The cage is designed so that no discharges will reach inside.The minimal distance between the bars ensures that a discharge will always find it easier to hit a bar than pass in between to another object. The person inside is therefore protected. The discharge is created by a vast Van der Graaff generator (columns and spheres) behind the cage, with a potential difference of 2.5 million volts. Faraday cages are used to protect electrical equipment from lightning strikes.
Electrostatic equilibrium in conductors
A charged conductor tends to redistribute its excess charges evenly all around itself.
We say that the conductor is in electrostatic equilibrium since there is no movement
of charges. A conductor in electrostatic equilibrium shows the following properties:
- There are no excess charges inside a conductor.
- The charges reside only on the surface of the conductor.
- The electric field on the surface of the conductor is perpendicular to the surface.
- The electric fields are strongest at locations along the surface where the object
is most curved.
Once a conductor is in electrostatic equilibrium, the electric field anywhere beneath
the surface of a charged conductor is zero. If an electric field did exist beneath
the surface of a conductor then the electric field would exert a force on the electrons
present there. This net force would begin to accelerate and move these electrons.
But since the conductor is in an electrostatic equilibrium, we should have no further
motion of charge. So if this were to occur, then the original claim that the object
was at electrostatic equilibrium would be a false claim. If the electrons within
a conductor have assumed an equilibrium state, the net force upon those electrons
The electric field lines either begin or end upon a charge and in the case
of a conductor, the charge exists solely upon its outer surface. The lines extend
from this surface outward, not inward. This concept of the electric field being
zero inside of a conducting surface was first demonstrated by Michael Faraday, a
19th century pioneer.
Faraday constructed a room within a room, covering
the inner room with a metal foil. He sat inside the inner room (now known as the
Faraday cage) with an electroscope and charged the surfaces of the outer and inner
room using an electrostatic generator. While sparks were seen flying outwards from
the walls of the outer room, there was no detection of an electric field within
the inner room. This clearly showed that charges in a conductor reside solely on
its outer surface. Inside a closed conductor, the electric field is zero.
Any closed, conducting surface can serve as a Faraday’s cage, shielding anything
that surrounds from the potentially damaging effects of electric fields. This principle
of electrical shielding is commonly utilized today. We protect delicate electrical
equipment by enclosing them in metal foils or cases. Even delicate computer chips
and other components are shipped inside of conducting plastic packaging, which shields
the chips from electric fields. In spacecrafts and satellites many delicate electrical
components are separated from each other by electrical shielding so that functioning
of one does not affect the other component.