Magnetism
Magnets
 Naturally occurring magnetic stones,
lodestones, known for about 3000 years
 Composed of iron-based mineral called
magnetite
 Lodestones: permanent magnets
 Magnetic all the time
 Some materials may be turned into
permanent magnets
 Rub several times with permanent magnet
 Place near strong magnet
 Permanent magnets can lose their
magnetism
 Strike with hammer
 Apply heat
 Some materials hold magnetism better
than others
 Classify substances as magnetically hard
or magnetically soft
 Hard: Hard to magnetize, but also hard to
demagnetize (Co, Ni)
 Soft: Easy to magnetize, but also easy to
demagnetize (Fe)
Magnetic Force
 Magnets exert forces on each other
 Attracting (opposite poles)
 Repelling (like poles)
 Impossible to isolate magnetic poles
 Can change motion of magnetic objects
 Non-magnetic objects not affected
Magnetic Field
 Magnetic force product of magnetic
fields
 Field force
 Magnetic field: extension of magnetic
property beyond body of magnet
 North and south poles both generate fields
 Magnetic field strength differs for
different substances and degree to
which substance has been magnetized
 Magnetic field lines used to represent
magnetic fields
 Magnetic field strongest closest to
poles
 More field lines = stronger field
 Field lines point from magnet’s north
pole to magnet’s south pole in closed
loop
 Compasses indicate directions of
magnetic fields
 Compass: magnet suspended on top of a
pivot so that magnet can rotate freely
 Compasses also align with Earth’s
magnetic field
 Compass points in direction tangent to, or
along direction of, field line at that point
 Earth’s magnetic field is like that of bar
magnet
 Field has both direction and magnitude
(strength)
 Magnetic poles are in northeastern Canada
and Antarctica
 Compass would point downwards at these
locations
South
 Source of Earth’s magnetic field is debated
 Iron core is molten: wouldn’t hold magnetic
properties
 Could be movement of ions and electrons in
liquid layer of core
 Electric charges associated with magnetic fields
 Could be combination of several factors
 Earth’s magnetic field has changed
directions throughout geologic time
 Over 20 reversals in last 5 million years
 Earth’s magnetosphere not symmetrical
 Solar wind balanced by Earth’s magnetic
field at magnetopause
 Magnetic poles not the same as
geographic poles
 Magnetic field lines run from geographic
south pole to geographic north pole
 Geographic south pole is a magnetic north pole
 Geographic north pole is a magnetic south pole
South
 Historically, poles of magnets named for
geographic pole to which they point
 Magnet points north with “north-seeking” pole
 South pole of magnet
 Magnet points south with “south-seeking” pole
 North pole of magnet
Magnetism from Electric Currents
 Moving electric charges produce
magnetic fields
 Compasses will indicate field lines
associated with current in a wire
 Use right-hand rule to predict direction
of induced magnetic field in straight
current-carrying wire
 Imagine gripping wire with right hand
 Grip with thumb pointing in direction of
current
 Fingers curl in direction of magnetic field
 Magnitude of magnetic field from currentcarrying wire is small
 Increase magnetic field by increasing current,
but method is hazardous
 Increase magnetic field by wrapping wire into a
coil - solenoid
 Magnetic field of coil of wire resembles
that of a bar magnet
 In solenoid, magnetic field of one coil of
wire adds to field of adjacent coil
 Field resembles that of bar magnet
 Solenoid has north and south poles
 Strength of field of a solenoid depends
on number of wire loops and magnitude
of current
 Solenoid magnetic field strength can be
increased further by inserting iron or other
potentially-magnetic metal rod in center of
wire coils
 Called an electromagnetic
 Magnetic field of solenoid magnetizes center
rod
 Magnetic field of rod adds to magnetic field of
solenoid
 All magnetism created by moving charges
 In magnets: movement of subatomic particles
 Electrons around cloud + electron spin
 Protons in nucleus
 Normally, magnetic fields cancel out and leave
substance nonmagnetic
 In some substances, not all fields cancel
 Each atom has its own magnetic field
 Magnetic atoms rotate to align with
magnetic fields of nearby atoms
 Alignment produces small regions in
material called domains
 Alignment and disorganization of domains
magnetizes and demagnetizes substance
Electromagnetic Devices
 Many modern devices make use of
magnetic fields produced by currentcarrying wires
 Hair dryers, speakers
 Galvanometers detect current or
movement of charges in a circuit
 Part of ammeters and voltmeters
 Galvanometers consist of coil of
insulated wire wrapped around iron
core that can rotate between poles of
permanent magnet
 Attach to circuit and current flows through
wire
 Wire and iron core act as electromagnetic
 Electromagnet produces magnetic field that
interacts with magnetic field of surrounding
permanent magnet
 Resulting forces turn the core and deflects
needle on gauge
 Magnitude of current determines
magnitude of force
