A Faraday cage
(or shield) is an enclosure formed by
a woven wire mesh of
conducting material. Such an enclosure blocks
out external static
electric fields. Faraday cages are named after Michael Faraday,
The cage's operation depends on the
theory that an external static
electrical field will cause the
electrical charges within the cage's conducting material to redistribute
themselves so as to cancel the field's effects in the cage's interior. This
phenomenon is used, for example, to protect electronic equipment from lightning strikes and other
Faraday cages cannot block static and slowly varying magnetic fields, such as Earth's magnetic
field (a compass will
still work inside). To a large degree though, they also shield the interior from
electromagnetic radiation if the conductor is thick enough and any holes are
significantly smaller than the radiation's wavelength. For example,
computer inspection and/ortest procedures of electronic components or systems that
require an environment not influenced by electromagnetic interference
are usually conducted
within a Faraday Cage (screen room). These screen rooms are essentially work areas that are
completely enclosed by one or more layers of fine
copper mesh. The
screen layers are often grounded to dissipate any electric currents
generated by the external electromagnetic fields and
should block a large amount
of the electromagnetic interference.
radio signals (electromagnetic radiation)
through an antenna within a cage can be greatly attenuated or even completely
blocked by the cage itself.
once observed that the charge on a
charged conductor resided only on its exterior and had no influence on
anything enclosed within it. To prove this theory
he constructed a room
coated with metal foil and applied high-voltage discharges
electrostatic generator to strike the rooms exterior.
Using an electroscope he
demonstrated that no electric charge present on the
interior walls of the room.
Several years prior to Faraday's,
Franklin observed the effect by lowering an uncharged cork
ball into a can which was suspended on a silk thread through an opening.
His conclusion was "the
cork was not attracted to the inside of the can as it would have been to
the outside, and though it did touch the bottom, when drawn out it was
not found to be electrified by that contact, as it would have been
by touching the outside." The fact is singular." Franklin had discovered
the behavior of what we now refer to as a Faraday cage or
|A Faraday cage is
similar to an ideal hollow conductor. Externally applied electric fields produce
forces on the
charge carriers (usually electrons) within the conductor, generating a
current that rearranges the charges. Once the charges have rearranged
so as to cancel the applied field inside, the current stops.
If a charge
is placed inside an ungrounded Faraday cage, the internal face of the cage
will be charged (in the same manner described for an external charge) to
prevent the existence of a field inside the body of the cage. However,
this charging of the inner face would re-distribute the charges in the
body of the cage. This charges the outer face of the cage with a charge
equal in sign and magnitude to the one placed inside the cage. Since the
internal charge and the inner face cancel each other out, the spread of
charges on the outer face is not affected by the position of the internal
charge inside the cage. Basically, the cage will
generate the same electric field it would generate if it was simply
charged by the charge placed inside.
the cage is
grounded, the excess charges will go to the ground instead of the outer
face, so the inner face and the inner charge will cancel each other out
and the rest of the cage will remain neutral.
Effectiveness of shielding of a static electric field depends upon the
geometry of the conductive material. In the case of a non-linearly varying
electric field, and hence an accompanying varying magnetic field, the
faster the variations are (i.e., the higher the frequencies), the better
the material resists penetration, but on the other hand, the better it
passes through a mesh of given size. In this case the shielding also
depends on the
electrical conductivity of the conductive materials used in the cages,
as well as their thicknesses.
electrical field causes the charges to rearrange, which cancels the
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January 02, 2017