Transcript Basic Magnetism

Basic Magnetism
November 2009
Physics 30S/40S
How Do We Use Magnets?
What is a Magnet?
• A magnet is an object which can attract either
iron or steel (other magnets)
• Magnets are designated with poles
• Poles: extreme ends of a magnet
• when a bar magnet is allowed to rotate freely,
the pole that seeks the northerly direction, is
called the north magnetic pole, the opposite
the south magnetic pole
Law of Magnetic Poles
• Like in electricity, opposite poles attract and
similar poles repel
– North attracts south
– Two north poles will repel
• Attraction and Repulsion?
– Magnets are able to exert a force on each other;
called a magnetic force
• Unlike in electricity, it is not possible to separate
two poles on a magnet
– There is no north without south!
History
• Not a modern
concept/Known since
ancient times
• Greeks (600 BC):
Magnesia stone would
attract iron
• China: used for navigation
– http://science.discovery.co
m/videos/what-theancients-knew-i-shortschinese-magneticcompass.html
• Lodestone (magnetite)
Sir William Gilbert
• De Magnete (1601)
• Earth is like a bar
magnet
• Earth has a core of iron
• Magnets can be cut
– Domains
• Presented magnetism
to Queen Elizabeth;
May 24, 1544 – November 30, 1603 used in navigation
Preliminary Compasses
• Leaf, Water, Wire
• http://dsc.discovery.com/videos/man-vs-wild1-shorts-3-min-mexico-water-compass.html
• A suspended magnet will point geographically
north
• Used for navigation
• Sign of physics to come
– Why do we rub the wire against hair?
Magnets Today
•
•
•
•
Artificial magnets are most common
ferrite - iron alloys
rare earth magnets - very strong alloys
alnico - alloy made of aluminum, nickel, iron,
and cobalt
– very stable
– supports 100 times it’s own weight (and costs
about $20/pound)
– used in guitar pickups, microphones,
loudspeakers and cow magnets
Geomagnetism – Short Overview
• Compasses can be used in navigation – why?
• Earth is essentially a giant magnet
• Naming conventions:
– North is South?
• Why is Earth a magnet? More to come later...
What is a Magnetic Field?
• The magnetic field is the region of space
around a magnet where another magnet will
experience a force
• Vector quantity: magnitude and a direction
• Magnitude: measured in Tesla or Gauss
– 1 T = 10,000 G
• Direction: the direction which the north pole
of a test compass would point; that is, towards
south
Mappings
• We can create a map of the magnetic field
around an object
• More concerned with direction of field as
opposed to magnitude
• Magnitude: More and denser field lines
indicate a stronger field
• Direction: Use arrows to indicate direction of
field
• Remember: arrows point towards south pole!
Bar Magnet
Horseshoe Magnet
• Think of bending a bar
magnet – what would
happen?
Multiple Bar Magnets
Look Familiar?
Notes on Drawing Field Lines
• Arrows always point towards the magnetic south
pole
• When drawing the field lines, remember law of
opposite poles
– Repulsion will push away field lines
– Attraction will pull field lines tighter
• Remember drawing the fields for static charges?
– Bar magnet is like a dipole
– Multiple bar magnets are like strings of dipoles
Homework
• Magnetic Field Lines Handout
Domain Theory
• Magnets are composed of small regions
(called domains) which behave as miniature
bar magnets
• Domains are around 1 μm
• Domains can be aligned to produce a net
magnetic field or unaligned so that no net
field is observable
What Would a Magnet Look Like if
Cut?
Types of Magnetic Material
• Ferromagnetic: permanent magnet
• Paramagnetic: magnetic only in the presence
of a magnetic field
• How can we explain these two types of
magnets in terms of domain theory?
Creation of Magnets
• Ferromagnetism: the phenomenon by which
materials become and remain magnetized
1. placing molten paramagnetic material in the
vicinity of a large magnetic field and allowing it to
cool
2. passing an electric field through the material
- More later…
3. rubbing a paramagnetic material with a magnet
– demo
Destruction of Magnets
• Aim is to break the alignment of the domains!
