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Lenz and The Haunting of Physics

Lenz and The Haunting of Physics. Lenz’s Lab. Dr. Lenz strikes again. Last Time. We looked at induced currents We indirectly covered Lenz’s Law Today we will explore induction further and learn about a new circuit element.

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Lenz and The Haunting of Physics

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  1. Lenz and The Haunting of Physics Lenz’s Lab Dr. Lenz strikes again.

  2. Last Time • We looked at induced currents • We indirectly covered Lenz’s Law • Today we will explore induction further and learn about a new circuit element. • Friday we will have a quiz. (Remember my threat … it might happen! Or not.)

  3. Last Time • We looked at currents and magnetic fields induced by changing magnetic fields. • We defined magnetic flux. • We can now state Lenz’s Law in the following way: • If you try to change the magnetic FLUX through a closed surface, the induced current will be in such a direction as to OPPOSE the change that you are trying to make! • Or – “The toast will always fall buttered side down!”

  4. HEY BINDELL – WAIT A SECOND ……. • You said that there is a conducting loop. • You said that there is therefore a current induced 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? • Or a current??? • ‘tis a puzzlement! Induction

  5. Induction

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

  7. 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

  8. 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? How can there be a current?? Induction

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

  10. 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

  11. 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

  12. 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

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

  14. 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

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

  16. Check Your Knowledge

  17. An inductor resists CHANGES in the current going through it. Induction

  18. An inductor resists CHANGES in the current going through it. Induction

  19. An inductor resists CHANGES in the current going through it. Induction

  20. Inductance Defined If the FLUX changes a bit during a short time Dt, then the current will change by a small amount Di. Faraday says this is the emf! This is actually a calculus equation Induction

  21. So … There should be a (-) sign but we use Lenz’s Law instead! E= The UNIT of “Inductance – L” of a coil is the henry. SYMBOL: Induction

  22. Series LR Circuit Induction

  23. RL or LR Series Circuit • Switch is open .. no current flows for obvious reasons. • Switch closed for a long time: • Steady current, voltage across the inductor is zero. All voltage (E) is across the resistor. • i=E/R Induction

  24. RL or LR Series Circuit When the switch opens, current change is high and back emf from L is maximum. i E/R t As the current increases, more voltage is across R, the rate of change of I decreases and as the current increases, it increases more slowly. Induction

  25. RL Circuit When L=0, the current rises very rapidly (almost instantly) As L increases, it takes longer for the current to get to its maximum. Induction

  26. RL Circuit - Kirchoff Stuff Induction

  27. The Graphic Result – Current Growth } 63% of maximum e= 2.71828… Induction

  28. Decay – Short out the battery • Magnetic field begins to collapse, sending its energy into driving the current. • The energy is dissipated in the resistor. • i begins at maximum (E/R) and decays. Induction

  29. Solution Induction

  30. Lab Time … if enough time!

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