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/ r2
E = k Q/ r2

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

9. Example
Find the total field acting on point P. If P is a proton initially at rest, what will its acceleration be?
20 cm
25 cm P
Q1 = -25μC
Q2 = +50μC

10. 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

11. 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

12. What will happen? +