3. Coulomb's Law

4. Outcomes
You will explain, qualitatively, the principles pertinent to Coulomb’s torsion balance Experiment
You will apply Coulomb’s law, quantitatively, to analyze the interaction of two point Charges
You will determine, quantitatively, the magnitude and direction of the electric force on a point charge due to two or more other point charges in a plane
You will compare, qualitatively and quantitatively, the inverse square relationship as it is expressed by Coulomb’s law and by Newton’s universal law of gravitation.

5. *Joseph Priestley – “Electric Forces Law”
Joseph Priestley suggested in 1767 that charged objects exert electric forces on each other, and that the mathematical relationship describing these forces is parallel to Newton’s law of universal gravitation.
Priestley came to this conclusion when he discovered that a charged pith ball placed inside a hollow charged sphere has no electrical force acting on it.

7. Charles Augustin de Coulomb & Torsion Balance
He discovered the inverse square law of forces between two charged particles.
as you move charges apart, the force between them starts to decrease faster and faster (exponentially)
He later showed that the force is also proportional to the product of the charges, a relationship now called “Coulomb’s Law”

8. The Torsion Balance
He charged one of the free moving spheres (A) by touching it to an already charged object (charging by conduction)
He then touched that one sphere to the other free moving sphere (B) (charging it by conduction)
Each of the free moving spheres was then touched to one of the spheres on the rod (guess what... charging by conduction!)
As the like-charged repel one another the angle of movement is measured. A B

9. Coulomb’s Law
From Coulomb’s experiment he was able to derive a formula which can calculate the electrostatic force between objects.
***Do not insert the type of charge into the equation!!!***

10. Electrostatic Forces in One Dimension
Determine the force on charge C

11. Multiple Charges in Two Dimensions
Determine the force on q₂
3.09x10⁹ at 222°

12. Comparing Electrostatic Forces and Gravitational Forces
Gravitational forces are very weak, while electrostatic forces are very strong
Gravitational forces are only attractive
Electrical forces can be attractive or repulsive

13. Homework
p. 538 #1-10 (omit #7 & #8)
p. 28 & 29

14. (Day #2) Homework
p. 32 #1, 3-6, 8, 10, 11, 13