1. Chapter 23 Electric Fields Two kinds + and – Charge is a scalar Like charges repel and unlike attract Conductor: charges relatively free to move Insulator: charges relatively fixed Charge is conserved in an isolated system – you can transfer charge, but not create or destroy charge The MKS unit of charge is 1 Coulomb Charge is quantized: e = 1.6 x 10 -19 Coulombs. Typically there are around 10 13 excess or deficient charges on a body – 10 13 /10 23 or 1 part in 10 10 23.1 Properties of Electric Charges

2. 23.2 Induced Charges

3. CT1: Two uniformly charged spheres are firmly fastened to and electrically insulated from frictionless pucks on an air table. The charge on sphere 2 is three times the charge on sphere 1. Which force diagram correctly shows the magnitude and direction of the electrostatic forces: A. B. C. D. E. F. G. None of these.

4. 23.3 Coulomb’s Law F 12 = k e q 1 q 2 /r 2 r k e = 8.99 x 10 9 Nm 2 /C 2 ^

5. CT2: P23.5 A hydrogen atom is composed of a nucleus containing a single proton, about which a single electron orbits. The electric force between the two particles is 2.3 x 10 39 greater than the gravitational force! If we can adjust the distance between the two particles, can we find a separation at which the electric and gravitational forces are equal? A.Yes, we must move the particles farther apart. B.Yes, we must move the particles closer together. C.No, not at any distance

6. Chapter 23 Electric Fields Superposition The effect of several electrical forces acting on a charge due to several other charges is the vector sum of the individual forces from each charge as if the remaining charges weren’t present. F 1 = F 21 + F 31 + F 41 + F 51 + F 61 + F 71 + ….. 23.3 Coulomb’s Law

7. P23.5 (p.666) P23.8 (p.667)

8. CT3: P23.8 Which statement is true? A.The charged bead is in stable equilibrium only if the charge is positive. B.The charged bead is in stable equilibrium only if the charge is negative. C.The charged bead is in stable equilibrium if the charge is either positive or negative. D.The charged bead is in never in stable equilibrium.

9. I. Definition Coulomb Force: Electric Field: E = lim F/q 0 q 0 is a positive q 0 0 test charge Ch23.4 The Electric Field Chapter 23: Electric Fields charge1charge2 charge1charge2 electric field

10. II. Calculation of E A.Point Charge B.Discrete Distribution of Point Charges Discrete sum – superposition (P 23.17 and P23.18) Note: E depends only on the source charge(s) Ch23.4 The Electric Field Chapter 23: Electric Fields

11. CT4: P23.18 For very large x A.The dipole field is stronger than the monopole (single charge) field. B.The dipole field is weaker than the monopole field. C.The dipole field is equal to the monopole field.

12. Continuous Distribution (P23.27)   ∫ Uniform volume charge density  = q/V and dq = dV Uniform surface charge density  = q/A and dq = dA Uniform linear charge density = q/ l and dq = d l Note: E depends only on the source charge distribution Ch23.5 Electric Field of a Continuous Charge Distribution

13. dl = rd  dq = dl  r dE x dE Ch23.27

14. Ch23.6 Electric Field Lines Electric field lines give a visualization of the electric force field The field points along the line in the direction of the arrow Field lines leave + charges and end on – charges The number of lines is proportional to the source charge, so the density of lines is proportional to field strength

15. Electric Field Lines – Isolated Positive Point Charge Ch23.6 Electric Field Lines

16. Electric Field Lines – Isolated Negative Point Charge

17. Electric Field Lines – Isolated Positive and Negative Point Charges Compared

18. Electric Field Lines – Two Isolated and Separated Equal Positive Point Charges

19. Electric Field Lines – Isolated and Separated Equal Positive and Negative Point Charges – Electric Dipole

20. Electric Field Lines – Two Parallel Oppositely, but Equally, Charged Plates - Capacitor

21. Electric Field Lines – Two Isolated Separated Charges of Opposite Sign and Different Magnitudes

22. P23.40 a) What is the ratio q 2 /q 1 ? b) Signs of q 1 and q 2 ? CT23.5 1 2 3 4 CT23.6 A.+,+ B.+,- C.-,+ D.-,-

24. Ch23.7 Motion of Charged Particles in a Uniform Electric Field Squeak Demo Do P23.41

25. The coin doesn’t have to be charged for the cup to attract the coin. The charged cup can induce a polarization of the charge which results in an attraction. Common misconceptions - Charged objects only interact with other charged objects. Charged objects do not interact with uncharged (neutral) objects. Uncharged (neutral) is considered to be a third type of charge. Charged and uncharged objects attract each other because they are opposites. Correct answer