β‘Conductors & Insulators
How Charge Moves in Materials
Conductors: materials where electrons move freely (metals like copper, silver, gold). Charge distributes itself on the surface until equilibrium.
Insulators: materials where electrons are tightly bound (rubber, glass, plastic). Charge stays where you put it.
Semiconductors: in between (silicon, germanium) β used in electronics.
In a conductor at electrostatic equilibrium: - Electric field INSIDE = 0 - All excess charge resides on the SURFACE - Electric field at surface is perpendicular to surface - Charge accumulates more at sharp points (lightning rods exploit this!)
Think About It
A hollow metal sphere is charged. Where does the charge go β on the inner or outer surface? What is the electric field inside the sphere?
Practical Applications
Faraday cage: A metal enclosure blocks external electric fields. The charges on the conductor rearrange to cancel the external field inside. Your car protects you from lightning.
Lightning rods: The sharp tip has a very high charge concentration (strong E field). This ionizes surrounding air, providing a safe path for lightning discharge.
Capacitors: Two conducting plates separated by an insulator, storing charge and energy.
π Key Facts
Conductors at equilibrium
E_inside\ conductor = 0Excess charge \to outer surfaceEβ β₯ surface at conductor boundaryβ οΈ Common Mistakes
Misconception: Charge distributes uniformly across any conductor surface.
β Correct thinking: Charge concentrates at sharp points and regions of high curvature. Flat or concave areas have lower charge density.
Why: The electric field near a pointed tip is much stronger β charge is "pushed" out toward extremities. This is how lightning rods work.
Misconception: A charged insulator will always repel a neutral conductor.
β Correct thinking: A charged object attracts a neutral conductor (regardless of the sign of the charge) through electrostatic induction.
Why: The charged object induces a separation of charge in the conductor. The near surface gets the opposite charge and the far surface gets the same charge. Since the opposite charge is closer, the net force is attractive.
Misconception: The electric field inside a hollow conductor is the same as outside.
β Correct thinking: At electrostatic equilibrium, the electric field everywhere inside a conductor (including hollow regions) is exactly zero.
Why: Any internal field would accelerate free electrons until they rearrange to cancel it. The "shielding" effect is perfect for static fields β the basis of the Faraday cage.
π Practice Problems
Try these problems. Check your answer when ready.
A solid metal sphere is given a charge of +10 ΞΌC. Where does the charge reside, and what is the electric field inside the sphere?
Why is it safer to be inside a car than outside during a lightning storm, even if the car is struck?
A hollow metal sphere has +20 ΞΌC on its outer surface. A β5 ΞΌC charge is placed in the hollow interior (not touching the sphere). What charge appears on (a) the inner surface and (b) the outer surface?
Two metal spheres, radius Rβ = 0.1 m and Rβ = 0.3 m, are connected by a thin wire and given a total charge of +8 ΞΌC. How does charge distribute between them?
A neutral metal rod is brought near (not touching) a positively charged balloon. Describe the charge distribution on the rod and explain why the rod is attracted to the balloon.
Finished reading through this lesson?