1 Magnetism The term magnetism comes from the name Magnesia, a
coastal district of ancient Thessaly, Greece. Unusual stones were
found by the Greeks more than 2000 years ago. These stones, called
lodestones, had the intriguing property of attracting pieces of
iron. Magnets were first fashioned into compasses and used for
navigation by the Chinese in the 12th century. Slide 2 2 Magnetic
Forces The force (F e ) between any two charged particles depends
on the magnitude of the charge (q 1 *q 2 ) on each and their
distance of separation (r 2 ), as specified in Coulombs law. F e =
k e *q 1 q 2 /r 2 But Coulombs law is not the whole story! When the
charged particles are moving with respect to each other, the
electrical force between electrically charged particles depends
also, in a complicated way, on their motion. There is a force due
to the motion of the charged particles that we call the magnetic
force. Slide 3 Magnetic Poles Magnetic force between a pair of
magnets Force of attraction or repulsion between a pair of magnets
depends on which end of the magnet is held near the other (N vs. S)
Strength of interaction depends on the distance between the two
magnets. AND, the magnitude of the magnets 3 Slide 4 Magnetic Poles
Magnetic poles Give rise to magnetic force Rule for magnetic forces
between magnetic poles: Like poles repel; opposite poles attract.
north pole (geographic north-seeking pole) south pole (geographic
south-seeking pole) (Note the word seeking above) 4 Slide 5
Magnetic poles (continued) In all magnetsyou cant have one pole
without the other No single pole is known to exist Example: simple
bar magnetpoles at the two ends horseshoe magnet: bent U shapepoles
at ends 5 Breaking a magnet in two produces two new magnets, each
with one south and one north seeing pole Slide 6 6 Permanent
magnets always have two poles; a north seeking pole and a south
seeking pole Unlike poles attract each otherLike poles repel each
other What type of pole (N or S) is near the north pole? Slide 7
Magnetic Forces 7 In this case a permanent magnet not only attracts
these nails, but magnetizes other nails. Thus magnetism can be
induced by magnets Slide 8 Magnetic Poles 8 Magnets attract metal
objects by magnetic induction just like charged particles The force
produced by two magnets is proportional the strength of the magnets
Slide 9 Magnetic Fields 9 Slide 10 10 The magnetic field lines go
from one pole to another What you cannot see is that the field
lines also exist inside the magnet and complete the loop Magnetic
Field Slide 11 Magnetic Fields Magnetic fields are similar to
electric fields Magnetic fields are ways to visualize what would
happen to a magnet in the field 11 The force pulling the magnets
pole will twist the magnet (TORQUE) Similarly an iron filing
becomes magnetized and aligns itself with the magnetic field
http://phet.colorado.e du/en/simulation/mag net-and-compass
http://phet.colorado.edu/ en/simulation/generator Slide 12 Magnetic
Fields 12 Note how the two north seeking poles and south seeking
poles repel each other in the left hand figure, but the opposite
poles attract on the right Slide 13 Magnetic Fields Magnetism is
created by moving charges In permanent magnets the moving charges
are electrons moving around the nucleus The magnetic field is
therefore due to the distortions that occur when electric fields
are in motion The magnetic field is relativistic. In other words
the magnetic field depends on the motion of the charge with respect
to the observer 13 Slide 14 Magnetic fields (continued) Produced by
two kinds of electron motion electron spin main contributor to
magnetism pair of electrons spinning in same direction creates a
stronger magnet pair of electrons spinning in opposite direction
cancels magnetic field of the other electron revolution 14 Slide 15
Magnetic Domains 15 3) When hot iron metal is in its liquid phase,
the domains are continually moving around in random directions.
4)Typically, as the iron cools and becomes solid, the domains are
frozen in an unorganized manner, where the domains will cancel each
other out. 5) But, IF there is a strong magnetic field nearby when
the metal is cooling, these domains will all align to the magnetic
field creating a permanent magnet. 1)Every atom has moving
electrons 2)Groups of atoms may have their electrons move
complementary to each which creates a magnetic field DOMAIN Slide
16 Magnetic Domains 16 Since the earth has a significant magnetic
field, molten iron may become magnetized naturally as the molten
iron cools. This is what creates a lodestone Slide 17 Magnetic
Domains Difference between permanent magnet and temporary magnet
Permanent magnet alignment of domains remains once external
magnetic field is removed Temporary magnet alignment of domains
returns to random arrangement once external magnetic field is
removed 17 Slide 18 Magnetic Domains 18 You can also create a
permanent magnet by taking another magnet and stroking a piece of
metal. After awhile the magnetic domains will align creating a
permanent magnet Slide 19 Electric Currents ( I ) and Magnetic
Fields (B) 19 When a current flows in the wire the magnet will move
the east or west determined by the direction of current Slide 20 20
Magnetic fields (B) form a pattern of concentric circles around a
current-carrying wire ( I ). When current reverses direction, the
direction of the field lines reverse. Connection between
electricity and magnetism *Moving charge (electric current) creates
a magnetic field I Slide 21 I Magnetic fields (B) form a pattern of
concentric circles around a current-carrying wire (I). I Slide 22
Electric Currents and Magnetic Fields 22 Magnetic field around a
wire Magnetic field in a coil (solenoid) Slide 23 The Right Hand
Rule 23 The direction of the magnetic field (B) can be determined
using the Right Hand Rule. Put your right hand on the wire with
your thumb pointing in the direction of the current (I). Your
fingers will be curled in the circular direction of the magnetic
field (B). Slide 24 24 *Moving charge (electric current) creates a
magnetic field Electric current moving through a coil (solenoid) of
wire will create a magnetic field = an ELECTROMAGNET Slide 25 25 A
current-carrying coil of wire is an electromagnet. The strength of
an electromagnet is increased by increasing the current through the
coil and increasing the number of turns in the coil. Industrial
magnets gain additional strength by having a piece of iron within
the coil. Where magnetic domains in the iron core are induced into
alignment, adding to the field. http://phet.colorado.edu/
en/simulation/generator Slide 26 26 Electromagnets Electromagnets
without iron cores are used in magnetically levitated, or maglev,
transportation. Levitation is accomplished by magnetic coils that
run along the track, called a guideway. The coils repel large
magnets on the trains undercarriage. Continually alternating
electric current fed to the coils continually alternates their
magnetic polarity, pulling and pushing the train forward.
