5-Magnetism

Slideshow: Magnetism

Textbook: Chapters 17 & 18

Right Hand Rule Practice: http://www.cstephenmurray.com

Objectives:

  • Magnet structure
    • explain magnetic domains in ferromagnets (permanent) compared to paramagnetic materials (weak & temporary) in terms of number of domains aligned and amount of alignment.
    • dipole is backwards from field line direction.
    • why no monopoles?
  • Magnets in fields
    • explain dipole alignment of a magnet in a field (example: compasses & earth as a dipole)
    • Explain metal filings & paper clips as induced dipoles
  • magnetic force on a charged particles/objects
    • Use the right hand rule and F = qvB to solve and explain problems
    • including 0º, 90º and 180º degree angles of v to field, but discuss effect at other angles
  • magnetic force created by a current carrying wire
    • Use grabbing right hand rule to find B fields tangential to concentric circles at specific points
    • compare strength of field at distances from the wire
    • Use B=(u/2pi)(I/r)
  • Electromagnetic Induction
    • Magnetic flux is kind of like magnetic field density (#lines passing through and area), but is mathematically B(cosθ)A
      • Changing flux induces an electric field
      • if there are charges (like in a wire), the E field created by the changing flux cancause a current. according to ɛ= –ΔB/t, where the negative sign indicates the EMF opposes the change in flux.
      • conservation of energy applies to magnetic forces
  • summary: 4 ways to cause a magnetic force
    • 2 magnets
    • 1 magnet with charged particle(s) moving nearby
    • 1 magnet moving near charged particle(s)
    • multiple charged moving objects (ex. 2 wires carrying current, straight or in loops)

Equations:

Common Misconceptions:

Fields

  • Misconception: North and south magnetic poles are the same as positive and negative charges.
    • Correct Principle: Magnetism and electric charge interactions are separate (although they can influence each other). Magnets are caused by a magnetic dipole based on spin , which will always have a N and S side, where electric charge interactions are based on the intrinsic properties of a particle. You can have single charges isolated from their opposite, but not single magnetic poles.
  • Misconception: Poles can be isolated.
  • Misconception: Flux is the same as field lines.
  • Misconception: Flux is actually the flow of the magnetic field.
  • Misconception: Magnetic fields are the same as electric fields.
  • Misconception: Charges at rest can experience magnetic forces.
  • Misconception: Magnetic fields from magnets are not caused by moving charges.
  • Misconception: Magnetic fields are not 3-dimensional.
  • Misconception: Magnetic field lines hold you on the Earth.
  • Misconception: Charges, when released, will move toward the poles of a magnet.
  • Misconception: Magnetic field lines start at one pole and end at the other.

Induction

  • Misconception: Generating electricity requires no work.
  • Misconception: When generating electricity only the magnet can move.
  • Misconception: Voltage can only be induced in a closed circuit.
  • Misconception: Magnetic flux, rather than change of magnetic flux, causes an induced emf.
  • Misconception: All electric fields must start on (+) and end on (-) charges.
  • Misconception: Water in dams causes electricity.

To Be Continued . . .