Problem
Qualitative: electrostatic shielding. Explain in five lines why, during a thunderstorm, you are safer inside a car with a metal body than outside in the open. And why the same thing makes it hard to receive a mobile phone signal inside that same car.
Solution
The car’s metal body behaves like a Faraday cage. When an external electric field hits the conductor, the free charges in the metal redistribute themselves almost instantaneously over the surface, arranging themselves so as to generate, inside, a field exactly opposite to the external one. The superposition of the two contributions cancels the field in the cavity: During a lightning strike, therefore, the current flows along the outer surface of the body and discharges to ground through the tyres or the air, without passing through the cabin: the passengers remain protected (it is not the rubber tyres that protect, but the metal shell itself).
The same mechanism, however, also shields the cabin from low-frequency electromagnetic waves: the metal walls strongly attenuate the mobile phone signal field, which is why reception inside the car is often worse than in the open. Protection and interference are two faces of the same phenomenon.
Connections
Topics: Electric field and potential Concepts: Conductors in electrostatic equilibrium · Electric field Skills: Applying Gauss’s theorem Methods: Gauss’s theorem Objects: Charged sphere