Title: Magnetism
1Magnetism
- A Strangely Attractive Topic
2History 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.
3History 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.
4Basically, we knew the phenomenon existed and we
learned useful applications for it. We did not
understand it.
5Finally, 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.
6The 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....
7A Sheep in a Cow Suit?
All magnetic phenomena result from forces between
electric charges in motion.
8Looking 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
9Top 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.
10Top 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.
11Top 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.
12For 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
13No 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
14Magnets 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!
15The Concept of Fields
Michael Faraday realized that ...
A magnet has a magnetic field distributed
throughout the surrounding space
16Magnetic 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.
17Field Lines Around a Magnet
18Field Lines Around a Doughnut Magnet
19Field Lines Around a Bar Magnet
20Field Lines Around a Magnetic Sphere
21Field Lines of Repelling Bars
22Field Lines of Attracting Bars
23Action 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.
24Force 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.
25Watch the Bending Fingers!
26Cyclotron
- 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
27How 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
28Making 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
29Magnetic 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.
30Electric 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.
31Electromagnet (Magnetism from Electricity)
An electromagnet is simply a coil of wires which,
when a current is passed through, generate a
magnetic field, as below.
32Magnetic 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.
33Magnet - 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.
34A 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?
35More 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.
36More 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.
37Lets Play!