1. Action at a Distance Physics 100 Chapt 12 + -

2. Coulomb’s law q1q1 M1M1 q2q2 M2M2 r F c = k q1q2r2q1q2r2 F N = G mempr2mempr2 Newton’s law of Gravitation All possible distances!

3. Moon-earth F N =G M M M E r 2 How does the Moon know that the Earth is where it is (& what M E is)?

4. Electroscope How do the electrons in the foil know that someone is moving a charged object up here? & that it’s negatively charged? & how much it is charged by?

5. Ball on an empty trampoline surface The ball feels no net force

6. Ball on a “loaded” trampoline surface The ball is “attracted” to the girl’s force

7. Luminiferous aether & E-Fields +

8. Faraday’s method for computing E fields + Imaginary small positive “test” charge q test F test =coulomb force on test charge F test E = F test /q test x +

9. Example ++

10. E-field from 2 plus charges

11. E-Field is the dir of force on + charge + -

12. E-Field for opposite charges

13. Better picture + -

14. Rules E-field lines start on + charges and end on - charges Direction of E-field lines = direction of force on a + charge placed at that point (opposite for – charge) Where E-field lines are close together, the E field is large; where they are far apart, the E-field is small.