Day 5: Objectives Electric Field Lines Point Charges Electric Diploes Parallel Plates Electrostatic Fields and Conductors

Electric Field Lines An electric field is a vector quantity An electric field of a (+) point charge points radially outward as lines of force An electric field of a (-) point charge points radially inward Figure 21-33. Electric field lines (a) near a single positive point charge, (b) near a single negative point charge.

Electric Field Lines The distribution of the electric field is called a vector field The number of field lines plotted is proportional to the magnitude of the charge. The nearer the charge, the stronger the electric field ( F α 1/r2 )

The Electric Dipole The electric field produced by 2 point charges of equal magnitude but opposite sign is called an electric dipole.

Electric field lines are directed from (+) to (-) and start out perpendicular to the surface of the charge

Parallel Plates In the central region of the electric field between two closely spaced, oppositely charged parallel plates, the magnitude is the same at all points and is:

Electrostatic Fields & Conductors The static electric field inside a conductor is zero – if it were not, the charges would move. The net charge on a conductor resides on its outer surface. Figure 21-36. A charge inside a neutral spherical metal shell induces charge on its surfaces. The electric field exists even beyond the shell, but not within the conductor itself.

Electric Fields & Conductors Free electric charges (electrons) move until they reach positions where the electric field, & therefore, the electric force on them becomes zero. (They raise to the surface to repel each other) Electric fields are always perpendicular to the outside surface of the conductor

Electrostatic Fields & Conductors A metal box or cage can be used as an effective device against external fields. This is called a Faraday Cage Figure 21-37. If the electric field at the surface of a conductor had a component parallel to the surface E||, the latter would accelerate electrons into motion. In the static case, E|| must be zero, and the electric field must be perpendicular to the conductor’s surface: E = E┴.