1. Textbook: 7.3 Homework: pg. 343-344 # 1 – 7 pg. 347 # 8 – 10
Electric Fields
Textbook: 7.3
Homework: pg # 1 – pg. 347 # 8 – 10
Review:
- Compare/Contrast electric and gravitational fields
- Concept of gravitational field strength

2. Electric Fields
A field can be represented with a field diagram which follows these rules:
Field lines come out of sources (+ve charge)
Field lines go into sinks (-ve charge)
Field strength depends on line density
Field lines cannot cross
Ex. Positive charge
Ex. Negative charge
Ex. Positive and negative charge
Ex. Capacitor
Ex. Pg. 344 #6
Ex. What is the magnitude of a field inside a conductor which is in a capacitor?
Discuss: Electrical shielding and coaxial cables.

3. Field lines: Ex. Positive charge Ex. Negative charge
Ex. Positive and negative charge
Ex. Capacitor
Ex. Pg. 344 #6 The electric field strength midway between a pair of oppositely charged parallel plates is 3.0 x 103 N/C. Find the magnitude of the electric field midway between this point and the positively charged plate.
Ex. What is the magnitude of a field inside a conductor which is in a capacitor?

4. Electric Fields
The electric field a distance r (in m) away from a charge q1 (in C) is:
k = 9.0 x 109 Nm2/C2
Pg. 343 #1, 5

5. Pg 343 # 1
Two negative charges of 2.4 x 10-6 C and 4.0 x 10-6 C are placed 0.5 m apart. Where would a positive charge of 1.0 x 10-6 C be placed to have a zero net electric force/field? Illustrate using field lines.
0.22 m

6. Pg 344 # 5
Determine the magnitude and direction of the electric field at point Z in Figure 14, due to the charges at points X and Y.
[1.2 x 105 N/C [up]]

7. Eight negative point charges of equal magnitude are distributed evenly around a circle.
Sketch the electric field in the region around and within this charge distribution.
Explain how this charge distribution can be used to model the electric field inside a coaxial cable.

8. What would happen to the uniform field strength inside a parallel-plate capacitor if the following changes were made independently of each other?
a) The distance between the plates is doubled.
b) The charge on each plate is doubled.
c) The plates are totally discharged and neutral.