1. Electric Force Chapter 15

2. Objectives What are the SI units of the proportionality constant k in the Coulomb force law? Two close electrons are released from rest and move away from each other with a decreasing acceleration. Why? How does the electric force between two point charges vary with the distance between them?

3. Electric Force Has direction. The directions of electric forces on interacting charges are given by the charge-force law (opposites attract, like repel) What about their magnitudes?

4. Coulomb’s Law The electrical force, like gravitational force decreases as the objects get farther apart. This relationship was discovered by Charles Coulomb in the 18 th century. He called this relationship Coulomb’s Law

5. Coulomb’s Law Coulomb did some research and found that two point charges q 1 and q 2 have forces dependent on each other. Given by: F e = kq 1 q 2 / r 2 COULOMB’S LAW

6. Electric Force Where: ◦ q 1 is the charge of the first point charge ◦ q 2 is the charge of the second point charge ◦ r is the DISTANCE between the two charges ◦ K is a constant equal to 9.00 x 10 9 Nm 2 /C 2

7. Coulomb’s Law The force acts along a straight line from one charged object to the other and then back again.

8. Coulomb’s Law The unit of charge is the coulomb, abbreviated C. A charge of 1 C is the charge associated with 6.25 billion billion electrons. This seems like a lot of electrons, but it is only the amount that passes through a common 100 Watt light bulb in a little over one second.

9. Questions The proton that is the nucleus of the hydrogen atom attracts the electron that orbits it. Relative to this force, does the electron attract the proton with less force, with more force, or with the same force?

10. Questions If a proton at a particular distance from a charged particle is repelled with a given force, by how much will the force decrease when the proton is three times farther away from the particle? When it is five times farther away? What is the sign of charge of the particle in this case?