1. SWBAT calculate the electrostatic force acting on an object.
Coulomb's Law
SWBAT calculate the electrostatic force acting on an object.

2. Do Now

3. HW Answers: Practice- Conservation of Charge
(-) in knob, (+) on leaves
Gains
-1.5 C
Loses
9.38x1018 C excess electrons
+7 C sphere gained electrons 13
+6.5 C
Negative
4.06x1019 C excess protons
-2.08x10-18 C
+9 C sphere gained electrons

4. Conductors & Insulators
A conductor’s electrons are free to move around the object (ex. metals). An insulator’s electrons are not free to move around the object, but can move around inside the individual atoms.

5. Conductors & Insulators
Charged objects are attracted to neutral objects.
Becomes
Polarized
Which example is like the one we saw in the beginning of class with the ruler and the paper?

6. Why do we get shocked more in the winter?
In this animation a charged object (the sphere in the center) is attracting neutrally charged particles from the air. Remember that neutrally charged objects (conductors or insulators) will always be attracted to a charged object. As the objects reach the charged object they end up picking up excess charge. Once they pick up excess charge they now have the same charge as the object so they are repelled by it. As they leave, they take charges with them. Given enough time, the particles in the air will remove the excess charge from the object leaving it neutrally charged. This explains why on dry days we tend to have more trouble with static electricity build-up than on humid (moist) days. On moist days there are more water molecules in the air to steal charge more rapidly. On dry days there are fewer particles in the air to steal charges so we accumulate charge until we touch something and get discharged (shocked).

7. Critical Thinking
Is there a force between two charged objects? How can you tell? Give examples you have seen in this class or in your everyday life.

8. We can see objects move when charged objects are brought close to them.
In other words a mass is being accelerated
F=ma
So there must be some electrostatic force between them.

9. Coulomb’s Experiment
Charles-Augustin Coulomb used a torsion balance to examine the relationship between electrostatic force and 2 different variables:
1. The Magnitude of Charges (q)
Video 1: Show so that students know what a torsion balance is.
Video 2: Show from 0:28-1:18 to show how lasers aid in measuring the twist of the torsion balance.
2. The Distance between Charges (r)

10. Electrostatic Force and Distance
Electrostatic Force and Magnitude of Charge
Data Analysis:
What is the relationship between Electrostatic Force and the Magnitude of Charges?
What is the relationship between Electrostatic Force and the Distance Between objects?
Write an equation for Electrostatic Force.
Where have we seen a relationship like this before?
Magnitude of Charge
Distance dot is displaced
0.5q
7 cm 1q
14 cm 2q
28 cm
Electrostatic Force and Distance
Distance between spheres
Distance dot is displaced
5cm
44 cm
10cm
22 cm
20cm
5.5 cm
These tables refer to the experiment shown in Video 2 of the previous slide. This video must be shown so that students know what the table’s values represent.

11. Where have we seen this relationship before?
Coulomb’s Law
F e = k q 1 q 2 r 2
Fe = Electrostatic Force k = Electrostatic Constant: 8.99x109 N*m2/C2 q1 and q2 = the charges of the 2 objects r = the distance between the centers of the 2 objects
Where have we seen this relationship before?

12. The Law of Universal Gravitation
Fg = Gravitational Force G = Gravitational Constant: 6.67x10-11 N*m2/kg2 m1 and m2 = the masses of the 2 objects r = the distance between the centers of the 2 objects
This relationship comes up again and again in physics!!!

13. Where is Coulomb’s Law? Where is the Electrostatic Constant (k)?
Reference Table
Where is Coulomb’s Law? Where is the Electrostatic Constant (k)?

14. 3 times one of the charges
Pop Quiz!
F e = k q 1 q 2 r 2 Fe Fe q1
3q2
3q2 r
3 times one of the charges

15. Pop Quiz!
F e = k q 1 q 2 r 2 Fe Fe q2 q1 3r
3 times the distance

16. 2 times both of the charges
Pop Quiz!
F e = k q 1 q 2 r 2 Fe Fe
2q1
2q2
3q2 r
2 times both of the charges

17. Pop Quiz!
F e = k q 1 q 2 r 2 Fe Fe q1 q2
½ r
½ the distance

18. 2 times both of the charges and twice the distance
Pop Quiz!
F e = k q 1 q 2 r 2 Fe Fe
2q1
2q2 2r
2 times both of the charges and twice the distance

19. 1 third the distance and 3 times one of the charges
Pop Quiz!
F e = k q 1 q 2 r 2 q1
3q2
1 third the distance and 3 times one of the charges

20. ½ one charge, 4 times the other, and twice as far
Pop Quiz!
F e = k q 1 q 2 r 2 Fe Fe
4q2
½ q1 2r
½ one charge, 4 times the other, and twice as far

21. Example 1
Two identical conducting spheres are placed with their centers 0.30m apart. One is given a charge of +12x10-9C and the other is given a charge of -18x10-9C.
Find the electric force exerted on one sphere by the other.
The spheres are connected by a conducting wire. After equilibrium has occurred, find the electric force between the two spheres.

22. Example 2
Two identical conducting spheres are placed with their centers 1.2 m apart. One is given a charge of +7.3 μC and the other is given a charge of -4.5 μC.
Find the electric force exerted on one sphere by the other.
The spheres are connected by a conducting wire. After equilibrium has occurred, find the electric force between the two spheres.

23. Exit Ticket
What is the electrostatic force between the two charged spheres shown below? What would happen to the electrostatic force is the distance between the spheres is doubled and one charge is tripled?
2 m
+3 C
3q2 Fe
-7 C

24. Practice- Coulomb’s Law