AP Physics C: E&M Unit 10 Cheatsheet: Conductors and Capacitors

Key principle: Excess charges move to the conductor surface and stop there almost instantly.

• Why: Mutual repulsion of charge carriers forces them outward until equilibrium is reached
• Result: Zero net charge and zero electric field inside the conductor
• Surface property: All points on the surface are at the same electric potential (equipotential)
• Field direction: Perpendicular to the outer surface

Charge distribution:

• Negative net charge → excess electrons on surface
• Positive net charge → electron deficiency on surface (model as positive charges)
• Higher density at points and edges than on flat areas

Polarization: Conductor remains equipotential even when external field is applied; charges redistribute to maintain this condition.

Field is uniform between plates (except near edges).

where σ = Q/A is surface charge density.

Key relationships:

• E is proportional to surface charge density
• E is perpendicular to plates
• E is constant in magnitude and direction (uniform field)

Charged particle motion: A particle between oppositely charged plates experiences constant acceleration, just like projectile motion in gravity. Use kinematics equations.

Connection to potential: E = ΔV/d, so ΔV = Ed