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 available and is due in one week. An extra credit assignment is also now available. Reading: Finish ch. 34. 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).

HW Questions/hints?

Magnetic Materials: If we put a material in 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 the force 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 vanishing force per charge. In magnetic fields, this modifies the “electrostatics” rule for inside conductors that E = 0: This rule is at the basis of electromagnetic induction, which is where EMF that isn't from batteries originates.

Example: problem 33.1

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:

The rail gun in reverse is a slidewire generator:

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

Clickers: It seems as though we could just allow the loop to keep moving, and we'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 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).

Clickers: The flux integral below must be for.... 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 a Weber (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 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.

This also generates an EMF! But if the magnetic field were uniform, it wouldn't. Works if the loop stays put and the magnet moves instead! Something fishy about that....

Clickers: when a magnet and circuit generate 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 the circuit moves toward the magnet, an EMF is generated!

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.

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).