1. Textbook: 7.1, 7.2 Homework: pg. 335-336 # 2 – 6
Coulomb’s Law
Textbook: 7.1, 7.2
Homework: pg # 2 – 6

2. Electrical Structure of Matter
The Law of Electric Charges:
Opposite Charges Attract
Similar Charges Repel
Charged objects attract some neutral objects
Law of Conservation of Charge:
For an isolated system the total charge is constant
Both gravity and the electrical force are “action-at-a-distance” forces

3. Coulomb’s Law Charles Augustin de Coulomb (1736 - 1806)
Two objects with charges q1 and q2 (in C) separated by a distance r (in m) experience a force of magnitude:
k = 9.0 x 109 Nm2/C2

4. Compare/Contrast Coulomb’s Law to Newton’s Law of Universal Gravitation

5. Pg 330
1. Two charged spheres, 10.0 cm apart, attract each other with a force of magnitude 3.0 x 10-6 N. What force results from each of the following changes, considered separately?
(a) Both charges are doubled, while the distance remains the same. [1.2 x 10-5 N]
(b) An uncharged, identical sphere is touched to one of the spheres and is then taken far away [1.5 x 10-6 N]
(c) The separation is increased to 30.0 cm. [3.3 x 10-7 N]

6. Two identical small spheres of mass ______ are fastened to the ends of an insulating thread of length ________. The spheres are given identical electric charges and hang in static equilibrium, with an angle of ________between the string halves, as shown above. Calculate the magnitude of the charge on each sphere.

7. Coulomb’s Law (Continue)
Textbook: 7.2
Homework: pg. 336 # 7 – 10

8. The principal of superposition
states that the resultant force is the vector sum of the forces that are coming together
Note: when finding the resultant force on a “free to move” charged object, keep all other charged objects stationary.

9. Pg 334 # 8
Three objects, carrying charges of -4.0 x 10–6 C, -6.0 x 10–6 C, and +9.0 x 10–6 C, are placed in a line, equally spaced from left to right by a distance of 0.50 m. Calculate the magnitude and direction of the net force acting on the first charge. (0.54 N [left])

10. Pg 334 # 9
Three spheres, each with a negative charge of 4.0 x 10–6 C, are fixed at the vertices of an equilateral triangle whose sides are 0.20 m long. Calculate the magnitude and direction of the net electric force on each sphere. (6.2 N [outward, 150º away from each side])

11. 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

12. 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

13. 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.

14. 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

15. 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

16. 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]]

17. 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.

18. Textbook: 7.4 Homework: pg. 358 – 359 #1 – 8
Electric Potential
Textbook: 7.4
Homework: pg. 358 – 359 #1 – 8

20. Electric Potential Energy
Work done is the change in the electric
potential energy of the charge in an electric field.
The electric potential energy stored in a system with two charges q1 and q2, separated by a distance r is:
q1q2 < 0 charges are bound
q1q2 > 0 charges are unbound

21. Electric Potential
Electric potential energy per unit charge is referred to as electric potential or just potential.
The electric potential (V) a distance r away from a charge q is:
The change in electric potential between two points is the electric potential difference V (also know as the voltage)

22. Important Relationships
In general
For a parallel plate
(electric field,  = constant)

23. A positive test charge of 1
A positive test charge of 1.5 x 10-6 C is placed in an electric field 10 cm from another charge of magnitude x 10-6 C that is anchored in place.
a) What is the electric potential energy of the test charge? [-6.8 x 10-1 J]
b) What is the potential difference between the test charge’s initial position and a point 5.0 cm closer to the negative charge? [-4.5 x 105 V]