1. Electric Forces and Fields
The Electric Field

2. Electric Field Strength
Electric field – the region in space around a charged object in which a stationary charged object experiences an electric force because of its electric charge
Any charged particle in that electric field will experience an electric force

3. Electric Field Strength
Electric field strength is a vector
Measured in Newtons/Coulomb (N/C)
The direction of the electric field depends on the sign of the charge producing the field
The strength of an electric field depends on charge and distance
Positive charges produce an outward electric field
Negative charges produce an inward electric field

4. Electric Field Strength

5. Electric Field Strength
Electric field strength is measured by placing a very small test charge near a larger charge
The test charge (q0) must be very small compared to the main charge (Q)
If not, is will cause a redistribution of charge on Q, which will effect electric field strength
Electric field strength = coulomb constant * charge producing the field/distance2
E = kCq/r2
Electric fields exist near any charged body, even in the absence of a test charge

6. Electric Field Strength

7. Electric Field Strength

8. Electric Field Lines Show electric fields with electric field lines
Lines that represent both the magnitude and the direction of the electric field
The stronger the field, the more lines are present
Point out from a positive charge
Point in toward a negative charge

9. Electric Field Lines

10. Electric Field Lines Rules for drawing electric field lines
The lines must begin on positive charges or at infinity and terminate on negative charges or at infinity
The number of lines drawn leaving a positive charge or approaching a negative charge is proportional to the magnitude of the charge
Electric field lines from the same field never cross

11. Electric Field Lines
Electric dipole – the electric field produced between to objects of equal and opposite charge
The number of electric field lines leaving the positive charge must be equal to the number of electric field lines entering the negative charge
If one charge or field is twice as strong as another, it should have twice as many field lines

12. Electric Field Lines

13. Electric Field Lines

14. Electric Field Lines

15. Conductors in Electrostatic Equilibrium
Good conductors contain charges (electrons) that are free to move about within the material
When there is no net motion of charge within a conductor, it is said to be at electrostatic equilibrium

16. Conductors in Electrostatic Equilibrium
Electrostatic equilibrium has the following conditions
The electric field is zero everywhere inside the conductor
Any excess charge on an isolated conductor resides entirely on the conductor’s outer surface
The electric field just outside a charged conductor is perpendicular to the conductor’s surface
On an irregularly shaped conductor, charge tends to accumulate where the radius of curvature of the surface is smallest, that is, at sharp points

17. Conductors in Electrostatic Equilibrium
The air surrounding a very strong electric field can become ionized
A bluish glow can be seen as charges leap from the surface to the air
Called a corona or St. Elmo’s fire
Van de Graaff generators collect electric charge
Uses static electricity to build up negative charge in the belt and ball and positive charge on the pulleys moving the belt

18. Conductors in Electrostatic Equilibrium

19. Conductors in Electrostatic Equilibrium