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MAGNETISM
Adapted from Mr. Dellibovi
Magnets- History and
Interesting Facts
• Lodestones
– 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!!!
YOU HAVE ROCKS IN YOUR HEAD!!!!!
History of Permanent Magnets
• By 2nd Century AD, Chinese were able to make
permanent magnets by repeatedly
______________________________________
______________________________________
______________________________________.
• Retained strength of a magnet depends on
___________________________________.
– ________: loses magnetism quickly
– _________________ (paper clips, nails): gradual loss
– ________: retains power for a long time and is
referred to as a “permanent magnet”
Magnetic Poles
• Magnets produce a force on other objects
• Poles are regions where the magnetic force is the
strongest
• Like magnetic poles _______________.
• Opposite magnetic poles _____________.
• Most magnets have ______ poles (dipole), but can have
three or more!
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!
• ____________________
____________________
• Do not pass GO.
Do not collect $200.
Magnetic Field
• Every magnet establishes in the space surrounding it, a
magnetic field (B-field)
• Map field with a _____________________
• Direction of field is direction in which the test-compass
needle will point at that location.
• Draw field lines so that compass always points _______
to the field lines.
• Field lines point from N to S __________ the magnet
• Field lines point from S to N __________ the magnet
• Field lines form closed loops
• Field lines never _____________________
• SI unit for B (magnetic field strength) is the tesla (T)
Magnetic Field
Mapping with
Test-Compass
Field Lines Form
Closed Loops
Field Mapped by
Iron Filings
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?
____________________
____________________
____________________
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!
Magnetism on an Atomic Level
• Charge ________________ (or electric
__________) produces magnetic force
• Electrons function as a subatomic dipole
– Electron “spin”
• Electrons existing in pairs: B-fields cancel
– Electron “orbit” around nucleus
• Random “orbits” of electrons: B-fields cancel
Diamagnetism
• Even “non magnetic” materials respond to an
applied B-field
– Applied B-field changes orbital motion of electrons
– Produces a field that _____________ applied field
– ______________ by applied field
• Diamagnetic materials have no _____________
atomic dipoles
• Occurs for ______ substances, but may be
swamped by other magnetic effects
Paramagnetism
• Paramagnetic materials are ___________
when placed in a strong B-field.
• Composed of atoms with ____________
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 _____________ as applied field
Ferromagnetism
• Naturally “magnetic”: magnetite, iron, nickel,
cobalt, steel, Alnico, other alloys
– Strongly attracted to poles of a magnet
– Easily magnetized
• Atomic dipoles ____________ with dipoles of
adjacent atoms
• Dipoles align spontaneously, w/o an applied field
• Many atomic dipoles cooperatively align
• Creates regions of __________ orientations
(_____________)
Magnetic Domains
• Domain: region where many atomic dipoles _________
• 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
Domains are not
aligned
Electrons in
domains align
with applied field
Substance is
Permanently
Magnetized
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
_________, fingers curl in direction of ____________ .
Note compass readings. Use for: ___________________
Magnetic field of a long straight wire
o I
B
2r
•
•
•
•
•
B: magnetic field strength (teslas)
I: current (amperes)
r: radius from wire
μo: ____________________________________
μo =
• The shape of this magnetic field is: _________________
_____________________________________________
1st Right Hand RuleThumb 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
_________________
_________________
______________
Fig 19.20b, p.682
Slide 19
Magnetic field of loops of wire (or a
coil) carrying current
n o I
B
2r
where n is the NUMBER of loops (in this example n=8)
•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.
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 ___________
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.
2ND Right Hand Rule
Magnetic Force on a Moving Charge
F = Bqv·sin Θ
B: field strength in _____________
q: charge in _________________
v: charge velocity in ____________
Θ: angle between __________
Magnetic Force on a CurrentCarrying Wire
F = B·I·L
B: field strength in _______________
I: current in ______________
L: length of current-carrying wire in __________