Illustrated Physics
physics, drawn
Concept

Electrostatic shielding — where it appears

The exclusion of external electric fields from the interior of a conductor, which happens because the surface charge rearranges until it cancels them. It works for any shape and any external arrangement, needs no earth connection, and fails only for the field of charge placed inside the cavity itself.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

A conductor in a field, with the surface charge solved for. Field lines approaching an isolated conducting cylinder. The surface charge was found by requiring the conductor to be an equipotential, and the lines then end on that charge, meeting the surface at right angles and leaving the interior empty.

The inside of a conductor, where the field is exactly nothing

Put a metal object in any electric field and the field inside it is zero. Not small — zero, by an argument that takes one sentence, and with consequences that reach from lightning to the most precise test of Coulomb's law ever made.

electromagnetism · Conductors The plane deleted, and one charge put in its place. A charge of 1 nC held 20 mm above an earthed conducting plane. The lines are traced through the field of the real charge plus an equal and opposite one at the mirror position, and then cut at the plane, because below it there is metal and no field whatever. Nothing in the tracing knows about the surface: each line follows the local field direction and stops where it arrives. That every one of them arrives perpendicular — the worst departure among the 9 drawn is 2.2° away from square — is the boundary condition showing itself rather than a rule imposed on the drawing. The image charge is drawn faint because it is not there: it is a way of writing a function that happens to satisfy the equation and the boundary values, which by the uniqueness theorem makes it the field and not a model of the field.

The charge that has to be somewhere else

Hold a charge above an earthed metal sheet and the field above it is exactly the field of two charges — the real one and an imaginary partner buried at the mirror position. The partner is not an analogy or an approximation. It is a legal guess, and a legal guess is a proof.

electromagnetism · Conductors

Named alongside it

The objects these essays reach for when they reach for this one.

Boundary conditionsElectric fieldEquipotentialSurface chargeConductorConductorsElectric potentialThe Faraday cageField linesGauss's lawImage chargeSuperposition

All concepts