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Magnetism. Force of Mystery demo. Magnetism Standards. Students know magnetic materials and electric currents (moving electric charges) are sources of magnetic fields and are subject to forces arising from the magnetic fields of other sources. (Ch 36)

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Force of Mystery


magnetism standards
Magnetism Standards
  • Students know magnetic materials and electric currents (moving electric charges) are sources of magnetic fields and are subject to forces arising from the magnetic fields of other sources. (Ch 36)
  • Students know how to determine the direction of a magnetic field produced by a current flowing in a straight wire or in a coil. (Ch 36)
  • Students know changing magnetic fields produce electric fields, thereby inducing currents in nearby conductors. (Ch 37)
famous 19th century quote
Famous 19th Century Quote
  • “The nation that controls magnetism controls the Earth”
magnetic poles
Magnetic Poles
  • North and South
  • Like poles repel N-N S-S
  • Unlike poles attract N-S
magnetic poles are not charges
Magnetic Poles Are Not Charges
  • Single poles cannot be isolated
  • Magnetic Monopoles do not exist in nature
  • Break a magnet:
  • Get two smaller ones






ferromagnetic materials
Ferromagnetic Materials
  • Show strong magnetic effects
  • Iron
  • Cobalt
  • Nickel
  • Gadolinium
  • Neodymium
permanent magnets
Permanent Magnets

Hi tech Neodymium iron boron magnets

magnetic field
Magnetic Field
  • Earth has field
  • Lines go from North to South
units of magnetic field b
Units of Magnetic Field B
  • Tesla (SI Unit)
  • Gauss (cgs unit)
  • 1 Tesla = 104 Gauss
  • Earth magnetic field about 0.5 gauss
direction of magnetic field
Direction of Magnetic Field

The direction the north pole of a compass would point when placed at that location

  • Magnet made of domains
  • 1 mm length
  • Each acts like tiny magnet
  • Normally domain cancel
  • External field aligns domains
  • Strong magnet can make other ferromagnetic materials into permanent magnets
electrons have spin
Electrons Have Spin
  • Even permanent magnets owe strength to “currents”
  • No way to divide a current and get N or S pole
  • Magnetism is electrical in origin
earth s magnetic field
Earth’s Magnetic Field
  • Very weak
  • Like bar magnet
  • North magnetic pole
  • South magnetic pole
electric currents produce magnetism
Electric Currents Produce Magnetism
  • Magnetic field around long straight wire

Right hand rule determines direction of magnetic field


right hand rule s
Right Hand Rule(s)
  • Long Straight Wire (Rule #1)
    • Point thumb in direction of current
    • Fingers wrapped around wire point in direction of magnetic field
  • Circular loop of Wire (Rule #2)
    • Curl fingers around wire with tips in field direction
    • Thumb points in direction of current
alternate preferred version of second rhr
Alternate (preferred) version of Second RHR
  • Put curled fingers in current direction around loop or loops; thumb points in field direction INSIDE loop or coil.
force on current carrying wire
Force on Current Carrying Wire

F = BIL sinQ

  • is angle between

field and wire



Force is perpendicular to both current and field direction

third right hand rule
Third Right Hand Rule
  • Long straight fingers in (positive) current direction (or direction of moving charged particle). Curled fingers in magnetic field direction, thumb points in direction of force on current carrying wire or positive charged particle
  • If particle is negative, change answer
force on moving charged particle in uniform magnetic field
Force on Moving Charged Particle in Uniform Magnetic Field
  • F = Bqvsinq
  • This force is perpendicular to the magnetic field and particle velocity vector
charge particle path in uniform magnetic field
Charge Particle Path in Uniform Magnetic Field
  • Circle or helix
  • F = ma
  • qvB = mv2/r (centripetal acceleration)
  • r = mv/qB
  • Direction follows right hand rule
how can f bil sin q be used to measure a field
How can F = BIL sinQ be Used to measurea Field?
  • Hint: use a rectangular loop of wire
force on a charged particle in a magnetic field
Force on a Charged particle in a Magnetic Field
  • Demo
  • F = qvB sinQ
  • Force perpendicular to both particle direction and field
magnetic field due to straight wire
Magnetic Field Due to Straight Wire
  • B = m0I/2pr
  • F = BIL
  • m0 permeability of free space
  • 4 p x 10-7


force between parallel wires
Force Between Parallel Wires
  • F/l = (mo/2p) I1I2/L
  • Force per unit length of wire
  • L is distance between wires
  • Parallel currents attract
  • Antiparallel currents repel
electrical vs magnetic forces
Electrical vs. Magnetic Forces
  • Similarities
    • Both involve attracting and repelling
    • Both decrease with distance
  • Differences
    • Isolated poles do not exist
    • Only electrical forces can be produced by stationary charges
    • Only moving charged particles experience magnetic force
    • Only electrical forces can do work
      • Magnetic forces on charged particles are perpendicular to field direction but electrical forces are in or opposite to electric field direction