Magnetism
Magnets
• Magnets create magnetic forces
– Unlike poles attract; like poles repel
• Electric charges can be separated, but
magnetic cannot be isolated
– Always a dipole, no magnetic monopoles
found
• Unmagnetized materials can be made
magnetic
– Stroke with permanent magnet
– Place for extended period in strong magnetic
field
– Reverse process through hammering or
heating and cooling
• Magnetic materials classified as
magnetically hard or soft
– Soft magnetic materials are easily
magnetized, but lose magnetism easily
• Iron, iron-silicon alloys, iron-nickel alloys
– Hard magnetic materials difficult to
magnetize, but retain magnetism
• Cobalt, nickel
Magnetic Fields
• Magnetic fields (B) extend beyond the
physical body of magnetic materials
• Direction of magnetic field at any location
is defined as the direction in which the
north pole of a compass needle points at
that location
• Magnitude greatest at magnetic poles
– Greatest density of magnetic field lines
• Direction of magnetic field lines
designated by lines, dots or crosses
– Line: field direction along plane of page
– Dot: field direction out of the page
– Cross: field direction into the page
• Poles of magnetic named according to the
pole of Earth’s magnetic field to which they
point
South
Earth’s Magnetic Field
– North pole of magnet points to Earth’s
geographic north pole or magnetic south pole
– South pole of magnet points to Earth’s
geographic south pole or magnetic north pole
• “True” north defined as the geographic
North Pole
– North indicated by a compass varies by
location on Earth
• Magnetic declination
• Earth’s field resembles field produced by
bar magnet
– 3-dimensional field
– Compass points horizontally only at Earth’s
equator
• If free to rotate vertically, compass needle would
point increasingly downward as move towards the
North Pole
• Source of Earth’s magnetic field not linked
to iron deposits
– Molten state prevents prolonged alignment of
magnetic domains
• Earth’s magnetic field likely caused by
moving charges in convection currents at
Earth’s core
– Charged ions and electrons
– Field strength may also relate to rotation rate
• Naturally-occurring magnetic materials
exposed to Earth’s magnetic field for very
long time
– Similar to placing non-magnetic material in
strong magnetic field
• Geologic time scale compensates for weaker
exposure
Electromagnetism
• Current-carrying conductors generate
magnetic fields
• Field orientation is in concentric circles
around wire
– Reverse current direction, reverse direction of
field circles
• Use right-hand rule to establish direction
of field in a current-carrying wire
• Magnetic field strength is proportional to
current wire and inversely proportional to
distance from wire
• If wire is bent into a loop, field lines
resemble those of a bar magnet
– Right-hand rule demonstrates that direction of
current in any part of loop is the same
• Collection of closely-spaced loops called a
solenoid
– Magnetic field increases with increasing
current and number of loops per unit length
– Further increase magnetic field by inserting
iron core into center of solenoid
• Electromagnet
• Magnetic field induced in rod adds to magnetic
field of solenoid
• Field inside solenoid is strong and nearly
uniform
– Field lines point in single direction and almost
parallel to each other
• Field outside solenoid is non-uniform and
much weaker
– Again, overall field pattern resembles that of
bar magnet
Magnetic Domains
• Spinning electrons represent charges in
motion that produce magnetic fields
– In most substances, electrons pair so that
spins cancel and net magnetic field does not
form
– In some materials, spins do not completely
cancel
• Ferromagnetic
• Ferromagnetic substances show strong
coupling between neighboring atoms
– Form large groups of atoms whose net spins
are aligned
• Magnetic domains
• Magnetic domains randomly oriented in
nonmagnetic substances
• When exposed to magnetic field, domains
may align with field or domains already
aligned may expand at expense of others
– Hard magnets: domains remain aligned when
external field is removed
– Soft magnets: random alignment of domains
is reestablished
Magnetic Force
• Stationary charges do not interact with
constant magnetic fields
– Charges moving through field experience
magnetic force
– Force maximum when charge moves
perpendicularly to field
– Force is zero when charge moves parallel
along field lines
• Properties of magnetic field described in
terms of the magnetic force exerted on a
test charge
– Test charge assumed to be positive
• Force strength depends on charge, charge
velocity and strength of magnetic field
– Fm = qvB or B = Fm/qv
– If particle moves at angle through field,
formula becomes Fm = qvBsinθ
• SI Unit for magnetic field strength: Tesla
(T)
– Usually, magnetic fields are much smaller
than 1 T
– Earth’s magnetic field near surface: 50 μT
• Direction of magnetic force is always
perpendicular to both direction of charge
motion and direction of magnetic field
– Use variation of right-hand rule
• Charges moving through uniform magnetic
fields follow a circular path
– Right-hand rule shows that, at any point,
magnetic force directed towards center of
circle
– Magnetic force acts like centripetal force and
maintains circular motion
• Changes direction of charge velocity, but not the
magnitude of the velocity
Current-Carrying Wires
• Current-carrying wires experience a force
when placed in magnetic field
– Force on wire is due to the sum of the
individual forces on the charged particles
moving through the wire
• Magnitude of force: Fm = BIl
– Equation only valid when current and
magnetic field are perpendicular
• Right-hand rule serves, again, to
determine direction of magnetic force
– Thumb in direction of current, rather than
velocity
• When current is parallel or anti-parallel to
magnetic field, magnetic force on wire is
zero
• Parallel wires exert magnetic force on
each other
– Fm = BIl can be used to determine
magnitude of force, where B is
magnitude of field generated by 2nd wire
– When current runs in same direction in both
wires: force is attracting
– When current runs opposite in wires: force is
repelling
Galvanometers
• Device used in the construction of
ammeters and voltmeters
• Operates based on torque that acts on a
current loop in the presence of magnetic
field
– Torque proportional to current in coil
– Reflected in amount of needle deflection