Magnetism

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Magnetism

• A Strangely Attractive Topic

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History 1

• Term comes from the ancient Greek city of
  Magnesia, at which many natural magnets were
  found. We now refer to these natural magnets as
  lodestones (also spelled loadstone lode means to
  lead or to attract) which contain magnetite, a
  natural magnetic material Fe3O4.
• Pliny the Elder (23-79 AD Roman) wrote of a
  hill near the river Indus that was made entirely
  of a stone that attracted iron.

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History 2

• Chinese as early as 121 AD knew that an iron
  rod which had been brought near one of these
  natural magnets would acquire and retain the
  magnetic propertyand that such a rod when
  suspended from a string would align itself in a
  north-south direction.
• Use of magnets to aid in navigation can be
  traced back to at least the eleventh century.

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Basically, we knew the phenomenon existed and we
learned useful applications for it. We did not
understand it.
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Finally, the Science

• Not until 1819 was a connection between
  electrical and magnetic phenomena shown. Danish
  scientist Hans Christian Oersted observed that a
  compass needle in the vicinity of a wire carrying
  electrical current was deflected!
• In 1831, Michael Faraday discovered that a
  momentary current existed in a circuit when the
  current in a nearby circuit was started or
  stopped
• Shortly thereafter, he discovered that motion
  of a magnet toward or away from a circuit could
  produce the same effect.

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Let This Be a Lesson!

• Joseph Henry (first Director of the
  Smithsonian Institution) failed to publish what
  he had discovered 6-12 months before Faraday

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The Connection is Made
SUMMARY Oersted showed that magnetic effects
could be produced by moving electrical charges
Faraday and Henry showed that electric currents
could be produced by moving magnets
So....
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A Sheep in a Cow Suit?
All magnetic phenomena result from forces between
electric charges in motion.
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Looking in More Detail

• Ampere first suggested in 1820 that magnetic
  properties of matter were due to tiny atomic
  currents
• All atoms exhibit magnetic effects
• Medium in which charges are moving has
  profound effects on observed magnetic forces

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For most of our discussions, we will assume the
medium is empty space, which is a reasonable
approximation of air in this context.
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Top Ten List
What We Will Learn About Magnetism
1. There are North Poles and South Poles. 2.
Like poles repel, unlike poles attract. 3.
Magnetic forces attract only magnetic materials.
4. Magnetic forces act at a distance. 5.
While magnetized, temporary magnets act like
permanent magnets.
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Top Ten continued
6. A coil of wire with an electric current
flowing through it becomes a magnet. 7. Putting
iron inside a current-carrying coil increases the
strength of the electromagnet. 8. A changing
magnetic field induces an electric current in a
conductor.
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Top Ten Continued
9. A charged particle experiences no magnetic
force when moving parallel to a magnetic field,
but when it is moving perpendicular to the field
it experiences a force perpendicular to both the
field and the direction of motion. 10. A
current-carrying wire in a perpendicular magnetic
field experiences a force in a direction
perpendicular to both the wire and the field.
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For Every North, There is a South
Every magnet has at least one north pole and one
south pole.  By convention, we say that the
magnetic field lines leave the North end of a
magnet and enter the South end of a magnet.  If
you take a bar magnet and break it into two
pieces, each piece will again have a North pole
and a South pole.  If you take one of those
pieces and break it into two, each of the smaller
pieces will have a North pole and a South pole. 
No matter how small the pieces of the magnet
become, each piece will have a North pole and a
South pole. 
S
N
S
N
S
N
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No Monopoles Allowed
It has not been shown to be possible to end up
with a single North pole or a single South pole,
which is a monopole ("mono" means one or single,
thus one pole).  Note Some theorists
believe that magnetic monopoles may have been
made in the early Universe. So far, none have
been detected.
S
N
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Magnets Have Magnetic Fields
We will say that a moving charge sets up in the
space around it a magnetic field,
and it is the magnetic field which exerts a
force on any other charge moving through it.
Magnetic fields are vector quantities.that is,
they have a magnitude and a direction!
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Defining Magnetic Field Direction
Magnetic Field vectors as written as B
Direction of magnetic field at any point is
defined as the direction of motion of a charged
particle on which the magnetic field would not
exert a force.
Magnitude of the B-vector is proportional to the
force acting on the moving charge, magnitude of
the moving charge, the magnitude of its velocity,
and the angle between v and the B-field. Unit is
the Tesla or the Gauss (1 T 10,000 G).
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Scientists Can Be Famous, Too!
Tesla
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Famous, continued
Gauss
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The Concept of Fields
Michael Faraday realized that ...
A magnet has a magnetic field distributed
throughout the surrounding space
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Magnetic Field Lines
Magnetic field lines describe the structure of
magnetic fields in three dimensions.They are
defined as follows. If at any point on such a
line we place an ideal compass needle, free to
turn in any direction (unlike the usual compass
needle, which stays horizontal) then the needle
will always point along the field line. Field
lines converge where the magnetic force is
strong, and spread out where it is weak. For
instance, in a compact bar magnet or "dipole,"
field lines spread out from one pole and converge
towards the other, and of course, the magnetic
force is strongest near the poles where they come
together.
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Field Lines Around a Magnet
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Field Lines Around a Doughnut Magnet
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Field Lines Around a Bar Magnet
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Field Lines Around a Magnetic Sphere
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Field Lines of Repelling Bars
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Field Lines of Attracting Bars
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Action at a Distance Explained
Although two magnets may not be touching, they
still interact through their magnetic fields.
This explains the action at a distance, say
of a compass.
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Force on the Charge
Right Hand Rule!
Put your fingers in the direction of motion of
the charge, curl them in the direction of the
magnetic field. Your thumb now points in the
direction of the magnetic force acting on the
charge. This force will bend the path of the
moving charge appropriately.
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Watch the Bending Fingers!
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Cyclotron

