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MAGNETISM. Lodestones. Natural Magnets Magnetite, Fe 3 O 4 (an oxide of iron) Ancient civilizations (Greek 590 BCE, Chinese 2600 BCE) realized that these stones would cling to iron tools. A suspended, pivoting lodestone always pointed along the North-South axis. Lodestones.

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  • Natural Magnets
  • Magnetite, Fe3O4 (an oxide of iron)
  • Ancient civilizations (Greek 590 BCE, Chinese 2600 BCE) realized that these stones would cling to iron tools.
  • A suspended, pivoting lodestone always pointed along the North-South axis
  • Magnetite crystals have been found in living organisms
    • Magnetotactic bacteria!
    • Migratory Bird brains!!
    • Other migratory animals: bees, fish
    • Human brains!!!
permanent magnets
Permanent Magnets
  • By 2nd Century AD, Chinese were able to make permanent magnets by repeatedly stroking an iron rod or needle from end to end along a lodestone, but always in the same direction.
  • Retained strength of a magnet depends on chemical properties of the metal.
    • Soft iron: loses magnetism quickly
    • Low-carbon soft steel (paper clips, nails): gradual loss
    • Hard steel: retains power for a long time and is referred to as a “permanent magnet”
magnetic poles
Magnetic Poles
  • Magnets produce a force on other objects
  • Poles are regions where the magnetic force is the strongest
  • Like magnetic poles repel.
  • Opposite magnetic poles attract.
  • Most magnets have two poles (dipole), but can have three or more!
monopole no not monopoly
Monopole? (No, not Monopoly!)
  • Monopole: piece of a magnet that is simply a north pole or a south pole
  • Many have tried to isolate a monopole by breaking magnets in half.
  • No matter how we break a magnet, the pieces are always dipoles!
  • A monopole cannot be isolated.
  • Do not pass GO.Do not collect $200.
magnetic field
Magnetic Field
  • Every magnet establishes in the space surrounding it, a magnetic field (B-field)
  • Map field with a test-compass
  • Direction of field is direction in which the test-compass needle will point at that location.
  • Draw field lines so that compass always points tangent to the field lines.
  • Field lines point from N to S outside the magnet
  • Field lines point from S to N inside the magnet
  • Field lines form closed loops
  • Field lines never intersect
  • SI unit for B (magnetic field strength) is the tesla (T)
magnetic field1
Magnetic Field

Mapping with Test-Compass

Field Lines Form Closed Loops

Field Mapped by Iron Filings

earth s magnetism
Earth’s Magnetism
  • Magnetic field has reversed direction ~300 times in the past 170 million years
  • Magnetic poles wander!
  • Magnetic & geographic poles not the same.
  • Magnetic declination: 11.5°
  • What’s strange about this picture? 
magnetic domains
Magnetic Domains
  • Domain: region where many atomic dipoles are aligned
  • Usually aligned randomly and effects cancel
  • BUT…
    • Place ferromagnetic material in strong B-field
    • Entire domains realign with applied field
    • Size & shape of domains remains the same
    • Causes irreversible re-orientation of domains
    • Creates permanent magnets
reorientation of domains
Reorientation of Domains

Electrons in domains align with applied field

Substance is Permanently Magnetized

Domains are not aligned

diagramming 3 d magnetic fields
Diagramming 3-D Magnetic Fields
  • Not everybody is an artist.
  • Use 2-D images to draw 3-D field vectors.
  • If field points perpendicularly into the page or board, use
  • If field points perpendicularly out of the page or board, use
  • Otherwise, draw the lines neatly.
  • Don’t forget, field lines are vectors!


electrodynamics the study of electromagnetism
Electrodynamics: The Study of Electromagnetism
  • Magnetism is caused by charge in motion.
    • Charges at rest have just an electric field
    • But, when they move, they generate both an electric field and a magnetic field
    • Can look at individual charges or electric current in a wire
  • Direction of current determines direction of the magnetic field.
  • Use right hand rules for analysis.

First Right Hand Rule: thumb points in direction of current, fingers curl in direction of magnetic field- note compass readings. Use for current-carrying wire.

Fig 19.15b, p.678

Slide 16

magnetic field of a long straight wire
B: magnetic field strength (teslas)

I: current (amperes)

r: radius from wire (meters)

μo: permeability constant in a vacuum

μo = 4π x 10-7 T·m/A

What is the shape of this magnetic field?

Magnetic field of a long straight wire

1stRight Hand Rule- Thumb points in the direction of the current, fingers curl in the direction of the created magnetic field – up through the coils and around the outside. Use for current-carrying loop or solenoid coil.

Fig 19.20b, p.682

Slide 20

magnetic field of loops of wire or a coil carrying current
How is this equation different from the mag field of a straight wire?

The strength of the field is more in a loop than in the straight wire and a single loop.

Magnetic field of loops of wire (or a coil) carrying current

where n is the NUMBER of loops (in this example n=8)

magnetism on an atomic level
Magnetism on an Atomic Level
  • Charge in motion (electric current) produces magnetic force
  • Electrons function as a subatomic dipole
    • Electron “spin” (Much More)
      • Electrons existing in pairs: B-fields cancel
    • Electron “orbit” around nucleus (Very Little)
      • Random “orbits” of electrons: B-fields cancel
  • Even “non magnetic” materials respond to an applied B-field
    • Applied B-field changes orbital motion of electrons
    • Produces a field that opposes applied field
    • Repelled by applied field
  • Diamagnetic materials have no permanent atomic dipoles
  • Occurs for all substances, but may be swamped by other magnetic effects
  • Paramagnetic materials are attracted when placed in a strong B-field.
  • Composed of atoms with permanent atomic dipoles
    • Atomic dipoles do not interact w/ one another
    • Atomic dipoles oriented randomly
    • Material has no dipole as a whole
  • A strong B-field re-orients these atomic dipoles in same direction as applied field
  • Naturally “magnetic”: magnetite, iron, nickel, cobalt, steel, Alnico, other alloys
    • Strongly attracted to poles of a magnet
    • Easily magnetized
  • Atomic dipoles interact strongly with dipoles of adjacent atoms
  • Dipoles align spontaneously, w/o an applied field
  • Many atomic dipoles cooperatively align
  • Creates regions of parallel orientations (domains)
2 nd right hand rule
2ND Right Hand Rule
  • Gives the direction of the FORCE exerted on a current (or charge) by an external magnetic field
  • Point thumb of RH in direction of current (or motion of positive charge)
  • Point fingers through in direction of magnetic field
  • Palm pushes in direction of force
2 nd right hand rule1
2ND Right Hand Rule

Fingers point to B, the direction of magnetic field lines.

Thumb points to v, which is direction of velocity of positive charge

Deflecting force is shown by direction of palm pushing.

magnetic force on a moving charge
Magnetic Force on a Moving Charge

F = qvB·sin Θ

B: field strength in teslas (T)

q: charge in coulombs (C)

v: charge velocity in m/s

Θ: angle between v & B

magnetic force on a current carrying wire
Magnetic Force on a Current-Carrying Wire

F = B·I·L

B: field strength in teslas (T)

I: current in amperes (A)

L: length of current-carrying wire in meters (m)