2. Physics 123

3. 16. Electric Charge & Electric Field 16.1 Static Electricity 16.2 Electric Charge in the Atom 16.3 Insulators and conductors 16.4 Induced Charge -- Electroscope 16.5 Coulomb’s Law 16.6 Problem Solving 16.7 Electric Field 16.8 Electric Field Lines

4. Example 16.1... Static When one rubs a plastic comb with a cloth A. The comb and the cloth acquire opposite charges B. The comb and the cloth acquire the same kind of charge C. The comb is charged but the cloth is not D. The cloth is charged but the comb is not

5. Solution 16.1... Static Plastic is negatively charged Cloth is positively charged - - - - + + + + +

6. Glass and Silk Glass is positively charged Silk is negatively charged + + + + + + + - - - -

7. Conservation of Charge Charge is transferred not created! In other words … Total Charge in the Universe is Constant!

8. Example 16.2... Material World Plastic and glass are examples of A.Conductors B.Semiconductors C.Insulators (non-conductors) D.Metals

9. Solution 16.2... Material World Plastic and glass are examples of A.Conductors B.Semiconductors C.Insulators (non-conductors) D.Metals Charges move freely in conductors Charges do not move in insulators

10. Example 16.3... Polarization A charged comb will attract small pieces of paper. This is because A. The comb has a net positive charge and the paper has a net negative charge B. The comb has a net negative charge and the paper has a net positive charge C. The comb has a net charge and the paper has a net negative charge. D. The comb has a net charge and the paper has no net charge.

11. Solution 16.3... Polarization A charged comb will attract small pieces of paper. This is because an opposite charge is induced by polarization in the uncharged (neutral ) piece of paper! D. The comb has a net charge and the paper has no net charge. - - - - + + - -

12. Electroscope Gold leaves separate to indicate the presence of charge. - - - -

13. Coulomb’s Law Like Charges repel Unlike Charges Attract F = k Q q / r 2 - + Q q r

14. Example 16.4... Coulomb’s Law The positive charge is located at a distance of 1 cm from the - 1  C charge and 2 cm from the + 4  C charge. What is the net force experienced by the positive charge? q 1 = - 4  Cq 2 = - 1  Cq 3 = + 1  C

15. Solution 16.4... Coulomb’s Law F = k Q q / r 2 F 31 = 9x10 9 x 4x10 -6 x 1x10 -6 / (2x10 -2 ) 2 F 31 = 90 N F 32 = 9x10 9 x 1x10 -6 x 1x10 -6 / (1x10 -2 ) 2 F 32 = 90 N Net Force is zippo … rhymes with hippo! q 1 = - 4  Cq 2 = - 1  Cq 3 = + 1  C

16. Electric Field E = F /q Electric Field at a point in space is the force per unit positive charge A charge Q will create an electric field around it. F = k Q q / r 2 E = k Q / r 2

17. Problem 16.5... Electric Field Where in the world is E = 0? q 1 = - 4  Cq 2 = - 1  C

18. Solution 16.5... Electric Field E = 0 where the is twice as far from the bigger charge! q 1 = - 4  Cq 2 = - 1  C

19. Problem 16.6... Electric Field Lines Which Electric Field Pattern is incorrect? + - ++

20. Solution 16.6... Electric Field Lines Correct Electric Field Pattern + +

21. That’s all folks!