Electric Charges and Forces: Lenses and Image Formation

1. p q Image Object From Last Time… Lenses and image formation Optical Instruments

2. Exam 1 Tue. Sep. 29, 5:30-7 pm, 145 Birge Covers Chap. 21.5-7, 22, 23.1-4, 23.7, 24.1-5, 26 + lecture, lab, discussion, HW 8 1/2 x 11 handwritten note sheet (both sides) allowed Students with scheduled class conflicts: stay after lecture today to arrange time • Study ideas: • Exam 1 Practice problems at Mastering Physics • Sample exams on website (blank & solutions) • Group/Quiz (blank & solutions). • Review lab question sheets. 2

3. Example: Lithium 3 protons in nucleus,3 electrons orbiting Chapter 26: Electric Charges and forces • Two different kinds of electric charges • Benjamin Franklin called these positive, negative • Negative charges are electrons • Positive charges are protons • Often bound in atoms: • Positive protons in central nucleus r~10-15 m • Negative electrons orbit around the nucleus r~10-10 m

4. + and - charges can be separated • Triboelectric • Charge is transferred as a result of mechanical (frictional) action • Conduction • charge transfer by contact (spark)

5. Separating charge • Rubber / fur: electrons transferred to rod • Rubber has negative charge • Glass / silk: electrons taken from plastic • Plastic has positive charge Charge is conserved. Can be moved around, but not created or destroyed.

6. Electric Chargesunits and quantization • The SI unit of charge is Coulomb (C ) • The electric charge, q, is said to be quantized • quantized = it is some integer multiple of a fundamental amount of charge e q = Ne • N is an integer • e is the magnitude of charge of electron = +1.6 x 10-19 C • Electron: q = -e • Proton: q = +e

7. - + + + + + - - + + - + + + + + + + + Neutral metal + + - Positively charged rod (too few electrons) - + + - - - + + + + + + + + + + + + + + + - electron flow + + Positively charged metal + + + + + + + + + + Less positively charged rod Charge by conduction (touching)

8. Positive charged rod results in positive leaves. Measuring charge • Touch charged rod to electroscope. • Charge transferred from rod to electroscope. • Everything positively charged. • Like charges on leaves repel. + + + +

9. Charge motion and materials • Insulators(e.g. plastic, wood, paper) • electrons bound to atoms, do not move around • Even extra charge is stuck • Extra charge cannot move around on insulator • Metals(e.g. copper, aluminum) • Some electrons free, positive ions stuck in place • Additional charge free to move, distributes over surface • Ionic solutions(e.g. saltwater) • Like conductor, but both positive, negative ions free to move

10. Forces between charges Like charges repel Unlike charges attract All of this without touching — a ‘noncontact’ force Attraction, repulsion decreases with distance

11. Induced charge • Charging by induction requires no contact with the object inducing the charge charged rubber rod Bring negative charge close. Electrons on sphere move away from rod. neutral metallic sphereElectrons (-) free to move

12. Quick quiz What is the sense of the force between these two objects? • Attractive • Repulsive • Zero

13. + + + + - - + + + + - - Lightning doorbell • Ben Franklin’s ‘door bell’. • Announced presence of lightning so knew to go out and do his experiments!

14. Positive charged rod results in positive leaves. Quick Quiz A charged rod is brought close to an initially uncharged electroscope without touching The leaves A. move apart B. only one moves away C. move closer together D. depends on sign of rod E. do nothing This is an induced dipole

15. Vector Nature of Electric Force a)The force is repulsiveif charges are oflikesign b)The force is attractive if charges are of opposite sign Multiple forces add as vectors. Electrical forces obey Newton’s Third Law: F21 = -F12

16. + + Quick Quiz Two charges are arranged as shown. What is the direction of the force on the the positively charged ‘test’ particle? C B D E A

17. Gravitational force: FG=GM1M2/ r2G=6.7x10-11 Nm2/kg2 Magnitude of force: Coulomb’s Law • Electrical force between two stationary charged particles • The SI unit of charge is the coulomb (C ), µC = 10-6 C • 1 C corresponds to 6.24 x 1018 electrons or protons • ke = Coulomb constant ≈ 9 x 109 N.m2/C2 = 1/(4πeo) • eo permittivity of free space = 8.854 x 10-12 C2 / N.m2

18. Quick Quiz Equal but opposite charges are connected by a rigid insulating rod. They are placed near a negative charge as shown. What is the net force on the two connected charges? Left Right Up Down Zero - + -

19. The electric dipole • Can all be approximated by electric dipole. • Two opposite charges magnitude qseparated by distance s Dipole moment Vector Points from - charge to + charge Has magnitude qs

20. How does the magnitude of the force depend on ? Force on an electric dipole • What is the direction of the force on the electric dipole from the positive point charge? Up Down Left Right Force is zero +

21. Induced dipoles (charge redistribution) charged rubber rod Bring negative charge close. Electrons on sphere move away from rod.

22. Induced dipole in insulators • A process similar to induction can take place in insulators • The charges within the molecules of the material are rearranged