1. Electric Fields

2. Force over a distance  Both gravity and electric force act over a distance without touching (unlike other forces)  Very difficult for early scientists to accept  Michael Faraday proposed the Electric Field to explain

3. Electric Field  Force that extends outward from any charged object and permeates through space  Any second charged object placed around the first charge will feel a force of attraction or repulsion due to this field  Tested with an imaginary positive test charge (q)

4.  Magnitude of the force acting on the test charge can be measured

5. Definition  Electric field (E) is defined as the amount of force (F) per unit of charge (q) E = F/ q units N/C

6. Another Equation  E = F/ q  F = k qQ/ r 2  E = k Q/ r 2

7. Example  Find the magnitude and direction of the electric field on a particle P which is located 30 cm right of a point charge of –3.0 x 10 -6 C.

8. Copying machines work by (+) charges on a drum attracting (-) charged ink particles. When a paper runs over the drum, the ink sticks to paper. Each ink particle has a mass of 9.0 x 10 -16 kg and carries 20 electrons. In order to stick, the force of attraction must be at least 2x greater than its weight. What is the field strength at the drum’s surface?

9.  If the field is due to more than one charge, the total field is the sum of the two individuals

10. Example  Find the total field acting on point P. If P is a proton initially at rest, what will its acceleration be? P Q 1 = -25μC Q 2 = +50μC

11. Field Lines  Lines of force can be drawn to visualize the force acting on a single point charge  Stronger fields have more lines  On point charges  On double points  Start on + and go to -  On plates

12. Fields and Conductors  The electric field inside any good conductor is zero - charge distributes itself evenly over the surface of a conductor making the net field inside zero  Electric field is always perpendicular to the surface of a conductor  Excess charge tends to accumulate on sharp points or areas of greatest curvature

13. What will happen? +