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Electric currents & Electromagnetism

This lecture delves into the foundational concepts of electric currents and electromagnetism, examining the motion of electric charges in both the micro and macro worlds. It covers key topics such as the behavior of positive ions, the function of batteries, the principles of electric current flow, and key equations governing electricity, like Ohm's Law and Faraday's Law. The concepts of magnetic fields produced by electric currents, the interaction of charges, and practical applications in technology, including motors and generators, are also discussed, providing a comprehensive understanding of electromagnetism's principles.

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Electric currents & Electromagnetism

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  1. Electric currents& Electromagnetism Micro-world Macro-world Lecture 9

  2. Electric currents (Motion of electric charges) Micro-world Macro-world Lecture 9

  3. Alessandro Volta

  4. Positive Ions Atoms with one or more electrons removed _ _ + + + _ _ _ + + _ _ _ ”net” charge = +2qe

  5. Battery C Zn Zn - - + + Zn Zn++ Zn++ Zn - - - - Zn Zn + + - - Zn++ - - Zn Zn Zn++ + + acid

  6. “Voltage” Cathode Anode - + - - + + - - + + - E - + + - Zn++ - F + + W = Fd “Voltage” F = 2qeE d W0 2qe V W = 2qeEd  W0 = 2qeE0d  =E0d

  7. Energy gained by the charge W = Fd =Q E0d = QV F=QE0 F=QE0 Q Q Anode - + Cathode - E0 - + + - - + + Zn++ - - + + - Zn++ - + + d

  8. Units again! W = Q V joules joules coulomb W Q  V = = Volt coulombs joule coulomb 1 V = 1

  9. Continuous charge flow = “electric current” Electrical “conductor” connected between anode & cathode Q Q Anode - + Cathode - - + + - - + + Zn++ - - + + - Zn++ - + +

  10. electric current Coulombs second Units: Q t I = =Amperes Q Q Anode - + Cathode - - + + - - + + Zn++ - - + + - Zn++ - + +

  11. The conductor can be a piece of wire Q t I = + + + Anode - + Cathode - - + + - - + + Zn++ - - + + - Zn++ - + +

  12. The energy can be used to run a gadget Energy time QV t P= = = I V + I + + I I Anode - + Cathode - - + + - - + + Zn++ - - + + - Zn++ - + +

  13. Electric light 60 Watts I=? T Power = P = I V P V 60 W 100V I = = J/s J/C 1/s 1/C = 0.6 = 0.6 C s V=100V = 0.6 = 0.6 A

  14. General circuit I Appliance - + I + - 12V Energy source (device that separates + & - charge)

  15. analogy Amt of water flow ~ current appliance Height ~ voltage Pump ~ battery pump pond

  16. Voltas’ 1st batteries

  17. Christian Oersted

  18. Electric currents produce B-fields B I

  19. Right-hand rule B

  20. Current loop S N

  21. Two current loops N S

  22. Even more loops S N

  23. Solenoid coil S N Looks like a bar magnet

  24. Atomic magnetism B + I - Some atoms are little magnets

  25. Permanent magnet-microscopic view-

  26. Magnetic forces on electric currents I

  27. Another right-hand rule I

  28. Forces on two parallel wires I I Current in same direction: wires attract B

  29. Forces on two parallel wires I Current in opposite directions: wires repel B I

  30. Force law of Biot & Savart I1 I2 I1I2l d F = k l N A2 k=2x10-7 B d

  31. Biot & Savart example I1I2l d 20A 20A F = k (20A)2 2m 0.01m N A2 F= 2x10-7 2m B F= 2 x 10-3N Small, but not tiny 0.01m

  32. Electric motor F I I B F

  33. Electric motor B I

  34. Speakers Solenoid Electro-magnet Permanent magnet

  35. Lorentz force B v F +q i=qv if v  B: F = iB = qvB direction by the right-hand rule

  36. Electromagnetism Michael Faraday Faraday’s Law

  37. Moving a Conductor in a B-fieldseparates + & - charges I

  38. Use this to drive an electric circuit + + + + I +

  39. Moving wire loop in a B field v + + An electric current is “induced” in the loop

  40. Either the magnet or the loop can move v + + an electric current is “induced” in the loop

  41. Magnetic flux (F) thru a loop F = BA┴

  42. Flux thru a coil of N loops F = NBA┴

  43. Faraday’s law Michael Faraday change in F elapsed time Induced voltage in a circuit = change in NBA┴ elapsed time EMF = “Electro-Motive Force”

  44. Rotating coil in B field B A┴ = 0  F =0

  45. Rotating coil in B field B A┴ = Acoil F = maximum

  46. Rotating coil in B field B A┴ = 0 (again)  F = 0

  47. AC voltage

  48. Lenz’ Law B B S B-field from induced current + + B-field from induced current I N v v the fall produces an induced current the B-field produced by the induced current tries to impede the fall

  49. Lenz’ law An induced voltage always gives rise to an electric current that creates a magnetic field that opposes the influence that produced it.

  50. Maglev trains

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