等電位面と導体

A tangential electric field component outside the conductor would create a corresponding tangential component inside, causing charges to move in loops and violating electrostatic equilibrium. Since charges are at rest in a conductor, the electric field must be perpendicular to the surface, making it an equipotential surface where no tangential forces act on charges.

No charge accumulates on the cavity's inner surface because any tangential electric field would cause charge movement, violating electrostatic conditions. This means you can safely touch the interior walls of a charged metallic enclosure without electrical shock, as the cavity remains charge-free and protected from external electric fields.

When two spherical conductors with different radii are connected by a conducting wire, the entire system becomes equipotential. Both spheres reach the same electric potential, but the smaller sphere develops higher surface charge density and electric field strength due to its tighter radius of curvature.

For connected conductors at equal potential, expressing charge in terms of surface charge density reveals that smaller radii of curvature concentrate more charge per unit area. This relationship between radius and charge density explains why sharp points on conductors accumulate higher charge densities than flat or gently curved regions.

A lightning rod is a grounded metal rod with a sharp point that exploits high charge density at small radii of curvature. The intense electric field around the sharp tip ionizes air molecules when it reaches approximately 3.0 × 10^6 N/C, allowing free electrons to flow through the rod to Earth, neutralizing positive ground charges and preventing lightning formation nearby.

Inside a conductor at electrostatic equilibrium, the electric field is zero because charges are at rest. Outside the conductor, the electric field varies inversely with the square of the radial distance and is always perpendicular to the surface, directly related to the surface charge density at that location.

Tangential electric field components would exert forces parallel to the conductor surface, causing charges to move and violating the electrostatic condition where all charges are at rest. This constraint ensures that only perpendicular field components exist outside the conductor, maintaining the equipotential nature of the surface.