When you get into a car on dry asphalt and touch it, you sometimes get a "shock". Explain what happens in terms of charge, potential and discharge. Why is the car considered a "Faraday cage"?
Solution
The charge. Rubbing your clothes on the seat (or walking on dry asphalt) transfers charge through triboelectricity: your body becomes electrostatically charged, building up an excess charge of a certain sign. Since dry air is a good insulator, this charge does not disperse easily and stays on the body.
The potential. A charged, isolated body rises to a high potential relative to objects connected to earth. Even modest charges (nanocoulombs) on a small capacitance like that of the human body ( pF) generate potential differences of thousands of volts:
The discharge. When you bring your finger close to the metal bodywork (at a different potential) the field between finger and metal grows until it exceeds the breakdown field of air ( V/m). The air ionises, a small spark jumps and the charge flows rapidly: that is the shock you feel. The phenomenon is more marked in winter, when the air is dry and conducts worse.
Faraday cage. In electrostatic equilibrium conditions the electric field inside a hollow conductor is zero and excess charges arrange themselves on the outer surface. The car’s metal shell therefore behaves like a Faraday cage: by shielding the interior, it protects the passengers even from a lightning strike, which flows along the outer surface and discharges to earth through the tyres or the air, without crossing the cabin.
Links
Topics: Electric field and potential Concepts: Conductors in electrostatic equilibrium · Electric potential Skills: Application of Gauss’s theorem Methods: Gauss’s theorem Objects: Charged sphere