1. How do electrons know what to do?
Electric Fields

2. - + Electric Field Maps Electric fields… exist near CHARGED OBJECTS
mapped using a POSITIVE test charge
1 – place test charge
2 – draw arrow in direction pushed
3 – this is the field direction + - + +

3. Electric Field Maps + -

4. Electric Field Maps + +

5. Electric Field Strength
Gets WEAKER with distance
FORCE exerted per unit CHARGE E r + A B C
A > C > B

6. Example #1
An object with a charge of -2 coulombs positioned in a uniform electric field experiences a force of -300 newtons. What is the strength of the electric field?
E= Fe/q
E = -300 N / -2 C
E = 150 N/C

7. Example #2
What is the force experienced by a 0.05 coulomb charge when it is placed in an electric field with a strength of 3 x 103 newtons per coulomb?
Fe = Eq
F = (3 x 103 N/C)(0.05 C)
F = 150 N

8. Parallel Plates + + + + + + + + + + - - - - - - - - - - - -
Field strength…
Between plates
E  Fe = SAME AT ALL POINTS
Outside plates = ZERO

9. Hollow Charged Spheres
Field strength…
Normal field pattern outside
ZERO inside
E = 0

10. End of PRACTICE