 Large deflection: high current
 Small deflection: low current
 Electric motors use magnetic force to
cause motion
 Electric energy converted to mechanical
energy
 Coil of wire spins when conducting
current
 Unlike galvanometer, wire continues to spin
 Coil attached to shaft: able to do work
 Shaft often attached to propeller or wheel
 Motors have component called
commutator made of 2 half rings of
metal
 Makes current change direction every time
flat coil makes half a revolution
 Brushes connect wires to commutator
 Because of slits in commutator, charges
must move through the coil of wire to
reach opposite half of ring
 As coil and commutator spin, current in coil
changes direction every time brushes come
in contact with different side of ring
 Magnetic field of coil changes direction
as coil spins
 Ensures that coil is repelled by both north
and south poles of surrounding magnet and
keeps rotating in single direction
 Stereo speakers consist of permanent
magnet, coil of wire and flexible cone
 Forces produce alternating back and forth
motion of speaker cone
 Varying magnitude of current changes how
much cone vibrates
 Vibrations produce longitudinal sound
waves
Electric Currents from Magnetism
 Current can be produced by pushing a
magnet through a coil of wire
 Moving a magnet in and out of a coil of wire
causes charges to move in the wire
 Process called electromagnetic induction
 Electromagnetic induction also called
Faraday’s Law
 Induction first described by Michael Faraday
in 1831
 Faraday’s Law: an electric current can
be produced in a circuit by a changing
magnetic field
 A current is induced in a loop of wire
moving in and out of a magnetic field
 As long as wire is moving, current flows
in circuit
 Maximum current when motion of wire is
perpendicular to magnetic field
 No current is produced if motion is parallel
to magnetic field
 Rotating the wire or changing the
strength of the magnetic field will also
induce current
 Any change of the magnetic field will induce
current
 Current will be induced as number of
magnetic field lines passing through wire
loop changes
 Magnetic flux
 Electromagnetic induction does not
create energy from nothing
 No violation of law of conservation of
energy
 Work must be done to move loop through
magnetic field
 Stronger field: more work needed to move loop
 Mechanical work converted to electric
energy
 Moving electric charges experience
magnetic force when in a magnetic field
 Force is zero when movement is parallel to
magnetic field
 Force is maximum when movement is
perpendicular to magnetic field
 Force varies with angle of particle motion in
magnetic field
Generators
 Convert mechanical energy to electric
energy
 Do work to turn a coil of wire in a
magnetic field
 For each half-rotation of loop, current
produced reverses direction
 AC generator
 Alternating current (AC): reverses
direction at specific frequencies
 Common household power
 Batteries: direct current (DC)
 Current flows in only one direction
 Amount of current produced by
generator related to orientation of loop
in magnetic field
 Perpendicular to magnetic field: no
current
 Parallel to magnetic field: maximum
current
 Current varies with angle
Household Power
 Power plants use generators to produce
electricity
 Sources of energy for generator vary:
water, coal, oil, wind, solar, geothermal,
nuclear
 Some mechanical energy lost as heat
and available electricity reduced due to
friction in power lines
Electromagnetic Force
 Moving charges produce magnetic field
and magnetic flux produces electric
field
 Forces are aspects of electromagnetic force
and the energy produced is electromagnetic
energy
 Electromagnetic waves are examples of
electromagnetic energy
 EM waves made of oscillating electric
and magnetic fields
 Fields oscillate perpendicularly to direction
energy travels
 Transverse waves
 As wave moves, changing electric field
generates magnetic field and changing
magnetic field generates electric field
 Each field regenerates the other,
allowing wave to travel through empty
space
Transformers
 Devices that increase or decrease the
voltage of alternating current
 Simple transformer is two wires coiled
around opposite sides of a closed iron
loop
 One wire attached to AC source and one
wire attached to appliance
 When there is current in the primary
circuit (connected to AC source), a
changing magnetic field is generated,
which magnetizes the iron loop
 Changing magnetic field in iron loop
generates electric current in secondary
circuit
 Direction of current in secondary circuit
changes every time the direction of the
current in the primary circuit changes
 Voltage induced in secondary circuit
depends on the number of loops or
turns in the circuit
 Number of loops in primary and secondary
circuits equal: voltage in circuits is equal
 Step-up transformer: number of coils in
secondary circuit greater than in primary
circuit and voltage is increased in secondary
circuit
 Step-down transformer: number of coils in
primary circuit is greater than in secondary
circuit and voltage is decreased in
secondary circuit