• Dropping
• Repeated exposure to opposing strong magnetic
fields
• Heating
– Curie Point (Pierre not Marie!):
– Magnetic Heat Machine:
• http://www.youtube.com/watch?v=RWrTvB-oK94
– Explanation from Walter Lewin (MIT):
• http://www.youtube.com/watch?v=X8ZHQQUusGo
Relation to Chemistry
• For the answer to why certain elements are
magnetic, while others are not, we turn to
Chemistry
• Pauli Exclusion Principle: for electrons in a
single atom, no two electrons can have the
same quantum numbers
– Spins of electrons gives certain elements
ferromagnetic or paramagnetic properties
Homework
• Domain Theory Handout
Geomagnetism – The Details
As postulated by Gilbert, the Earth does
act like a bar magnet. It has magnetic north
and south poles. However, our best
explanation of Earth’s magnetic phenomena is
described by:
Dynamo Theory
• Molten core is composed primarily of iron;
surrounds solid iron core
• Fluid motion of iron creates magnetic poles
• Poles are not stationary; they wander
– Fluid dynamics
• Poles are able to flip; on average, this happens
every 300,000 years (variable)
• Evidence in ocean floor
• Last flip occurred 780,000 years ago
Computer simulation: Blue signifies a magnetic south pole; yellow signifies a north pole
From: http://www.es.ucsc.edu/~glatz/geodynamo.html
See more simulations here!
Location of Magnetic Poles
• Earth’s magnetic poles wander
• Currently, magnetic pole near
geographic north is headed north
at about 40 km per year
• Located about 600 km north of
Resolute Bay
• Magnetic pole can move daily;
sometimes dramatically (80 km)
From: http://gsc.nrcan.gc.ca/geomag/nmp/northpole_e.php
Magnetic Declination
• Angle between magnetic pole and geographic north
• Must be known for navigation
• Dependent on location
– Victoria: 20o East
– St. John’s: 23o West
(Means that in Victoria, magnetic north is 20o East of
geographic north)
• For Elm Creek (geographical coordinates: 49° 40' 0"
North, 98° 0' 0" West), the magnetic declination is ?
• http://geomag.nrcan.gc.ca/apps/mdcal-eng.php
Magnetic Declination Map
Angle of Dip
• Earth’s magnetic field is
not 2D, but 3D
• True pole is actually
located in Earth’s core
– Results in an angle of
dip
Circle of Dip
• Directly above the pole,
the angle of dip is 90o
• At the equator, the
angle is 0o
• Can be used to
determine latitude
Magnetosphere
• Earth’s magnetic field is not bound to the surface; it
extends upwards into space
• Magnetosphere: a region of the upper atmosphere
beyond 200 km which the motion of charged
particles is governed by the Earth’s magnetic field
• Solar winds: streams of charged particles that travel
away from the sun
• extends to 57 000 km towards the sun (10 Earth
radii) and hundreds of Earth radii away from the
Sun
Auroras
• Magnetic field diverts charged particles and
protects Earth from radiation
• Earth’s magnetic field means that charged
particles from solar winds are unable to contact
Earth’s surface, except for near the poles
• Results in Aurora Borealis and Aurora Australis
Explanation of the Auroras
• In 1958, regions of intense radiation were discovered in
the magnetosphere by a team headed by Dr. J. Van
Allen
– contain energetic protons and electrons
– create the northern lights
• high energy particles from solar winds are trapped in
the belts
• energetic particles collide with oxygen and nitrogen
molecules near the poles
– collisions excite the molecules and they emit light we see in
the auroras
• Interestingly enough, the colours which we see are
described by the atomic structure of the atoms
– green light - oxygen, pink light - nitrogen
Electromagnetism
• Why does electricity keep popping up in the
magnetism unit?
• Remember how magnetism had to do with
electron spins.... And electricity is the
movement of electrons, so...
Vocabulary
• Magnet: object which can attract iron or steel (other magnets)
• Magnetic field: the region of space around a magnet where another magnet
will experience a force
• Domain: small magnet (region) within a larger object
• Domain theory: small magnetic domains contribute to the larger magnetic
properties of the object
• Ferromagnetic: permanent magnet i.e. iron
• Paramagnetic: becomes magnetized only in the presence of a magnetic field
• Curie point: temperature at which an element loses magnetic properties
• Dynamo theory: Molten core is composed primarily of iron which surrounds
solid iron core; fluid motion of iron creates magnetic poles
• Magnetic declination: angle between pole and geographic north
• Angle of dip: the angle beneath Earth’s surface at which the magnetic pole
is located
• Magnetosphere: a region of the upper atmosphere beyond 200 km which
the motion of charged particles is governed by the Earth’s magnetic field