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What is Electricity?

What is Electricity?. Flow of electrons. Electromagnetic force. BASIC MODEL. Charge: Modern view. charge comes in two flavors (positive and negative) normally balanced (neutral) the amount of positive charge in an object is fixed! imbalance causes “charge”. Problem:.

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What is Electricity?

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  1. What is Electricity? • Flow of electrons. • Electromagnetic force

  2. BASIC MODEL

  3. Charge: Modern view • charge comes in two flavors (positive and negative) • normally balanced (neutral) • the amount of positive charge in an object is fixed! • imbalance causes “charge”

  4. Problem: • A bushel basket contains 50 apples. How much of a bushel is just one apple?

  5. Measuring charge • we measure in units called Coulombs (C) • 6.25x1018 bare protons is required to make 1C. • What is the charge on a single proton?

  6. elementary charge • we define the charge on one proton e=1.6 x10-19C • the charge on the electron is then -e

  7. #removed fraction removed = original # Problem • A penny contains about 2 x1023 protons and an equal number of electrons. If the penny is charged to +1C, what fraction of the electrons have been added or removed?

  8. Problem • A penny contains about 2 x1023 protons and an equal number of electrons. If the penny is charged to +1C, what fraction of the electrons have been added or removed? #removed fraction removed = 2 x 1023

  9. Problem • A penny contains about 2 x1023 protons and an equal number of electrons. If the penny is charged to +1C, what fraction of the electrons have been added or removed? 6.25 x 1018 fraction removed = 2 x 1023

  10. Problem • A penny contains about 2 x1023 protons and an equal number of electrons. If the penny is charged to +1C, what fraction of the electrons have been added or removed? ! .00003 fraction removed =

  11. Conductivity • conductors: loosely bound electrons; charge flows freely • insulators: tightly bound electrons; charge hard to move

  12. EBONITE Induction in a conductor How will the +’ve charge move? How will the -’ve charge move?

  13. EBONITE Induction in a conductor I see a problem; do you see a problem?

  14. EBONITE Induction in a conductor

  15. EBONITE Induction in a conductor Where is the +’ve charge? Where is the -’ve charge?

  16. EBONITE Induction in an insulator

  17. EBONITE Induction in an insulator

  18. EBONITE Fnet Induction in an insulator Look at the surface.

  19. Induction • conductors: charge flows • insulators: charge shifts

  20. Constant of proportionality M1 M2 r Newton’s Law of Gravity

  21. (5.98 x 1024 kg) Mbook Wearth,book = (6.67 x 10-11Nm2) (6.38 x 106 m)2 kg2 r= 6.38 x 106 m What is the force between the Earth and a book? Wearth,book = Mbook(9.8 N/kg) Mearth = 5.98 x 1024 kg

  22. Gravitational Field consider a 1kg block F = ? 5 N F = ? 10 N

  23. gravitational field strength Gravitational Field consider a 3kg block What is the gravitational field strength at the Earth’s surface? F = ? 3 kg x 5 N/kg = 15 N F = ? 10 N

  24. 7 N/kg Gravitational Field What is the gravitational field strength at this point? m = 5 kg F = 35 N

  25. F = m g Gravitational Field 5 N/kg 7 N/kg 10 N/kg

  26. Electric Force • Definition: Coulomb’s Law • Electric field is the force that +1C would feel if it were placed at this location.

  27. What units? F is in units of: Newtons (N) q is in units of: Coulombs (C) k : 9 x 109 N.m2/C2 E is in units of: N/C

  28. Example #1 Anthea rubs two latex balloons against her hair, causing the balloons to become charged negatively with 2.0 x 10-6 C. She holds them a distance of 0.70 m apart. A) what is the electrical force between the two balloons? B) Is it one of attraction or repulsion?

  29. Example 2 Anthea rubs two latex balloons against her hair, causing the balloons to become charged negatively with 2.0 x 10-6 C. She holds them a distance of 0.70 m apart. A) what is the electrical force between the two balloons? B) Is it one of attraction or repulsion? C) What is the electrical field of the 1st balloon?

  30. F +q E 3 20 60 Example #3 At the location marked with an x, the electric field is 2000 N/C and points right. What is the electric force (size and direction) on a 6 x 10-6 C charge that is placed at the x? 20 3 F = qE = (6x10-6)(2000) = 1.2 x 10-2 N (to the right)

  31. -q F What if a charge were placed somewhere else? E E Example #3 cont. What if the charge were the same size but negative? Same size F = 60 N (to the left) F = Who Knows?

  32. Electric Fields(again)

  33. Electric Field • Operational definition: • Electric field is the force that +1C would feel if it were placed at this location.

  34. F E -q Example #2 A charge of -5 x 10-8 C feels a force of 0.2 N to the right. What is the electric field (magnitude and direction) at the charge’s location? F = qE E = F/q E = 4 x 106 N/C (to the left)

  35. Field at q’s location due to other charges at other locations(source charges) Force on q due to the field, E,at q’s location The charge that is feeling the force(test charge) Where does E come from?

  36. E What is the direction of the electric field at x? E The direction of E What is the direction of the electric field at x? HOW BIG?

  37. E r Coulomb’s Law k qs E = r2 qs k = 9 x 109 Nm2/C2

  38. 5 cm qs Example #last A 2.5 x 10-6 C charge is placed as shown below. What is the electric field at a point 5 cm to the right? Which way does E point? E = kqs /r2 = (9 x 109)(2.5 x 10-6)/(5 x 10-2)2 = +9 x 106 N/C

  39. E r Coulomb’s Law k qs E = r2 qs k = 9 x 109 Nm2/C2

  40. 3x10-9C -6x10-9C 0.20 m What is the electric field strength at the location of Q1 due to Q2? Q1 Q2 Which charge do we care about?

  41. What is the electric field strength at the location of Q1 due to Q2? -6x10-9C 0.20 m Q2 Which charge do we care about? How does this change the problem?

  42. -6x10-9C 1350 N/C Q2 E = kqs /r2 = (9 x 109)(6 x 10-9)/(0.20)2 = 1350 N/C

  43. What force does Q1 experience? 1350 N/C 4 x 10-6 N Q2 F = qE = (3 x 10-9 C)(1350 N/C) = 4 x 10-6 N (right)

  44. What is the electric field strength at the location of Q2 due to Q1? 3x10-9C -6x10-9C 0.20 m Q1 Q2 Which charge do we care about?

  45. What is the electric field strength at the location of Q2 due to Q1? 3x10-9C 0.20 m Q1 Which charge do we care about? How does this change the problem?

  46. 3x10-9C 675 N/C Q1 E = kqs /r2 = (9 x 109)(3 x 10-9)/(0.20)2 = 675 N/C

  47. What force does Q2 experience? 675 N/C Q1 4 x 10-6 N F = qE = (6 x 10-9 C)(675 N/C) = 4 x 10-6 N (left)

  48. 4 x 10-6 N 4 x 10-6 N F2,1 F1,2 due to Q2 due to Q1 1350 N/C 675 N/C

  49. What is the electric field at the midpoint between the charges? 0.20 m Q1 Q2 Which charge do we care about?

  50. E2,X E1,X What is the electric field at the midpoint between the charges? Q1 Q2

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