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Magnetism
Historical
• Early magnets called lodestones, naturally
occurring iron ore magnetite
• Named magnets by Greeks since they were
found in the region Magnesia
• Important for early navigation
• Coulomb showed magnetic force has
similar relationship to electric and
gravitational forces
Historical
• Oersted showed electric current and
magnetism related in 1820
• He found electric current created magnetic
field which would deflect a compass needle
Magnetic Poles
• Like electric charges, there are two types
• Called north and south because of the way
they react to the earth’s magnetic field
• Unlike electrical charges, can’t have
isolated N or S pole; always have both
• like electrical charges, opposite poles
attract, like poles repel
Magnetic Poles
• Forces between poles are directly
proportional to strength of the poles,
inversely proportional to the square of
distance between them.
• Magnets can be large: a planet, or small: a
spinning electron
Magnetic Fields
• Magnetic forces act over a distance by
means of a field, like electric and
gravitational forces
• Magnetic field is represented by lines of
force like electric field
• Field lines can be seen by using bits of iron
sprinkled on paper placed near magnet
Magnetic Fields
• Direction of field is direction of force on N
pole placed in field
• Magnetic field lines shown as arrows going
out of N poles, into S poles
Cause of Magnetism
• Magnetic fields created by moving
electrical charges
• In common magnets, moving charges are
spinning electrons
• Movement of charges in wires (electric
current) creates electromagnets
Ferromagnetism
• In most materials, magnetic effects due to
motion of electrons cancel out
• Some materials have several spinning
electrons without an opposite spinning
partner
• Called ferromagnetic from the Latin word
for iron
• Fe, Co, Ni, Dy, Pa, Nd
Magnetic Domains
• Since each atom of ferromagnetic material
is magnet, groups of atoms will line up
together -- called domains
• In unmagnetized iron, domains are
randomly oriented
• When magnet is brought near, domains
become aligned together, iron becomes
magnetized
Magnetic Domains
• Domains can be rearranged and magnetism
lost by heating or vibration
Electric Current and Magnetism
• For straight wire with current, magnetic
field lines form concentric circles around
wire
• Changing direction of current changes
direction of magnetic field
• If wire is bent into loop, magnetic field is
concentrated inside loop
Electric Current and Magnetism
• If more loops are added, magnetic field
strength increases with each loop in coil
• Long coil called a solenoid
• If iron rod is placed inside solenoid,
magnetic field is concentrated inside iron
and electromagnet results
Magnetic Field of Loop
Magnetism and Moving Charges
• Charged particle moving in magnetic field
will experience deflecting force due to
interaction of two magnetic fields
• Force is greatest when motion is
perpendicular to external magnetic field,
zero when parallel
• Direction of force is perpendicular to both
velocity and external magnetic field
Magnetism and Moving Charges
• Electrons in TV sets are aimed to screen
using magnetic fields
• Earth’s magnetic field deflects incoming
charged particles from sun and outer space
• Current carrying wires in magnetic fields
experience similar sideways force because
of charges moving in wires
Magnetism and Moving Charges
• Two nearby current carrying wires will
exert forces on each other because each
creates a magnetic field
• So current carrying wire will deflect
compass and will be deflected by strong
external magnetic field
Meters and Motors
• Analog (not digital) meters use coil of wire
with magnets on each side to measure
current: galvanometer, ammeter, or
voltmeter
• Current through coil creates magnetic field;
coil will try to line up with field of magnets
• If coil is allowed to rotate, motor can be
made
Galvanometer
Meters and Motors
• As coil becomes aligned with magnets,
current direction changes through
commutator and brushes
• In large motors, permanent magnets replace
with electromagnets
• Armature is rotating coil in motor
Earth’s Magnetic Field
• Thought to be caused by convection
currents in liquid interior of planet
• Geologic evidence shows magnetic field has
moved around and reversed polarity
• Magnetic poles don’t line up with
geographic poles
Earth’s Magnetic Field
• Difference between magnetic north and true
north called magnetic declination
• Since north poles of magnets and
compasses point to north, Earth’s north
magnetic pole is really a south type pole;
south pole is really north type pole
North Magnetic Pole