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Take out a piece of paper

Take out a piece of paper. Jot down what you know about magnets… Where do we find magnets? What metals are magnetic? How can you make or destroy magnets? How do we use magnets? And…what would life be like without them?. Magnets. Why do we discuss magnets along with electricity?.

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Take out a piece of paper

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  1. Take out a piece of paper • Jot down what you know about magnets… • Where do we find magnets? • What metals are magnetic? • How can you make or destroy magnets? • How do we use magnets? • And…what would life be like without them?

  2. Magnets • Why do we discuss magnets along with electricity?

  3. MAGnetIC Fields Why are we attracted to magnets? Or… Animal Magnetism: Myth or Misunderstanding

  4. Discovery of Magnets • Some debate – one of the great controversies of our time! • Story of sheep herder in Magnesia region of Greece. • Or did the Chinese discover it thousands of years prior?

  5. Magnetic Fields – Overhead • Look at magnets on overhead with iron filings • Magnetic flux lines – flow out of the north poles of magnets and into the south poles. • Closed loops • No overlap between 2 magnetic fields • Remind you of anything?

  6. Magnetic Fields – Overhead • Paper clips • Soft vs hard • How do you destroy a magnet? • Can you see the magnetic fields? • Magnetic Domains

  7. Magnets interact with electricity • Ampere’s – the guy that current is named for… • Studied magnetism and electricity • Observed current through a wire • In a magnetic field • Bounces or jumps

  8. Current makes magnets? • Current causes a magnetic field around the wire • Ampere: interaction between 2 magnetic fields • One from a magnet • One from electricity • Electricity can create an electromagnet • Like at the junk yard or

  9. Magnetic Fields – now that we’ve seen them… • The conventional way to think about magnetic fields… • Direction of concentric field lines based on RHR • Grab the wire with your right hand • The current flow lines up with your thumb • Your fingers curve in the direction of the magnetic fields.

  10. Magnetic field around a coil • Take one loop of wire • Trace your right hand around the loop and look where your fingertips go • Resulting magnetic force inside the loop combines in one direction • What happens when you have 2 loops?

  11. Magnetic fields – odds and ends • Magnetic Fields • Strength of the magnetic field • B (vector) • Into or out of page (x or .) • Arrows

  12. What next? • Start Video?

  13. Magnetic Force March 30/31, 2010

  14. POTD • Check homework • Go over questions • Magnetic Force Lecture • Lab

  15. repel, attract, attract two a, b parallel to the earth’s surface pointing towards the geographic north pole Hard Concentric circles Smaller region inside Opposite spin – magnetic fields cancel HW

  16. Last time’s Lab • Please turn it in now! ; )

  17. What’s the point? • Last time: • Magnetism from Electricity • We can see what happens! • Must be a force acting on it… • Today • Calculate the FORCE on a charge in a magnetic field

  18. Magnetic Force • Consider a charged particle • Like an electron or a proton… • In the presence of a magnetic field • The force felt by the charge • F = qvB • q= charge (Coulombs) • V = velocity (m/s) • B = strength of magnetic field (Tesla)

  19. Strength of B • The strength of a typical lab magnet • 1.5 T • The strength of the earth’s magnetic field • Is it stronger or weaker than a lab magnet? • Yes!! Nice call!! • Earth’s is about 50 μT • Or… 5.0 x 10-5 T

  20. Force as in VECTOR • What direction is the force? • Assume B is a uniform magnetic field • The particle moves at a velocity of v • Right hand rule #2 • Force comes out of wall at you

  21. Force as in VECTOR • Right hand rule #2 • Based on a positive charge • If it’s negative… • It’s the other direction

  22. Example • A charged particle • Charge = 1.7 x 10-6 C • Is traveling north at 47 m/s • Through a uniform magnetic field • B = 1.5 T straight up • What is F? • F = qvB = (1.7 x 10-6 C) x (47 m/s) x 1.5 T • F = 1.2 x 10-4 N (east)

  23. Uniform Magnetic Field • B is directed into the wall • A particle travels perpendicular to B • The force on the particle moves it • Perpendicular to travel direction • What is the path of the particle?

