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Magnetism

Magnetism. Chapters 36 & 37. Magnetism. A brief history Lodestones were found in Greece some 2000 years ago. The Chinese later used them for navigating ships. In the 18 th century, Charles Coulomb conducted a study of the forces between lodestones. Magnetism. A brief history:

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Magnetism

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  1. Magnetism Chapters 36 & 37

  2. Magnetism A brief history Lodestones were found in Greece some 2000 years ago. The Chinese later used them for navigating ships. In the 18th century, Charles Coulomb conducted a study of the forces between lodestones.

  3. Magnetism A brief history: Until early 19th century, electricity and magnetism were considered to be separate fields. Hans Christian Oersted, in 1820, discovered a relationship between the two during a classroom demonstration. This led to new technology that would bring electric power, radio and television.

  4. Magnetic Poles Magnets apply forces on each other similar to charges. Magnets can attract and repel each other. Magnets have poles that are the regions in the magnet that apply forces. Magnetic poles are not positive and negative, but rather North and South.

  5. Magnetic Poles There is a rule when it comes to the poles of magnets: Like poles repel; opposite poles attract.

  6. Magnetic Poles What would happen if you were to cut a bar magnet in half?

  7. Magnetic Field Every magnet produces a magnetic field. A magnet’s magnetic field is similar to a planet’s gravitational field. When another magnet is near, or even a compass, it will lie in a line with the magnetic field. Similarly, iron filings become tiny bar magnets in the presence of a magnetic field.

  8. Magnetic Field What did we learn in our lab about the shape of a magnetic field around a bar magnet?

  9. Magnetic Field What did we learn about the direction of the magnetic field lines around a bar magnet? Field lines go out from the North and into the South.

  10. Magnetic Field What would happen to the magnetic fields of two like poles placed next to each other? N N

  11. Magnetic Field What would happen to the magnetic fields of two like poles placed next to each other? N N

  12. Magnetic Field What would happen to the magnetic fields of two opposite poles placed next to each other? N S

  13. Magnetic Field What would happen to the magnetic fields of two opposite poles placed next to each other? N S

  14. Magnetic Field What can you tell about the two magnets in each of these situations?

  15. Earth’s Magnetic Field This is a drawing of Earth’s magnetic field and its direction. What do you notice? So which magnetic pole is which?

  16. Magnetic domains = a microscopic cluster of atoms with their magnetic fields aligned. Magnetic Domains

  17. Magnetic Domains In our lab yesterday, how were you able to pick up the paperclips with the nail? What did the magnet do to the nail? Induced magnetism = metals (particularly iron) exhibiting magnetic properties due to contact with another magnet.

  18. Electric Current & Magnetic Field Remember Oersted? What did he discover during a classroom presentation? A moving charge/current produces a magnetic field, deflecting a compass. No current: Current:

  19. Electromagnetism I • First Right-Hand Rule • Thumb points in direction of current • Fingers follow magnetic field lines (direction of magnetic field) I

  20. Electric Current & Magnetic Field These are examples of a current-carrying wire, a current-carrying loop and a coil of loops.

  21. Electric Current & Magnetic Field If a current-carrying wire is bent into a loop, the magnetic field lines bunch up. If you add another loop and another, the magnetic field becomes more and more concentrated. This coil is called an electromagnet.

  22. Electromagnetism • What about a coil of wire? • The RHR still applies! I

  23. Electromagnets • Coil has a field like any permanent magnet with N and S poles • Advantage: can be turned off and on

  24. Electromagnets • 2nd Right-Hand Rule • Determine magnetic field of electromagnets • Fingers follow current as it curls in the coil • Thumb points in direction of N pole

  25. Magnetic Force A magnetic field will also apply a force on a current-carrying wire. To determine direction, we use the Right Hand Rule.

  26. Forces caused by Magnetic Fields • Vectors • Perpendicular to magnetic field lines and current

  27. Forces caused by Magnetic Fields • 3rd Right-Hand Rule • Determine direction of Force on a current-carrying wire in a magnetic field I N S

  28. Magnetic Force Let’s try another one… Thumb points in direction of current. Fingers point in direction of magnetic field. Palm points in direction of force.

  29. Magnetic Force Give this one a try: I S N Force: into the page

  30. Magnetic Force N S I Force: out of the page

  31. Magnetic Force N S X I Force: down

  32. Forces caused by Magnetic Fields • F = BIL • B = strength of magnetic field • I = current in the wire • L = length of wire in magnetic field • We know how to measure F, I and L, but not B so instead we use…

  33. Forces caused by Magnetic Fields • B = F / (IL) • Magnetic induction – strength of the magnetic field • Units: Tesla (T) • 1 T is very strong • Most lab magnets are 0.01 T • Earth’s magnetic field is 5 X 10-5 T

  34. A Simple DC Motor

  35. A Simple DC Motor

  36. Important Definitions • Magnetic flux • Number of magnetic field lines passing through a surface

  37. Faraday discovered that electric current could be produced in a wire simply by moving a magnet in and out of a coil of the wire. This is called electromagnetic induction. Electromagnetic Induction

  38. Electromagnetic Induction The greater the number of loops of wire that move in a magnetic field, the greater the induced voltage and the greater the current in the wire.

  39. Magnetic Force A magnetic field applies a force on a moving charge.

  40. Force on a single charged particle • Cathode ray tube – TV! • Electrons deflected by magnetic fields to form pictures

  41. Cathode Ray Tube Electric fields pull electrons off atoms, then more electric fields gather, and focus electrons into a beam. • Magnetic fields deflect electrons side to side and up and down across the screen • Screen coated with phosphorous that glows when struck

  42. Force on a single charged particle • F = BIL • F = B(qv/L)L • F = Bqv • q = charge of electron • v = particle velocity

  43. Magnetic Force The magnetic field of Earth deflects many charged particles that make up cosmic radiation.

  44. Van Allen Radiation Belts • Electrons trapped in Earth’s magnetic field • Solar storms send high-energy charged particles toward Earth • They knock electrons off VA belts • The electrons excite nitrogen and oxygen in the atmosphere creating a “halo” • The halo surrounds geomagnetic north

  45. Van Allen Belts • The Van Allen radiation belts are formed as a result of earth’s magnetic field and shield us from radiation. We can see the aurora borealis as a result.

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