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Electromagnetic Induction

Taking It To The Maxwell. Electromagnetic Induction. Michael Faraday (1791 – 1867). Introduction. We’ve discussed two ways in which electricity and magnetism are related: (1) an electric current produces a magnetic field.

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Electromagnetic Induction

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  1. Taking It To The Maxwell ElectromagneticInduction

  2. Michael Faraday (1791 – 1867)

  3. Introduction We’ve discussed two ways in which electricity and magnetism are related: (1) an electric current produces a magnetic field. (2) a magnetic field exerts a force on an current carrying wire or moving electric charge.

  4. Introduction These discoveries were made in 1820 – 1821. Scientists wondered: If electric currents produce a magnetic field, does a magnetic field produce electric currents? Joseph Henry (1797 – 1878) and Michael Faraday (1791 – 1867) independently found, ten years later, that this is so.

  5. Hey Mikey! Faraday found that changing a magnetic field produces a current. Such a current is called an induced current. To move charges requires a force and this force is Called the ELECTROMAGNETIC FORCE, EMF for short. You know it as VOLTS of potential.

  6. Induced Current Faraday suspected that a magnetic field would induce a current, just like a current produces a magnet field. Mikey found that a steady current in X produced no current in Y. Only when the current in X was starting or stopping (i.e., changing) was a current produced in Y.

  7. The Big Idea To induce a current in a wire …. The MAGNETIC FIELD MUST BE CHANGING WITH RESPECT TO TIME There are a number of differential equations to describe this, but why complicate things?

  8. Since there is no change in the magnetic flux, no current is induced.

  9. The Status Quo I. Distance between coil and magnet decreases. So the magnetic field (therefore the flux) through the coil increases. III. Current is induced. II.To oppose this upward increase in the magnetic filed (flux), the field produced by the induced current points downward.

  10. I.Distance between magnet and coil increases. So the magnetic field (and therefore the flux) decreases. III.Current is induced in the opposite direction as the previous case. II.To oppose the decrease in the upward magnetic field (flux), the induced current produces an upward magnetic field, trying to maintain the “status quo.”

  11. The Big Idea! The induced current moves such that its magnetic field tends to oppose and resist the bar magnet’s moving field. Essentially it wants to DAMPEN the other field. Nature wants the two fields to be in harmony. This is LENZ’S LAW and it explains why the magnet falls slowly through the copper pipe!

  12. Lenz’s Law An induced emf always gives rise to a current whose magnetic field opposes the original change in flux. An induced emf is always in a direction that opposes the original change in flux that caused it.

  13. The Flux Capacitor The measure of how the field changes has to do with an amount, or area, of coil or loop of wire exposed to the field. It doesn’t matter if the field is in motion, changing in intensity, or if the coil is moving. So long as – FROM THE POINT OF VIEW OF THE WIRE - the B field appears to be changing with respect to time.

  14. The Flux Capacitor Magnetic flux is a measure of field strength B over an area measured in m2. Think of it this way. The absolute amount of rain fall is 2” per hour over the entire state. We only care about a small part, namely a square foot.

  15. The Flux Capacitor

  16. Phi, Fi, Pho, Phum? The unit of flux is the Henry, named for Joseph Henry. The symbol is the capitol Greek letter Phi…. Φ = B X A Φ = B A cosθ Henry = Tesla • m2

  17. Case 1 I Motion A current can be induced by changing the area of the field exposed to the coil. Here the area depends on moving the coil into the field.

  18. Case 2 Area through the coil decreases Therefore A current can be induced by changing the area of the coil exposed to the field by collapsing the coil. Here, the area is changed by shrinking the ring.

  19. Case 3 – A Generator This side is coming toward you I A current can be induced by changing the area of the coil exposed to the field. Here, the area is changed by rotating with respect to the field.

  20. How to Induce an EMF An emf can be induced whenever there is a change in flux. Since B = BA cos  an emf can be induced in three ways: by a changing magnetic field B by changing the area of the loop in the field by changing the loop’s orientation  with respect to the field.

  21. Fleming’s LHR Revisited

  22. I

  23. I Area has increased

  24. dx =

  25. I F

  26. Umm….Sorry But we simply have to have a differential equation. Emf = change in Flux = B Field x Change in Area change in time change in time Emf = B Field x length x velocity x change in time change in time

  27. EMF Induced in a Moving Conductor dBB dA Blv dt = = = = Blv dt dt dt This equation is valid as long as B, l, and v are mutually perpendicular.

  28. F = qvxB is the force on the charges in the wire that produces a current. Rotating clockwise

  29. Faraday’s Law of Induction = N Lenz’s law N = Number of loops of wire dB dt Induced emf Faraday found, experimentally, that the magnitude of the induced emf is proportional to the: rate of change of magnetic flux.

  30. I

  31. Example 29-5 An ac generator. The armature of a 60-Hz ac generator rotates in a 0.15-Tmagnetic field. If the area of the coil is 2.0 x 10-2 m2, how many loops must the coil contain if the peak output is to be 0 = 170 V?

  32. DC Generator A dc generator is much like an ac generator or alternator, except the slip rings are replaced by split-ring commutators, just as in a dc motor.

  33. 29-6 Transformers and the Transmission of Power A transformer is a device for increasing or decreasing an ac voltage. It consists of two coils of wire known as the primary and secondary coils.

  34. 29-6 Transformers and the Transmission of Power A transformer is a device for increasing or decreasing an ac voltage. A transformer may be a step-up transformer (increasing voltage) or a step-down transformer (decreasing voltage). A transformer consists of two coils of wire known as primary (voltage input) and secondary (voltage output) coils. = Transformer equation: VSVP NSNP

  35. Example 29-8 Portable radio transformer. A transformer for home use of a portable radio reduces 120-V ac to 9.0-V ac. The secondary contains 30 turns and the radio draws 400 mA. Calculate (a) the number of turns in the primary; (b) the current in the primary; and (c) the power transformed.

  36. Example 29-9 Transmission lines. An average of 120 kW of electric power is sent to a small town from a power plant 10 km away. The transmission lines have a total resistance of 0.40 . Calculate the power loss if the power is transmitted at (a) 240 V and (b) 24,000 V.

  37. 29-7 Changing Magnetic Flux Produces an Electric Field A changing magnetic flux induces an emf and a current in a conducting loop. Therefore, it produces an electric field.

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