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Magnetism Notes
CP Physics
Ms. Morrison
General Information
► Greeks
discovered magnetic rocks called
lodestones more than 2000 years ago
► Chinese used lodestones to help navigate
ships
► Lodestones contain iron ore (magnetite)
Magnetic Poles
► Regions
of the magnet that produce
magnetic force
► Pole pointing northward – North
► Pole pointing southward – South
► Breaking a magnet in half will not separate
the poles – only makes two smaller magnets
Magnetic Poles vs. Electric Charges
Magnetic
Poles
Electric
Charges
likes repel,
opposites attract
√
√
force acts over a
distance
√
√
can separate
charges (poles)
from each other
NO
√
Property
Magnetic Fields
► Space
around a magnet through which
magnetic force is exerted, can be seen with
iron filings (shows magnetic field lines)
► Magnetic field lines move from north to
south pole
► More lines = stronger field
► Field strongest at poles
Magnetic Fields, pg 2
► Moving
charges create magnetic fields, ex.
Spinning electrons
► Electrons = tiny magnets
 Pair spin in same direction = stronger magnet
 Pair spin in opposite direction = no magnet
because their magnetic fields cancel out
Magnetic Fields, pg 3
► Most
materials NOT magnetic because
electron pairs cancel out their magnetic
fields
► Iron, nickel, and cobalt – have atoms with
unpaired electrons whose magnetic fields
are not entirely canceled out – so have
magnetic properties
► Iron has four unpaired electrons so each
iron atom is a tiny magnet
Magnetic Domains
► Large
clusters of aligned atoms – their
magnetic fields are so strong that they line
up with each other
► Magnets have domains that are lined up
while ordinary iron and other materials do
NOT have aligned domains
Magnetic Domains, pg 2
► Can
create permanent iron magnets by
placing the iron in a strong magnetic field
► Can create temporary magnets by placing
materials with iron in them near a strong
magnetic field, once field gone domains will
revert to original state
► Can destroy permanent magnets by
dropping them or heating them up – causes
domains to be jostled out of alignment
Magnetic Domains, pg 3
Electric Current and Magnetic Fields
► Moving
charges produce magnetic fields
► Current through a wire creates a magnetic
field around the wire – Oersted (1820)
► Reversing current flow reverses direction of
the magnetic field
Electromagnets
►A
current-carrying coil of wire with many
loops
► Has a north pole and a south pole
► Strength of electromagnetic affected by
three factors
 # of loops in the coil (more loops = stronger)
 Amount of current (larger current = stronger)
 Presence of iron core (present = stronger)
Magnetic Forces and Charged
Particles
► Charged
particles at rest are not affected by
magnetic field (stationary)
► When moving, charged particles are
deflected by magnetic fields
► Used in TV tubes to create picture on screen
► Earth’s magnetic field deflects charged
particles from outer space (cosmic rays)
Magnetic Forces and CurrentCarrying Wires
► Current-carrying
wire placed in magnetic
field is deflected
► Reversing current cause deflection in
opposite direction
► Greatest deflection occurs when current is
perpendicular to magnetic field
Meters
► Meters
use magnetic needle
 Current moving through wires in meter creates
magnetic field that interacts with magnetic field
of needle
 Cause needle to be deflected (like repel, etc.)
 Amount of deflection indicates amount of
current
Motors
► Motors
 Electric current through coil of wire inside a
permanent magnet
 Coil of wire creates magnetic field that interacts
with magnet and begins spinning
 Current must reverse every half turn so that
two magnetic fields keep repelling each other
 So electrical energy is converted into
mechanical energy
Earth’s Magnetic Field
► Earth
is huge magnet (magnetic poles not
same as geographic poles)
► Thought to result from moving charges
within molten part of Earth beneath crust
► NOT stable – wanders, diminishes to zero
and then reverses itself
► Holds Van Allen Radiation belts around
Earth (aurora borealis)