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Induced Voltages And Inductance

Induced Voltages And Inductance. Chapter 20 Hans Christian Oersted. Introduction. Oersted’s discovery was the first evidence of a link between electricity and magnetism. Symmetry in Physics. Symmetry in nature helps scientists to make new discoveries. Generating Electricity.

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Induced Voltages And Inductance

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  1. Induced Voltages And Inductance Chapter 20 Hans Christian Oersted

  2. Introduction • Oersted’s discovery was the first evidence of a link between electricity and magnetism.

  3. Symmetry in Physics • Symmetry in nature helps scientists to make new discoveries.

  4. Generating Electricity • Could magnetic fields produce electric currents? • Joseph Henry made this discovery • Power plants were the result of his efforts

  5. Induced emf And Magnetic Flux • Faraday’s experiment • A current can be produced by a changing magnetic field. 20.1

  6. Primary and Secondary Coils • A steady magnetic field in the primary coil cannot produce a current in the secondary coil.

  7. Induced emf And Magnetic Flux • Magnetic flux (F) • The flux is proportional to the number of lines passing through the loop • Changes in the flux induce a change in the emf. • 20.2

  8. Magnetic Flux Formula • q is the angle between the B field and • the normal to the plane of the loop

  9. Faraday’s Law of Induction • A simple demonstration • Wire loop, galvanometer, magnet 20.4

  10. Induced Current • A current is set up in the loop as long as there is relative motion between the magnetic field and the loop. • This is an induced current. 20.4

  11. Faraday’s Law of Induction • The induced emf (e) in a circuit equals the rate of change of the magnetic flux through the circuit.

  12. Formula for Faraday’s Law of magnetic induction

  13. Faraday’s Law of Induction Since F = B.A.cosq Ande = -N.DF/Dt Thene = -N.D (B.A.cosq) /Dt

  14. Lenz’s Law • The induced current tries to maintain the original flux through the circuit. • It sets up a magnetic field that opposes any change in the original magnetic field.

  15. Applications of Faraday’s Law • Electric guitar • Metal strings • Pickup coil • Cruise control • Ground fault interrupter (GFI) • Protects against electrical shock • Where is it used? • How does it work? • SIDS monitor 20.10a, 209, 20.8, 214, 20.5

  16. Motional emf • Motional emf is the emf induced in a conductor moving through a magnetic field. • Example: A straight conductor moving through a magnetic field 20.8, 20.13

  17. Formula:

  18. Motional emf • A potential difference (DV) is maintained across a conductor as long as there is motion through the field. • If the direction of the motion is reversed, the polarity also reverses.

  19. Motional emf • Motion involving a closed conducting path • Current flows through the circuit 162, 211

  20. Important Formulas:

  21. Lenz’s Law Revisited • Another example (Figure 20.17): • A stationary conducting loop and a bar magnet 215, 20.13, 165

  22. Applications of Magnetic Induction • Cassette tape recorders/players • Magnetic tape • Recording head • Playback head • VHS recorders/players • Computer hard drives

  23. Questions 1 - 7 Pg. 689

  24. AC Generators • The alternating current (ac) generator • Converts mechanical energy to electrical energy • Has a coil rotating in a magnetic field • Slip rings • Insure that the output voltage changes polarity • Stationary brushes 20.20, 216

  25. Types of Commercial Power Plants • Fossil Fuel • Hydroelectric • Nuclear

  26. Total emf • Formula for total emf: Note: wt = q and w = 2pf (f = 60 Hz in USA) qis measured between the magnetic field and the normal to the loop. • Maximum emf occurs when the plane of the loop is parallel to the magnetic field 213

  27. DC Generators • The direct current (dc) generator • Uses a split ring or commutator • This insures that the output voltage does not change polarity. 20.22

  28. Motors and Back emf • A motor is the opposite of a generator. • Because of Lenz’s Law, back emf is generated in a motor. • Back emf increases with rotational speed. • Back emf tends to reduce current flow in the windings. • Maximum current is present when the motor starts up. 217

  29. Back emf vs. Speed

  30. Equation for back emf

  31. Eddy Currents • Eddy currents are circular currents which occur in a piece of metal when it moves through a magnetic field

  32. Magnetic Damping • The magnetic fields produced by the eddy currents try to prevent motion of the metal through the field.

  33. Effects of Eddy Currents • The effects of eddy currents are undesirable in motors and generators. • These effects can be reduced • Laminations are used

  34. Laminations in an Armature

  35. Self-Inductance • A changing flux through the circuit arises from the circuit itself. • This occurs in coils and solenoids • Solenoid Video

  36. Self-Induced emf • The self induced emf is proportional to the time rate of change of current L is a constant representing the inductance of the device and is measured in Henries (H).

  37. Inductance • Formula for inductance (L)

  38. Inductors • An inductor is a circuit element which is used to provide inductance. • Usually a closely wrapped coil of many turns

  39. Inductance • Inductance (L) is a measure of the opposition to the rate of change of current. • Schematic symbol 20.27/20.28

  40. RL Circuits • RL time constant (t) • The time that it takes for the current in the circuit to reach 63.2 % of its maximum value.

  41. Energy Stored In A Magnetic Field • Current flowing through a solenoid produces a magnetic field. • The battery must do work to produce a current in a coil. • This energy is stored in the magnetic field of the coil. 37-1, 14, 78

  42. Questions 8,10, 11, 13, 14 Pg. 689

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