Conductors, dielectrics and polarisation

Conductors and dielectrics

Conductors (such as metals) contain free electrons that can move through the material. Insulators or dielectrics (such as glass, mica, paper) have electrons bound to their atoms, so charge cannot flow freely.

Electrostatic properties of a conductor at equilibrium:

• The electric field inside the conductor is zero.
• Any excess charge lives only on the outer surface.
• Just outside the surface the field is perpendicular to the surface, with magnitude σ/ε₀.
• The whole conductor is at one potential (its volume is equipotential).
• Charge density is higher at sharp points; this is why lightning conductors are pointed.

Electrostatic shielding: the field inside a hollow conductor (a cavity with no charge in it) is zero, whatever the outside field. This is why a car body can protect people inside during lightning.

Dielectrics and polarisation: in a dielectric, an external field slightly separates the positive and negative charges in each molecule (non-polar molecules) or lines up existing dipoles (polar molecules such as water). This is polarisation. The result is bound charges on the surfaces of the dielectric which set up a field opposite to the applied field, so the net field inside is reduced. The factor of reduction is the dielectric constant K (also called relative permittivity, ε_r = ε/ε₀). For vacuum K = 1; for all other materials K > 1. For a conductor, K is effectively infinite.

Key points

• Free charges move in conductors; bound charges only shift slightly in dielectrics.
• E inside conductor = 0; E just outside = σ/ε₀, normal to surface.
• Field inside a dielectric = applied field ÷ K.
• Polar molecules have a permanent dipole moment; non-polar molecules get an induced one.

Exam tips

• Statement-type questions about conductors are very common; learn the five properties above.
• Know the difference between polar (H₂O, HCl) and non-polar (O₂, H₂, CO₂) molecules.

Common mistakes

• Saying the potential inside a conductor is zero; it is constant, not necessarily zero.
• Confusing bound charges (in dielectrics) with free charges.
• Thinking charge spreads uniformly on any conductor; it gathers at sharp points.