Phys. 122: Tuesday, 17 Nov. Written HW 9 returned: please pick up yours in front. Written HW 12: due Thursday. Mast. Phys.: Assign. 8 due this evening. Assign. 9 is availableand is due in one week. An extra credit assignment is alsonow available. Reading: Finish ch. 34. Exam 3: will cover chapters 29, 30, 31, and maybe part or allof ch. 32. Likely to happen the Tuesday after Thanksgiving(Dec. 01).

HW Questions/hints?

Magnetic Materials: If we put a materialin a magnetic field, three types of behavior

are possible: Diamagnetism: the original field can be

reduced slightly (by induced currents – Lenz' Law). For a superconductor, the original field

is cancelled completely (perfect diamagnetism).

Paramagnetism: the original field can be increased slightly, by alignment of small permanent current loops in the material.

• Ferromagnetism: The original field can be increased dramatically, and the magnetism can persist when the original field is gone

(this makes a “permanent” magnet).

Clickers: Why didn't we give you theforce rule between two permanent magnets?

• a) There is no force between them

• b) The force is only explainable by invoking special relativity

• c) The force rule is too complicated to be put in simple form

• d) Permanent magnets aren't important enough

• e) The force rule is top-secret, guarded by descendants of the medevial Physics

Guild

Conductors in equilibrium have a vanishingforce per charge. In magnetic fields, thismodifies the “electrostatics” rule for insideconductors that E = 0:

This rule is at the basisof electromagnetic induction, which iswhere EMF that isn't from batteriesoriginates.

Example: problem 33.1

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:

The rail gun in reverse is a slidewiregenerator:

This also generates an EMF! But if themagnetic field were uniform, it wouldn't.

Clickers: It seems as though we couldjust allow the loop to keep moving, andwe'll keep getting power from it! Can

this solve some of our energy problems?

• a) Yes, but the materials haven't yet been perfected.

• b) Yes, but the power companies are greedy and won't allow it.

• c) No, because you can't make a magnetic field in one region only like the diagram.

• d) No, because a force is required to keep it moving.

• e) Yes, and I'm leaving class now to go make a fortune in selling this electrical power!

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).

Clickers: The flux integral below must befor....

a) A closed surface b) An open surface c) The surface of the battery d) Calculus experts only e) Electric field lines piercing the surface

Clickers: The SI unit of magnetic flux,(Tesla) times (meters)² is often called aWeber (Wb). One Wb/s equals one...

a) Light year b) billion million c) foot-pound d) Watt e) Volt

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.

This also generates an EMF! But if themagnetic field were uniform, it wouldn't.

Works if the loop stays put and the magnet movesinstead! Something fishy about that....

Clickers: when a magnet and circuitgenerate EMF through relative motion,

the EMF comes from...

a) The E field b) v x B forces

c) Either (a), (b), or both, depending upon which object is moving

d) A hidden battery e) Coulomb's Law

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.

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).