E1 Basic Electrostatics

1. E1 Basic Electrostatics

2. Electrostatics • Complete the tutorial on electrostatics (both sides of handout) • Tape activity • www.sos.siena.edu/~rfinn/phys140s09/tape.pdf

3. Electrostatics • electric charges that are not moving • Definition of new symbols • e- = electron • p = proton • n = neutron • e+ = positron

4. Nature of charge Amber is a natural substance that can be naturally charged Few naturally occurring objects can be electrically charged Static electricity is easily shown with plastic, rubber, and synthetic fibers Electrical charges behave like positive and negative numbers

5. Electric Force • Similar form as gravitational force (weird, huh?), except • positive and negative charges • Like charges repel; opposite charge attract

6. A proton is located just north of an electron. The net force that the electron feels due to the proton is in which direction? • North • South • West • East

7. Coulomb’s Law - magnitude • k = 9.0 x 109 N m2/C2 • Coulomb’s contant • q1, q2 = charges • measured in Coulombs (C) • 1 C is a lot of charge! • r = distance between q1 and q2

8. Charge Carriers • The electron • 1 e- = -1.602 x 10-19 C • smallest unit of charge, also referred to as e • negatively charged • Charge of 1 e- is -1e • mass = 9.11 x 10-31 kg • The proton • 1 p = 1.602 x 10-19 C • positively charged • Charge of 1 p is +1e • mass = 1.67 x 10-27 kg • Charge is quantized!

9. Coulomb’s Law with different constants ε0 =permittivity of free space (more relevant to future chapters)

10. Illustrations of Coulomb’s Law • Comb & Paper • Balloon on wall

11. Illustrations of Coulomb’s Law • Electroscope induction conduction

12. Charge Conservation • The total charge of an isolated system is conserved. • Rubbing a balloon on hair • Triboelectric series • Decreasing tendency to lose electrons

13. Conductors and Insulators • Charges aren’t free to move around in an insulator • Why they may be charged • If charge is left on them due to rubbing another material, the charge can’t go anywhere • Examples – amber, plastics, synthetic materials • Charges are free to move very easily in materials called conductors • Examples – metals, salty water

14. Conductor versus Insulator Conductor Insulator charge separation within molecules e-’s travel

15. Charging by Induction

16. Net Force from Multiple Charges • An electrostatic force exists between each pair of charges according to Coulomb’s law • Add components of forces to get net force when adding multiple charges

17. Adding Vectors Magnitude and direction of Net Force given by:

18. The Atomic Model

19. How objects become charged • A macroscopic object is composed of a huge number of tiny atoms • Each atom has a tiny nucleus (includes protons and neutrons) surrounded by a cloud of electrons • Equal number of electrons and protons • Avogadro’s number of protons or neutrons have a mass of ~ 1 g. • Total mass of electrons is negligible • Most of the volume of an atom is occupied by the electron clouds

20. Coulomb’s Law – examples • Review Example E1.2 • Determine direction of force • Review Example E1.3 • Determine direction and magnitude of force

21. Van de Graff Generator • Demonstration • In-class • Pie plates • Ion motor • Extra Credit - see schedule and moodle

22. Group Problems E1S.3 E1S.5 E1S.10

23. Charges in a line • What is the net force on the 48 C charge?

24. Charges in a line - Solution +x Q1 Q2 Q3

25. Charges in a line - Solution +x Q1 Q2 Q3 Do signs make sense?!!

26. Q1 Q2 Q4 Q3 Charges in a square +y +x Find net force on Q1

27. Q1 Q2 Q4 Q3 Charges in a square The x and y components of the net force are given by: Do signs of forces make sense? Does trig make sense?

28. Charges in a square

29. Charges in a square