Electromagnets that utilize superconducting coils produce extremely
strong magnetic fieldsand they do so very economically because
there is no heat losses. Slide 27 Slide 28 Magnetic Forces acting
on Moving Charges 28 (+q) Charged particles (+q) that move through
a magnetic field (B) experience a perpendicular deflecting force
(F) Slide 29 Magnetic Force on Moving Charged Particles 29 A
charged particle moving through a magnetic field will be effected
by the magnetic field. The charged particles will experience a
force perpendicular to the original direction (velocity) of the
particle and perpendicular to the magnetic field (B) I (e - ) F B
Slide 30 Magnetic Force on Current-Carrying Wires 30 A current
carrying wire will have a force that is proportional to the current
(I) and magnetic field (B). N S N S B B F F I (e - ) Slide 31 31
Who likes to watch TV? Slide 32 32 F I (e - ) B Slide 33 The reason
that an electron moving in a magnetic field doesnt pick up speed is
A.magnets only divert them. B.only electric fields can change the
speed of a charged particle. C.the magnetic force is always
perpendicular to its motion. D.All of the above. 33 Magnetic Force
on Moving Charges Slide 34 The reason that an electron moving in a
magnetic field doesnt pick up speed is A.magnets only divert them.
B.only electric fields can change the speed of a charged particle.
C.the magnetic force is always perpendicular to its motion. D.All
of the above. Explanation: Although all statements are true, the
reason is given only by C. With no component of force in the
direction of motion, speed doesnt change. 34 Magnetic Force on
Moving Charges Slide 35 Magnetic Force on Current- Carrying Wires
Electric meters detect electric current Example: magnetic compass
compass in a coil of wires 35 Slide 36 Electric Meters 36 Slide 37
Electric Motors 37 An electric motor differs from galvanometer in
that each time the coil makes a half rotation, the direction of the
current changes in cyclic fashion to produce continuous rotation.
Slide 38 Earths Magnetic Field 38 Slide 39 39 Earths Magnetic Field
Earth is itself a huge magnet. The magnetic poles of Earth are
widely separated from the geographic poles. The magnetic field of
Earth is not due to a giant magnet in its interiorit is due to
electric currents. Most Earth scientists think that moving charges
looping around within the molten part of Earth create the magnetic
field. Earths magnetic field reverses direction: 20 reversals in
last 5 million years. Slide 40 40 Earths Magnetic Field Universe is
a shooting gallery of charged particles called cosmic rays. Cosmic
radiation is hazardous to astronauts. Cosmic rays are deflected
away from Earth by Earths magnetic field. Some of them are trapped
in the outer reaches of Earths magnetic field and make up the Van
Allen radiation belts Slide 41 Cosmic Rays 41 Slide 42 42 Earths
Magnetic Field Storms on the Sun hurl charged particles out in
great fountains, many of which pass near Earth and are trapped by
its magnetic field. The trapped particles follow corkscrew paths
around the magnetic field lines of Earth and bounce between Earths
magnetic poles high above the atmosphere. Disturbances in Earths
field often allow the ions to dip into the atmosphere, causing it
to glow like a fluorescent lamp. Hence the aurora borealis or
aurora australis. Slide 43 Cosmic Rays 43 Cosmic Rays that dip into
the atmosphere cause the aurora borealis (Northern Lights) or
aurora australis (Southern Lights) Slide 44 44 Biomagnetism Certain
bacteria biologically produce single-domain grains of magnetite (a
compound equivalent to iron ore) that they string together to form
internal compasses. They then use these compasses to detect the dip
of Earths magnetic field. Equipped with a sense of direction, the
organisms are able to locate food supplies. Pigeons have multiple
domain magnetite magnets within their skulls that are connected
with a large number of nerves to the pigeon brain. Pigeons have a
magnetic sense, and not only can they discern longitudinal
directions along Earths magnetic field, they can also detect
latitude by the dip of Earths field. Other similar animals: Bees,
butterflies, sea turtles and fish.