• Developed in 1931 by E. O. Lawrence and M. S.
  Livingston at UC Berkeley
• Uses electric fields to accelerate and
  magnetic fields to guide particles at very high
  speeds

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How a Cyclotron Works

• Pair of metal chambers
• shaped like a pillbox cut
• along one of its diameters
• (cleverly referred to as Ds)
• and slightly separated
• Ds connected to
• alternating current
• Ions injected near gap
• Ions are accelerated as long as they remain
  in step with alternating electric field

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Magnetic Force on Current-Carrying Wire
Since moving charges experience a force in a
magnetic field, a current-carrying wire will
experience such a force, since a current consists
of moving charges. This property is at the heart
of a number of devices.
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Electric Motor
An electric motor, is a machine which converts
electrical energy into mechanical (rotational or
kinetic) energy.    A current is passed through
a loop which is immersed in a magnetic field. A
force exists on the top leg of the loop which
pulls the loop out of the paper, while a force on
the bottom leg of the loop pushes the loop into
the paper.
The net effect of these forces is to rotate the
loop.
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Electromagnet (Magnetism from Electricity)
An electromagnet is simply a coil of wires which,
when a current is passed through, generate a
magnetic field, as below.
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Magnetic Properties of Matter
In other words.materials which produce magnetic
fields with no apparent circulation of charge.
All substances - solid, gas, and liquid - react
to the presence of a magnetic field on some
level. Remember why? How much they react causes
them to be put into several material types.
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Magnet - isms

• Ferromagnetism - When a ferromagnetic material
  is placed near a magnet, it will be attracted
  toward the region of greater magnetic field. 
  This is what we are most familiar with when our
  magnet picks up a bunch of paperclips.  Iron,
  cobalt, nickel, gadolinium, dysprosium and alloys
  containing these elements exhibit ferromagnetism
  because of the way the electron spins within one
  atom interact with those of nearby atoms.   They
  will align themselves, creating magnetic domains
  forming a permanent magnet.   If a piece of iron
  is placed within a strong magnetic field, the
  domains in line with the field will grow in size
  as the domains perpendicular to the field will
  shrink in size. 

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Making a Magnet from a Ferromagnetic Material
domains in which the magnetic fields of
individual atoms align orientation of the
magnetic fields of the domains is random no
net magnetic field.
when an external magnetic field is applied, the
magnetic fields of the individual domains line up
in the direction of the external field this
causes the external magnetic field to be enhanced
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A Ferromagnet in the Middle
If we look at a solenoid, but rather than air,
wrap it around a nice iron core. What happens to
the change in flux for a given current? Can you
see why ferromagnetic materials are often put in
the middle of current-carrying coils?
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More Magnet - isms

• Diamagnetism - When a diamagnetic material is
  placed near a magnet, it will be repelled from
  the region of greater magnetic field, just
  opposite to a ferromagnetic material.  It is
  exhibited by all common materials, but is very
  weak.   People and frogs are diamagnetic.  Metals
  such as bismuth, copper, gold, silver and lead,
  as well as many nonmetals such as water and most
  organic compounds are diamagnetic.

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More Magnet - isms

• Paramagnetism - When a paramagnetic material
  is placed near a magnet, it will be attracted to
  the region of greater magnetic field, like a
  ferromagnetic material.  The difference is that
  the attraction is weak.  It is exhibited by
  materials containing transition elements, rare
  earth elements and actinide elements.  Liquid
  oxygen and aluminum are examples of paramagnetic
  materials. 

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Lets Play!