1. Phys. 122: Thursday, 19 Nov. Written HW 12: due today (by 5:00 pm rather than 2:00 pm). Written HW 13: ch. 33, problems 18, 22, and 68, and ch. 34, probs. 10, 16, and 58. Due one week from Tuesday (same as exam 3). (This is your LAST written HW assignment!) Mast. Phys.: Assign. 9 due on Tuesday. An extra credit assignment is also now available. Reading: Finish ch. 34. Concentrate on sections 3, 4, 5, 6, and 7. Exam 3: will cover chapters 29, 30, 31, and maybe part or all of ch. 32. Likely to happen the Tuesday after Thanksgiving (Dec. 01). I will make a study guide and sample formula sheet by next week. Next week: No class on Thursday (Thanksgiving break). No scheduled recitation (we may have an optional “study” one if Mark decides to). No more labs this semester after today.

2. Special Announcement: The Physics Club and Civil Eng. Club students are hosting a BBQ outside of Fidel today (around lunchtime). Hamburgers and Hot Dogs for sale, with chips and drinks included. Please come and support their fundraising!

3. HW Questions/hints?

4. The key to generalizing EMF (ElectroMotive Force, which is actually VOLTAGE rather than a force) when magnetic fields are present: integrate the Electro-Magnetic Force (which IS a force) per charge, F/q, over distance: UNIVERSAL formula for EMF in a circuit:

5. Every motor can also be used as a generator!

6. This version of Faraday's Law is almost always true! There are some EMFs which don't have changing magnetic flux, however. It's usually a great shortcut, though. As with the previous EMF definition, the convention is that positive EMF is “downhill”: in the direction of the current. There is yet another right-hand-rule for this: if your right-hand thumb points along the convention of positive flux (on the right of the equation), then your right hand fingers curl in the positive EMF convention (on the left side of the equation).

7. Lenz' Law: Induced EMF always acts in a direction to oppose the change in magnetic flux. After a bit of practice, Lenz' Law is MUCH easier to use than the complicated right-hand-rule version given earlier! You are advised to become a “black belt” master user of Lenz' Law.

8. Clicker Question 4 A long wire carries a current I as shown. What is the direction of the current in the circular conducting loop when I is decreasing? A.The current flows counterclockwise. B.The current flows clockwise.

9. Faraday's Law in one final form, which is valid for any STATIONARY loop in space: This says that whenever a magnetic field changes with time, electric fields are generated! The electric fields are there in space no matter what (whether a conductor is there to make a current flow, or not).

10. Comparing Ampere's Law with Faraday's:

11. If the magnet moves toward the circuit OR the circuit moves toward the magnet, an EMF is generated!

12. The key to generalizing EMF (ElectroMotive Force, which is actually VOLTAGE rather than a force) when magnetic fields are present: integrate the Electro-Magnetic Force (which IS a force) per charge, F/q, over distance: EMF = ∮ (E + v x B) ∙ dl Always true.

14. Clickers: if the electric field points in the direction shown, the magnetic field must actually be... a) Out of the page b) Pointing clockwise c) Counterclockwise d) Decreasing with time e) Increasing with time

16. Clickers: If the magnetic field in the previous slide were DECREASING instead, which direction would the current flow? a) There would be no current; only increasing fields create current. b) It would still flow counterclockwise, since the magnetic field still points inward. c) It would flow clockwise, to create inward flux d) It would flip direction rapidly, creating AC in the loop. e) Out of the page.

17. Magnetic braking: without the applied force, the motion will come to a stop!

18. Clickers: What is responsible for the braking action of the conductor? a) Horizontal currents inside the conductor b) Vertical currents inside the conductor c) Horizontal electric fields inside the conductor d) Magnetic attraction of the conductor to the magnet e) Friction in the pivot bearings

19. Clickers: Which of the following actions will increase magnetic braking effects? a) Using a better conductor b) Using a weaker magnet c) Moving the objects more slowly d) Heating up the conductor e) All of the above actions are correct.

20. If the magnet moves toward the circuit OR the circuit moves toward the magnet, an EMF is generated! If the circuit's coil moves because it's attached to a flexible cone that wiggles when sound waves impact it, we have a microphone!

21. Clickers: a microphone is a type of generator, and we've said that every generator can also behave as a motor. What is the “motor” version of a microphone called? a) A blender b) A coffee maker c) An amplifier d) A speaker e) A lawn mower

22. Clickers: We've seen two examples where induction caused braking forces on conductors (or magnets). Can induction ever accelerate a conductor instead? a) No b) Yes c) Definitely maybe

23. Clickers: The electromagnet in the “jumping rings” demonstration has a changing magnetic field with both upward and outward components. Which was responsible for the induced EMF in the rings? a) The upward component b) The outward component c) Both d) Neither e) Who knows? My main thought is that I'm ready for lunch.

24. Clickers: The electromagnet in the “jumping rings” demonstration has a changing magnetic field with both upward and outward components. Which was responsible for the net force on the rings? a) The upward component b) The outward component c) Both d) Neither e) I will blame Obama.

25. Another way to understand the “jumping rings”: Two nearby “north poles” push one another apart!

27. Mutual Induction: A change in current in one circuit generates an EMF in the other. This principle is behind the transformer, and is one form of the wireless transmission of power. Having the pickup be a coil of N turns rather than a single loop multiplies the induced EMF by N.

28. Mutual Induction: A change in current in one circuit generates an EMF in the other. For Self-Induction, the original circuit is the SAME as the induced-EMF circuit! The SI unit of induction is the Henry, abbreviated H. Inductors give inertia to currents in circuits. ↱ L (or M)