charge inside cage. intents , purposes, cage generates same dc electric field generate if affected charge placed inside. same not true electromagnetic waves.

if cage grounded, excess charges go ground instead of outer face, inner face , inner charge cancel each other out , rest of cage retain neutral charge.

exterior fields

skin depth vs. frequency materials @ room temperature, red vertical line denotes 50 hz frequency:

effectiveness of shielding of static electric field largely independent of geometry of conductive material, however, static magnetic fields can penetrate shield completely.

in case of varying electromagnetic fields, faster variations (i.e., higher frequencies), better material resists magnetic field penetration. in case shielding depends on electrical conductivity, magnetic properties of conductive materials used in cages, thicknesses.

a idea of effectiveness of faraday shield can obtained considerations of skin depth. skin depth, current flowing in surface, , decays exponentially depth through material. because faraday shield has finite thickness, determines how shield works; thicker shield can attenuate electromagnetic fields better, , lower frequency.

faraday cage

faraday cages faraday shields have holes in them , therefore more complex analyze. whereas continuous shields attenuate wavelengths shorter skin depth, holes in cage may permit shorter wavelengths pass through or set evanescent fields (oscillating fields not propagate em waves) beneath surface. shorter wavelength, better passes through mesh of given size. work @ short wavelengths (i.e., high frequencies), holes in cage must smaller wavelength of incident wave. faraday cages may therefore thought of high pass filters.