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

2. 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 Electric field lines (a) near a single positive point charge, (b) near a single negative point charge.

3. 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 )

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

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

6. 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:

7. 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 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.

8. 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

9. 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 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┴.