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Chapter 23 Electric Fields

Chapter 23 Electric Fields. 23.1 Properties of Electric Charges. Two kinds + and – Charge is a scalar Like charges repel and unlike attract Conductor: charges relatively free to move Insulator: charges relatively fixed

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Chapter 23 Electric Fields

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  1. Chapter 23 Electric Fields 23.1 Properties of Electric Charges • 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 1013 excess or deficient charges on a body – 1013/1023 or 1 part in 1010

  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: D. A. B. E. C. F. G. None of these.

  4. 23.3 Coulomb’s Law F12 = keq1q2/r2 rke = 8.99 x 109 Nm2/C2 ^

  5. CT2: P23.5 A hydrogen atom is composed of a nucleuscontaining a single proton, about which asingle electron orbits. The electric force between the two particles is 2.3 x 1039 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? • Yes, we must move the particles farther apart. • Yes, we must move the particles closer together. • No, not at any distance

  6. Chapter 23 Electric Fields 23.3 Coulomb’s Law 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. F1 = F21 + F31 + F41 + F51 + F61 + F71 + …..

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

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

  9. Chapter 23: Electric Fields Ch23.4 The Electric Field • DefinitionCoulomb Force: Electric Field: E = limF/q0 q0 is a positiveq0 0 test charge charge1 charge2 electric field charge1 charge2

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

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

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

  13. dl = rd  dq = dl r dEx 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. Ch23.6 Electric Field Lines Electric Field Lines – Isolated Positive Point Charge

  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. CT23.5 • 1 • 2 • 3 • 4 • CT23.6 • +,+ • +,- • -,+ • -,- P23.40 a) What is the ratio q2/q1? b) Signs of q1 and q2?

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

  24. 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

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