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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 asexam 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 allof ch. 32. Likely to happen the Tuesday after Thanksgiving(Dec. 01). I will make a study guide and sample formula sheet bynext 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.
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Special Announcement: ThePhysics Club and Civil Eng. Clubstudents are hosting a BBQ outsideof Fidel today (around lunchtime).Hamburgers and Hot Dogs for sale,with chips and drinks included. Pleasecome and support their fundraising!
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HW Questions/hints?
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The key to generalizing EMF (ElectroMotiveForce, which is actually VOLTAGE ratherthan a force) when magnetic fields are present:integrate the Electro-Magnetic Force (whichIS a force) per charge, F/q, over distance:
EMF=∮ ( E⃗+ v⃗× B⃗ )⋅d x⃗
UNIVERSAL formula for EMF in a circuit:
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Every motor can also be used as a generator!
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This version of Faraday's Law is almostalways true! There are some EMFswhich don't have changing magnetic flux,however. It's usually a great shortcut, though.
As with the previous EMF definition, the convention isthat positive EMF is “downhill”: in the direction of thecurrent. There is yet another right-hand-rule for this:if your right-hand thumb points along the convention ofpositive flux (on the right of the equation), then yourright hand fingers curl in the positive EMF convention(on the left side of the equation).
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Lenz' Law: Induced EMF always acts ina direction to oppose the change inmagnetic 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.
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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.
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Faraday's Law in one final form, whichis valid for any STATIONARY loop inspace:
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).
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Comparing Ampere's Law with Faraday's:
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If the magnet moves toward the circuit OR thecircuit moves toward the magnet, an EMF isgenerated!
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The key to generalizing EMF (ElectroMotiveForce, which is actually VOLTAGE ratherthan a force) when magnetic fields are present:integrate the Electro-Magnetic Force (whichIS a force) per charge, F/q, over distance:
EMF = ∮(E + v x B) ∙ dl Always true.
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Clickers: if the electric field points inthe direction shown, the magnetic fieldmust actually be...
a) Out of the page b) Pointing clockwise c) Counterclockwise d) Decreasing with time e) Increasing with time
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Clickers: If the magnetic field in the previousslide 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.
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Magnetic braking: without the applied force, themotion will come to a stop!
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Clickers: What is responsible for thebraking 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
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Clickers: Which of the following actionswill 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.
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If the magnet moves toward the circuit OR thecircuit moves toward the magnet, an EMF isgenerated!
If the circuit's coil moves because it'sattached to a flexible cone that wiggleswhen sound waves impact it, we have amicrophone!
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Clickers: a microphone is a type of generator,and we've said that every generator can alsobehave as a motor. What is the “motor” versionof a microphone called?
a) A blender b) A coffee maker c) An amplifier d) A speaker e) A lawn mower
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Clickers: We've seen two exampleswhere induction caused braking forces onconductors (or magnets). Can inductionever accelerate a conductor instead? a) No b) Yes c) Definitely maybe
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Clickers: The electromagnet in the “jumpingrings” demonstration has a changingmagnetic field with both upward and
outward components. Which was responsiblefor 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.
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Clickers: The electromagnet in the “jumpingrings” demonstration has a changingmagnetic field with both upward and
outward components. Which was responsiblefor the net force on the rings?
• a) The upward component
• b) The outward component
• c) Both
• d) Neither
• e) I will blame Obama.
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Another way to understand the “jumpingrings”:
Two nearby “north poles” push one anotherapart!
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Mutual Induction: A change in current inone circuit generates an EMF in the other.This principle is behind the transformer, andis one form of the wireless transmission ofpower.
Having the pickup be a coil of N turns rather thana single loop multiplies the induced EMF by N.
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Mutual Induction: A change in current inone circuit generates an EMF in the other.
For Self-Induction, the original circuit is theSAME as the induced-EMF circuit! The SIunit of induction is the Henry, abbreviated H.Inductors give inertia to currents in circuits.
Inductance=−( EMF ) / (dI /dt ) ↱L (or M)