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.

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!

HW Questions/hints?

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:

Every motor can also be used as a generator!

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).

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.

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.

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).

Comparing Ampere's Law with Faraday's:

If the magnet moves toward the circuit OR thecircuit moves toward the magnet, an EMF isgenerated!

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.

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

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.

Magnetic braking: without the applied force, themotion will come to a stop!

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

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.

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!

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

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

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.

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.

Another way to understand the “jumpingrings”:

Two nearby “north poles” push one anotherapart!

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.

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)