Problem — Faraday cage

Problem

Faraday cage. A person inside a car during a thunderstorm is protected from a possible lightning strike. Explain why, referring to the electric field inside a conductor and the role of the metal bodywork.

Solution

Zero field inside a conductor. In a conductor in electrostatic equilibrium the free charges redistribute over the surface until the field inside the material vanishes. The same holds for the cavity enclosed by a closed conductor: the internal electric field is zero.

$\vv{E}_{\text{interno}} = 0$

Role of the bodywork. The car’s metal shell behaves like a Faraday cage: when lightning strikes it, the current flows along the outer surface of the bodywork and from there to earth (through the tyres or the ionised air). Inside the cabin the field remains zero, so the occupants are not traversed by current.

Conditions. For the protection to work, the bodywork must be sufficiently conductive and enclose the cabin (a closed or near-closed cage). It is the metal, not the rubber tyres, that provides the shielding.

$\ev{\vv{E}_{\text{interno}} = 0}$

Linked atoms

Topics: Elettrostatica Concepts: Conduttori in equilibrio elettrostatico · Campo elettrico