Chapter 17 Electric Forces and Fields

1. Chapter 17 Electric Forces and Fields

2. 17.1 Electric Charge Objectives • Understand the basic properties of electric charge • Differentiate between conductors and insulators • Distinguish between charging by contact, charging by induction, and charging by polarization

3. How / Why does electric charge happen?

4. Electrostatics - Properties • Two kinds of charge: positive negative • Electric charge is conserved • Like charges repel, unlike charges attract • Charges travel between unlike materials

5. Electrostatics – More Properties • The fundamental unit of electric charge is denoted by the small letter “e” • An electron has a charge of –e • A proton has a charge of +e • Quantized: Electric charges are always a multiple of e. (+/- 1e, +/- 2e, +/-3e etc.) • Value of e = 1.602 x 10-19 Coulombs and 1.0 C = 6.2x1018electrons

6. Electrostatics • Charge was discovered by Robert Millikan (oil drop experiment)

7. Electrostatics • Negatively charged drop suspended inside had no net force

8. Electrostatics - Transfer • CONDUCTORS – electric charge moves freely • INSULATORS – electric charge does not move freely • Semiconductors • Superconductors

9. Electrostatics – Methods of Charging • By contact • Two objects touching each other • Conductors or insulators • By induction • No contact required • Grounding source needed • Conductors only • By polarization • Realignment of charge on surface • Contact or no contact • Insulators only

10. Charging by Contact

11. Triboelectric Series • Rubbing two different materials together results in the transfer of electrons • The prefix “tribo” means “to rub” • The Triboelectric Series is a list that ranks materials according to their tendency to gain or lose electrons

12. Electroscope

13. Charging by Induction

14. Charging by Polarization PhET Interactive, UC-Boulder

15. Example of static electricity in nature?

17. Electroscope • A device that can be used to detect and demonstrate the presence of a static charge

18. 17.2 Electric Force Objectives • Calculate electric force using Coulomb’s Law • Compare electric force with gravitational force • Apply the superposition principle to find the resultant force on a charge and to find the position at which the net charge is zero.

19. Coulomb’s Law: Electric Force Felectric = kC(q1q2) r2 kC = Coulomb constant = 8.99x109Nm2 C2 q1 = charge of charge 1 (C) q2 = charge of charge 2 (C) r = distance between charge 1 & charge 2 (m)

20. Felectric is a vector quantity • Magnitude • Direction

22. Comparison of forces Electric (Felectric) Gravitational (Fg) Felectric = kCq1q2 r2 Fg = G m1m2 r2 G = gravitational constant = 6.67x10-11 Nm2/kg2 kC = Coulomb constant = 8.99x109 Nm2/C2

23. Resultant ForceThe sum of all individual vector forces What is the resultant (net) force C will feel? FAC = 0.067 N A is pushing C away FBC = 0.022 N B is pulling C toward it Fnet-C = FAC – FBC = 0.067 – 0.022 = 0.045 N

24. Resultant Force – Another Example A charge of -2.00nC is located at the origin. Another charge of 5.00nC is located at a position x = 4.00m. Another charge of 6.00nC is located at a position y = -3.00m. What is the magnitude and direction of the net force on the 5.00nC charge? ANS: F3 = 7.16nC at an angle of 65.2o

25. Charges in Equilibrium Example Bookwork #22, set up

26. 17.3 Electric Field Objectives • Calculate electric field strength • Draw and interpret electric field lines • Identify the four properties associated with a conductor in electrostatic equilibrium

27. Electric Field • Electric Field is the field that permeates the space around a charged object and in which another charged object experiences an electric force • Electric Field is a ratio of Force to Charge E = Felectric / q0 • The direction of E is defined as the direction of the electric force that would be exerted on a small positive test charge (q0)

28. Electric Field Strength

29. Electric Field A vector quantity with….. E = kCq N/C r2 MAGNITUDE determined by: The direction of the force that charge (q) would exert on a small positive test charge placed in it’s vicinity DIRECTION determined by:

30. Electric Field Lines • Electric FieldLines are lines that represent both the magnitude and the direction of the electric field. • The number of lines shown is proportional to the electric field strength. As lines get closer together, the field strength increases.

31. Electric Field Lines – Rules for Drawing • Lines begin on + charges (or at infinity) and terminate on – charges (or at infinity) • The number of lines drawn leaving a + or terminating on a – is proportional to the magnitude of the charge • No two field lines can cross one another.

32. Electric Field Lines - Monopoles The electric field from an isolated positive charge The electric field from an isolated negative charge

33. Electric Field Lines - Dipoles

34. E-Field Lines: Point source vs Uniform source

35. Electrostatic Equilibrium An isolated conductor with no net motion of charge is said to be in a state of…..

36. Four Properties of Electrostatic Equilibrium • The electric field is zero inside a conductor 2. Any excess charge resides entirely on the surface of the conductor 3. The electric field just outside a charged conductor is perpendicular to the conductor’s surface 4. On an irregularly shaped conductor, charge tends to accumulate where the radius of curvature is smallest.

38. Van de GraaffGenerator

39. The Van de Graaff Generator

41. Lab – Balloon Electrostatics

42. Lab – Balloon Electrostatics • Sum the vectors in the x-direction • Sum the vectors in the y-direction • From the diagram, Felectric can be found • Felectric is equal and opposite Tx (tension) • Q can be found • Finally, the number of electrons can be found