phys. 122: tuesday, 17 nov. written hw 9 returned: please pick up yours in front. written hw 12: due...
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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).
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HW Questions/hints?
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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).
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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
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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.
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Example: problem 33.1
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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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The rail gun in reverse is a slidewiregenerator:
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This also generates an EMF! But if themagnetic field were uniform, it wouldn't.
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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!
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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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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
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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
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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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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....
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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
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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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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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