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Schedule

Schedule. Today: Check Exam I Circuit Problems Magnetism Turn in Lab 6 Thursday Do Lab 8 (in lab book). Exam Trouble Spots. Difference between field, potential, force, potential energy Vectors (GPII) Conservation of Energy. Electric Field. Starts at + Ends at –

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Schedule

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  1. Schedule • Today: • Check Exam I • Circuit Problems • Magnetism • Turn in Lab 6 • Thursday • Do Lab 8 (in lab book)

  2. Exam Trouble Spots • Difference between field, potential, force, potential energy • Vectors (GPII) • Conservation of Energy

  3. Electric Field Starts at + Ends at – Can also go to “infinity” Field makes force Indicates direction of force on + charge

  4. Magnetic Fields Start at North  Go to South Unlike charges  Every North has a South Make a force – but more complicated

  5. Magnetic Fields • Magnetism discovered ages ago • Fields like electrostatics • Terminology  “pole”arization • More complicated than electric forces • Forces Related (will see, eventually…)

  6. Magnetic Interaction Magnets have poles  N & S Like Electrostatic Forces  Opposites Attract, Like Repel, & non-magnetic unaffected “Magnetic” can be polarized… S S S N N N S N

  7. Compass • Floating Magnetic Needle • North end attracted to “North Pole” • Points North • Handy outside • Work inside?

  8. Field of a bar magnet • Field lines start at N and end at S • Notice no infinity! N S

  9. Field of a Horseshoe Magnet • Stronger at Poles

  10. Fridge Magnet • Material like many horseshoe magnets • “Domains”

  11. Fridge Magnet:is Fridge Magnetic? • What are magnetic materials? • Why do magnets stick to “magnetic materials”?

  12. Magnetism: Macroscopic ViewFerromagnetic Domains Arrows Point from North to South Individual “Little Magnets” Unpolarized Ferro-magnet  Fridge, Chunk of Iron, etc…

  13. Magnetism: Macroscopic ViewFerromagnetic Domains Polarized Material Bar Magnet, Compass Needle, Earth How to Polarize : Use magnetic Field! Magnet polarizes nearby magnetic material (Think Charge) Always attractive

  14. Non-Magnetic Materials • Paramagnetism • Some things just don’t care about fields • Wood, Paper, Aluminum • Diamagnetism • Actually small repulsion does occur • (non-ferro) • In a large enough field “anti-polarization” • 16 T floats a frog…

  15. Ferromagnetic Properties • Iron, Nickel, Cobalt are ferromagnets • Aluminum, silicon, argon are not • Similar properties between 3 elements? • Are they neutral? • Hint: magnetism & electric forces related • What gives rise to magnetic properties?

  16. Electrons • 90% of anything’s properties due to Electrons • Determine insulator / conductor • Heat & Electricity • Determine Magnetism • Determine Color, etc…

  17. Magnetism • Created by & Acts on MOVINGcharges • Magnetic Fields  B, units T (Tesla) • Tesla Coil

  18. Magnetic Materials must be Metals? • Magnetic force acts on MOVING charges • What moves in a metal? • Total charge of a metal? • Do protons move? • Magnetic Force on electrons? • Magnetic Force on protons?

  19. Moving Charges Make Magnetic Fields (Straight Wire) m0 is magnetic permittivity of free space r is distance to wire I current in wire Like Electrostatic Permittivity

  20. Field Obeys R.H.R.

  21. Fielf From a Loop? B? B? Current Current Current

  22. Magnetic Field from a Loop Field at Center Like a Bar Magnet

  23. Magnetic Field from Many Loops Field at Center for one loop What if a small coil, say 5 loops? Hint: Superposition

  24. Solenoids: Capacitor of Magnetism Field Constant Inside Field ~ zero outside Direction R.H.R. L

  25. What are electrons doing in a bar magnet? N What are they doing In a non-magnetic Metal? S

  26. Magnetic Force on Moving Charge Q = 0  F = 0 V = 0  F = 0

  27. Force is PerpendicularEverything Perpendicular • Force perpendicular to BOTH v & B

  28. Magnetic Field from Moving Charge Definition of magnetic Field

  29. Right Hand RuleWorks for wrenches too! Sign Important How to determine directions Quickly Negative Charges Opposite “Left Hand Rule”

  30. What is trajectory? Force Perp. To v & B Straight Line? Bendy? How?

  31. Motion of Charged Particle in a Magnetic Field • Force perpendicular to velocity & field Velocity Perp. To B B is pointing “out” Circular Motion R.H.R.

  32. Circular Motion Review • Centripetal Acceleration / Force r

  33. If a velocity not quite perpendicular: • Helical Motion • Separate Components • vP2 + vL2 = v2

  34. Mass Spectrometer Simple Design 2 Isotopes, Ionized Same Charge (+e) Different Mass Same velocity r1 r2

  35. Force on a Current Carrying WireCharges moving in Wires Remember  Current is positive charge flow Electron flow opposite (negative charges) Work with current: everything positive & R.H.R.

  36. Force Obeys R.H.R. Point in current flow direction, rest same

  37. Try Problems • Circuits • Magnetism problems Thursday before lab • Read over chapter & lab

  38. Additional Examples • In the circuit shown below: • Rank in order, from most to least bright, the brightness of bulbs A–D. Explain. • Describe what, if anything, happens to the brightness of bulbs A, B, and D if bulb C is removed from its socket. Explain. Slide 23-41

  39. Additional Examples In the circuit shown below, rank in order, from most to least bright, the brightness of bulbs A–E. Explain. Slide 23-42

  40. Additional Examples • In the circuit shown below: • How much power is dissipated by the 12 Ω resistor? • What is the value of the potential at points a, b, c, and d? Slide 23-43

  41. Clicker Question • The diagram below shows a segment of a circuit. What is the current in the 200 resistor? • 0.5 A • 1.0 A • 1.5 A • 2.0 A • There is not enough information to decide. Slide 23-13

  42. There is a current of 1.0 A in the circuit below. What is the resistance of the unknown circuit element? What is the current out of the battery? Slide 23-19

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