Electric current

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Electric current

  1. 1. Chapter 34 – electric current<br />
  2. 2. 34.1 – flow of charge<br /><ul><li>Charge moves when a conductor has a potential difference
  3. 3. Charge flows until no difference in potential
  4. 4. To sustain flow of charge, something must keep one end at a higher potential
  5. 5. Compare this to water flowing from a reservoir
  6. 6. Something must continually pump water to maintain a difference in height</li></li></ul><li>34.2 – electric current<br /><ul><li>The flowing of electric charge
  7. 7. Only electrons
  8. 8. Variable: I, I = q/t
  9. 9. SI unit: ampere (A), the “amp”, 1 A = 1 C / sec
  10. 10. The same number of e enter conductor as leave
  11. 11. The net charge is always zero</li></li></ul><li>34.3 – voltage sources<br /><ul><li>The “pump” for the charges,
  12. 12. causes a potential difference
  13. 13. Must have capacity to maintain
  14. 14. constant flow
  15. 15. Batteries  chemical reaction
  16. 16. Generators  convert mech. work to electrical energy
  17. 17. The voltage (potent. diff) is what forces charges to move – “electric pressure”
  18. 18. 120 V give 120 J to each coulomb of charge
  19. 19. Current is the flowing of charge through a circuit, voltage causes the flowing</li></li></ul><li>34.4 – Electric resistance<br /><ul><li>Current (charge flow) depends on:
  20. 20. Voltage & resistance (R) – the tendency to slow movement of charges
  21. 21. We can ↑ current by either:
  22. 22. ↑ voltage (electric pressure) or
  23. 23. ↓ resistance (or both)
  24. 24. Resistance of conductor depends:
  25. 25. Conductivity (how well it conducts)
  26. 26. Thickness – thicker = < resistance
  27. 27. Length – shorter = < resistance
  28. 28. Temperature – cooler = < resistance</li></li></ul><li> 34.5 – ohm’s law<br /><ul><li>At very low temperatures, some materials loose all resistance & become superconductors
  29. 29. High temp superconductor > 100 K
  30. 30. Ohm’s “law” states: current is directly related to voltage and resistance  I = V/R
  31. 31. 2 x V = 2 x I, 2 x R = I/2
  32. 32. Therefore: small R = large current
  33. 33. current produces heat, ↑ current = ↑ heat (like toaster)</li></li></ul><li>34.6 – Ohm’s law & electric shock<br /><ul><li>Damage is caused by current – not voltage
  34. 34. Depends upon voltage & body resistance
  35. 35. Rwith salt water ~ 100 Ω, Rdry ~ 500,000 Ω
  36. 36. Voltage drives current: ↑ voltage  ↑ current
  37. 37. Touching outlet while dry (120 V)  small current
  38. 38. Wet while grounded ↑ current dramatically, poss. Fatal
  39. 39. Distilled water – good</li></ul>insulator<br /><ul><li>Adding salt ↓ resistance</li></li></ul><li>High voltage wires<br /><ul><li>Parts of body at the same elect. potential – no shock
  40. 40. Why birds sit on high voltage wires
  41. 41. Charges move down path of least resist. – the wires, not bird
  42. 42. Safe to hold onto wire – as long at you do not touch anything else</li></li></ul><li>Grounding wires<br /><ul><li>If surfaces of appliances are at different potential, touching them creates a path for current to flow (a shock)
  43. 43. To prevent this, a third wire of plug is grounded and connected to appliance
  44. 44. Any “short” will be “grounded”</li></ul>Health Effects<br /><ul><li>Shock causes: overheating of tissue & disrupt nerve functions</li></li></ul><li>34.7 – direct vs. alternating current<br /><ul><li>Direct (DC) – charges flow in only one direction
  45. 45. Alternating (AC) – charges move back and forth
  46. 46. In US, occurs 60 times/sec (60 Hz) at 120 V
  47. 47. The wires constantly change polarity
  48. 48. Positive ↔ Negative
  49. 49. AC used because: voltage easily changed, produced naturally as AC</li></li></ul><li>34.9 – speed of electrons<br /><ul><li>Electrons move in random directions w/i conductor until an E - field is created by a potential difference
  50. 50. The e experience a force, moving them along E – field
  51. 51. Constant collisions (w/ rigid particles of conductor) cause heating & slow the motion of e – drift velocity
  52. 52. AC the e oscillate back and forth (60 times/sec) from one location, delivering energy</li></li></ul><li>34.10 – source of electrons<br /><ul><li>The e that power circuits come from the conductors that make up the circuit
  53. 53. e do not come from power companies, they are already in the conductors (wires)
  54. 54. Power companies provide the energy (via an E-field) that causes the charges to move
  55. 55. This energy is converted to heat, light, sound, etc.</li></li></ul><li>34.11 – electric power<br /><ul><li>The rate at which electric energy is converted to mechanical energy
  56. 56. P = I V
  57. 57. Provides a relationship between power & current
  58. 58. A kilowatt • hour = energy
  59. 59. Energy companies charge some amount per kilowatt • hour</li>

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