  24. Is the particle + or - ?

  25. Wire in a Uniform Magnetic Field • Similar to a charge…except… • Thumb along current’s path • Fingers up magnetic field • RHR

  26. The calculation… • Force = qvB • qv = i x length • Force = Bil • B – magnetic field strength • i – current • l – length of wire in field

  27. Example • A 6.0 m wire carries a current of 7.0 A in the x direction. • A magnetic force = 7.0 x 10-6 N in the –y direction • What is the direction and magnitude of the magnetic field? • 7.0 x 10-6 N = B x 7.0 A x 6.0 m • B = F/(i l) • B = 1.7 x 10-7 T

  28. Can you see it? • What happens when you have 2 wires carrying current in the same direction. • Check your answer with a friend… • What about if they are in the opposite direction? • Same thing…

  29. Think about a loop… • In a magnetic field…

  30. Lab intro • Discovery lab • A magnet • A solenoid • A galvanometer • To measure current (μA) • Determine as much as you can • Your grade will be based on your discoveries • Don’t skimp

  31. Induced Current March 31 & April 1, 2010

  32. Lab Get homework out 0.081 T Up 0.75 N Attracts Out of page Turn in …

  33. Today’s Learning Goals • What are the factors that affect induced current? • Understand that induced current generates its own magnetic field • Calculate the magnitude of an induced voltage

  34. Right hand rules • What are they? • Check your understanding with a neighbor

  35. Induced current • Relative motion between a wire and a magnetic field… • Creates a potential difference in the wire • Current flows – if the circuit is complete! • Recall: a charged particle moving in a magnetic field experience a force. • F = qvB

  36. F = Bil: Consider a wire • Relative motion is required • due to the force • Pull it through a magnetic field • Perpendicular to the field • The magnetic force causes the positive ones to move • According to the RHR • And the negative charges to move • According to the “left” hand rule

  37. F = qvB: Consider a wire • Separation of charges • Like a potential difference or voltage • If there is a completed circuit • Then current flows

  38. Motion is essential • The voltage (and current) is maintained • As long as the wire is moving relative to the magnetic field. • The faster it moves – the greater the current • The greater the magnetic field, the greater the current

  39. Orientation matters • In a complete circuit/loop of wire • more current flows if motion is perpendicular to the magnetic field. • The number of field lines that you “cut” as it moves… • Increase that and you increase current! • This means you can also rotate the coil in a magnetic field.

  40. Faraday’s Law of Induction • Factors that affect the induced voltage: • The number of coils in the circuit (N) • The angle between B and the coil (θ) • Area of the coil (m2) • B = magnetic field strength (T) • emf = -N Δ(AB(cos θ))/ Δt

  41. Faraday’s Law of Induction • emf = -N Δ(AB(cos θ))/ Δt • If you vary the: • cross sectional area • Magnetic field • The angle of orientation • With respect to a change in time • THEN you will see an induced emf

  42. Example • A coil with 25 turns of wire • Cross sectional area of 1.8 m2 • Magnetic field applied at a right angle to the plane of the coil • The field is increased from 0.00 T to 0.55 T in 0.85 seconds • What is the magnitude of the induced emf?

  43. The answer, please: • emf = -N Δ(AB(cos θ))/ Δt • emf = - 25 (1.8m2)cos0 (ΔB/Δt) • emf = - 45 (0.55 – 0.00T/0.85 s) • emf = - 29 V • Note that all factors are included in the answer. • But something has to change!

  44. Lenz’s Law • If you induce a current in a wire • That current creates a magnetic field of its own! • Lenz tells us that the induced current creates a magnetic field that opposes the applied magnetic field. • The induced current tries to keep the field strength constant.

  45. Today’s Goals • Factors that affect induced voltage/current • Induced current generates its own _________________? • Calculate the magnitude of an induced voltage

  46. Lab! • Today’s lab DOES NOT connect directly with the lesson!! • It does relate to magnetic fields generated by current carrying wire… • Have fun!

  47. Transformers April 5/8, 2010

  48. Transformers: Saving the Planet • We generate electricity at Bonneville • Alternating current • We use it in our homes • 40 miles away • Is that a problem? • Line losses… • Big “i”… big losses… • So how do we change the current??

  49. How indeed… • Remember the door bell example… • The bell only works when there is a change in electrical current. • The system is DC • When connected, a constant current flows. • What would happen if it was AC?

  50. Same idea… • 2 coils connected with a soft metal core • The current in the first coil creates a magnetic field. • AC circuit - current changes • 60 times a second (Hertz) • The changing magnetic field induces a current in the secondary coil

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