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Conclusion – Chapter 20 & Chapter 21 – Electromagnetic induction

Conclusion – Chapter 20 & Chapter 21 – Electromagnetic induction. Chapter 21. Quick Review I B. r. Force Between Two Current Carrying Conductors. First wire produces a magnetic field at the second wire position. The second wire therefore feels a force = Bi l. Solenoid.

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Conclusion – Chapter 20 & Chapter 21 – Electromagnetic induction

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  1. Conclusion – Chapter 20 &Chapter 21 – Electromagnetic induction Chapter 21

  2. Quick Review IB r

  3. Force Between Two Current Carrying Conductors First wire produces a magnetic field at the second wire position. The second wire therefore feels a force = Bil

  4. Solenoid B=~0 outside

  5. The Toroid– B=0 outside

  6. A rectangular loop has sides of length 0.06 m and 0.08 m. The wire carries a current of 10 A in the direction shown. The loop is in a uniform magnetic field of magnitude 0.2 T and directed in the positive x direction. What is the magnitude of the torque on the loop? 8 × 10–3 N × m

  7. A solenoid of length 0.250 m and radius 0.0250 m is comprised of 440 turns of wire. Determine the magnitude of the magnetic field at the center of the solenoid when it carries a current of 12.0 A. 2.21 × 10–3 T

  8. The drawing shows two long, thin wires that carry currents in the positive z direction. Both wires are parallel to the z axis. The 50-A wire is in the x-z plane and is 5 m from the z axis. The 40-A wire is in the y-z plane and is 4 m from the z axis. What is the magnitude of the magnetic field at the origin? 3 × 10–6 T

  9. I I Moving On to the next chapter……………….

  10. INTRODUCTION TO INDUCTION Induction

  11. Important Definition – Magnetic Flux Magnetic Field AREA Induction

  12. Induction

  13. The Essence of this Topic • Consider a conductor that is shaped in a loop but is continuous. • The conductor has a magnetic field through the loop that is not necessarily uniform. • There is a MAGNETIC FLUX through this loop. • If the FLUX CHANGES, an “emf” will be induced around the loop. • This emf can cause a current to flow around the loop. Induction

  14. How Can You Change the Magnetic Flux Going Through The Loop? Huh? • Divide the area of the loop into a very large number of small areas DA. • Find the Magnetic Field through each area as well as the angle that it makes with the normal to the area. • Compute the total flux through the loop. Induction

  15. The Magnetic Flux Going Through The Loop: Add up all of these pieces that are INSIDE the loop. Induction

  16. Changing F • Change any or all of the • Bi • DAi • fi • Change the SHAPE of the loop • Change the ANGLE that the loop makes with the magnetic field (subset of above) • And the Flux will change! Induction

  17. WAIT A SECOND ……. • You said that there is a conducting loop. • You said that there is therefore a VOLTAGE or emf around the loop if the flux through the loop changes. • But the beginning and end point of the loop are the same so how can there be a voltage difference around the loop? • ‘tis a puzzlement! Induction

  18. Induction

  19. REMEMBER when I said E Fields start and end on CHARGES??? DID I LIE?? Induction

  20. The truth • Electric fields that are created by static charges must start on a (+) charge and end on a (–) charge as I said previously. • Electric Fields created by changing magnetic fields can actually be shaped in loops. BUT WAIT, YOU STILL LIE! Induction

  21. Why do you STILL think I am a liar? Because you said that an emf is a voltage so if I put a voltmeter from one point on the loop around to the same point, I will get ZERO volts, won’t I Induction

  22. Yes … but this doesn’t make me a liar! Let Me Explain. Induction

  23. The POTENTIAL between two points • Is the WORK that an external agent has to do to move a unit charge from one point to another. • But we also have (neglecting the sign): Ds Induction

  24. So, consider the following: x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x E Conductor Induction

  25. THEREFORE WHAT WILL A VOLTMETER READ FROM A to A? x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x A The emf B Zero C Can’t tell E  A Conductor Induction

  26. How Big is the emf? Faraday’s Law MINUS???? Induction

  27. Q: Which way does E point? A: The way that you don’t want it to point! (Lenz’s Law). Lenz’s Law Explains the (-) sign! Induction

  28. OK. LET’S DO THE PHYSICS NOW Induction

  29. DEMOs Induction

  30. Is there an induced current??? Induction

  31. Induction Effects A changing magnetic field INDUCES a current in a circuit loop.

  32. Faraday’s Experiments ? ?

  33. Insert Magnet into Coil

  34. Remove Coil from Field Region

  35. Summary

  36. Does the Flux Change?

  37. In the Previous Example, if there are N coils rather than a single coil, A The current is increased by a factor of N B The current is decreased by a factor of N C The current stays the same. Induction

  38. Push a magnet into a coil of two wires and a current is produced via an emf. In this case, 2 coils, each has the SAME emf. Ohm’s Law still works, so Induction

  39. If we go from 2 to 4 coils, the current A Stays the same B Doubles C Is halved D Is four times larger Induction

  40. A rectangular circuit containing a resistor is perpendicular to a uniform magnetic field that starts out at 2.65 T and steadily decreases at 0.25 T/s. While this field is changing, what does the ammeter read? Induction

  41. ••The conducting rod ab shown makes frictionless contact with metal rails ca and db. The apparatus is in a uniform magnetic field of 0.800 T, perpendicular to the plane of the figure. (a) Find the magnitude of the emf induced in the rod when it is moving toward the right with a speed 7.50 m/s. Induction

  42. Application: AC Voltage Generator Induction

  43. Almost DC Induction

  44. The Strange World of Dr. Lentz

  45. Lenz’s Law Induced Magnetic Fields always FIGHT to stop what you are trying to do! i.e... Murphy’s Law for Magnets

  46. Example of Nasty Lenz The induced magnetic field opposes the field that does the inducing!

  47. Don’t Hurt Yourself! The current i induced in the loop has the direction such that the current’s magnetic field Bi opposes the change in the magnetic field B inducing the current.

  48. Let’s do the Lentz Warp again !

  49. OR The toast will always fall buttered side down! Again: Lenz’s Law An induced current has a direction such that the magnetic field due to the current opposes the change in the magnetic flux that induces the current. (The result of the negative sign that we always leave out!) …

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