1. Aim: How can we explain the forces that occur between two charges?
Coulomb’s Law
Aim: How can we explain the forces that occur between two charges?

2. Do Now
Explain what Newton’s Law of Universal Gravitation is.

3. Coulomb’s Law
Recall: Like charges repel and unlike charges attract. For objects to attract or repel, a push between them must occur and therefore a force exists.
Suppose two charges, q1 (+) and q2 (-), are separated in space by a distance r.

4. Coulomb’s Law
This electrostatic force that attracts each charge can be determined by Coulomb’s Law.
Coulomb’s Law: q1 and q2 are the two charges, represented in Coulombs. r is the distance between the charges. k is called the electromagnetic constant =8.99 x 109

5. Coulomb’s Law example
Two positive charges, q1 = 3.0 x 10^-6 coulombs and q2 = 6.0 x 10^-5 coulombs, are separated by a distance of 9.0 meters. Calculate the magnitude of the force between the two positive charges.
Fe=kq1q2/r2=(8.99 x 109 N m2/C2)(3 x 10-6 C)(6x10-5 C)/(9m)2
Fe=0.02 N

6. Thought Question
Using Coulomb’s Law and knowing that like charges repel and unlike charges attract, what can we say about the force between:
Two like charges (Positive - Positive and Negative - Negative)? The force has a positive sign
Two unlike charges (Positive - Negative)?
The force has a negative sign

7. Thought Question 2
Does Newton’s Third Law (Action - Reaction) still apply to the electrostatic forces? YES

8. Problem 2
Two charges, q1 and q2, are separated by a distance of d. Which of the following would decrease the electrostatic force between the two charges?
Doubling charge q1, only.
Doubling d.
Doubling charge q1 and q2.
Doubling charge q2, only.

9. Problem 3
What is the magnitude of the electrostatic force between the two charged spheres? Fe=kq1q2/r2=(8.99 x 109 N m2/C2)(1 x 10-6 C)(1 x 10-6 C)/(0.4m)2
Fe =0.056 N