1. 1. Electrostatics Electric Force

2. First review the gravitational force… Any two masses are attracted by equal and opposite gravitational forces: m1m1 m2m2 r F-F Newton’s Universal Law of Gravitation where…… G=Universal Gravitation Constant = 6.67x10 -11 Nm 2 /kg 2  This is an Inverse-Square force  Gravity is a very weak force

3. r Charge (Q) Coulombs (C) 1 C = 6.2421 x 10 18 e e = 1.602 x 10  9 C Distance (m) Coulomb’s constant (k) k = 8.988 x 10 9 N  m 2 /C 2 Force (N)

4. Notes on Coulomb’s Law 1) It has the same form as the Law of Gravitation: Inverse-Square Force 2) But… (can you spot the most basic difference between these two laws?) 3) The electrostatic constant (k) in this law is derived from a more fundamental constant:  0 = permittivity of free space = 8.85 x 10 -12 C 2 /Nm 2 4) Coulomb’s Law obeys the principle of superposition

6. 1.Compare the electric force holding the electron in orbit (r = 0.53 × 10 -l0 m) around the proton nucleus of the hydrogen atom, with the gravitational force between the same electron and proton. What is the ratio of these two forces?

7. Vector addition review: Two forces acting on an object tail-to-tip method Components method

8. Coulomb’s law strictly applies only to point charges. Superposition: for multiple point charges, the forces on each charge from every other charge can be calculated and then added as vectors.

9. 2.At each corner of a square of side L there are four point charges. Determine the force on the charge 2Q. Q 2Q 3Q4Q L

10. 3.A +4.75  C and a  3.55  C charge are placed 18.5 cm apart. Where can a third charge be placed so that it experiences no net force? Q1Q1 Q2Q2 d x Q

11. [Extra] Two 6.7 kg bowling balls are placed in a vertical cylinder. Charge is added to each ball until they repel with enough force to separate the two balls by a distance of 43.7 cm. Assuming the two bowling balls are charged positively and equally, determine the amount of charge (to four significant digits) added to each individual ball.

12. More information http://ap-physics.david-s.org