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Faraday Cage: Definition and Function

Faraday cage: definition and function

A Faraday cage is a box or cage-like structure made of electrically conductive material, such as copper or iron, that prevents static electric fields from penetrating inside the cage. The Faraday cage is named after its inventor, Michael Faraday.

The cage is permeable to static magnetic fields such as the Earth's magnetic field and to external electric fields. For this reason, the Faraday cage provides protection from static electrical discharges such as lightning in a thunderstorm.

In practice, the term 'Faraday cage' is used today mainly for rooms that exclude electromagnetic waves. These spaces become dead spaces without electromagnetic fields.

As a general rule, a Faraday cage is impervious to electromagnetic radiation if the size of its metal mesh is less than one-tenth of the wavelength of that radiation.

If the wavelength of a Faraday cage is greater than the size of the metal mesh, it will not be possible to receive or transmit radio signals within the cage.

How Does the Faraday Cage Work?

The Faraday cage works thanks to the properties of an electrical conductor that is in electrostatic equilibrium. As soon as an external electric field comes into contact with the cage, the positive electric charges become fixed. However, the electrons that are free in a metal begin to move from an atom pushed by the force of the electric field. 

Faraday cage: definition and functionElectrons, having a negative charge, move in the opposite direction to the electric field. Although the total electrical charge of the conductor is zero, an excess of negative charge is concentrated on one side of the cage and the opposite side remains positively charged. This movement of electrons causes an electric field in the opposite direction to the external field to be created inside the box.

The result of the two electric fields in the opposite direction causes that inside, the electric field is zero. Inside the box there is no electric field and therefore no charge can pass through it.

Examples of the Faraday Cage in Everyday Life

 

  • Microwave ovens consist of a metal box, with a transparent door on the front where a metal film has been deposited, with holes to make it transparent. This structure forms a Faraday cage for microwave radiation. Microwave radiation has a wavelength of about 12 centimeters, which is too long to escape through the holes.

  • The metal structure of an airplane forms a Faraday cage. For this reason, if an aircraft is struck by lightning , the passengers do not perceive any electric field.

  • The room in which an MRI scanner is located is protected with a Faraday cage.

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Published: January 12, 2022
Last review: January 12, 2022