Transcript Magnetism

Magnetism
A Strangely Attractive Topic
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.
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 property…and 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.
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 motion of a
magnet toward or away from a circuit could produce the same
effect.
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
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
A Sheep in a Cow Suit?
All magnetic
phenomena result
from forces between
electric charges in
motion.
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
Magnets
What do we already
know?
What are we going to learn today?
What the characteristics of magnets
are
 That magnets can repel and attract
each other
 How magnets behave
 What happens when you put two
magnets together

What are the characteristics of
magnets?
They can attract some materials
 They can also repel other magnets
 They are usually made of iron
 They have two ends called magnetic
poles

What do all these new words
actually mean?
Magnet
A stone or a piece of metal that attracts
some other metal.
 Attract
To pull towards each other.
 Repel
To push away from each other.
 Poles
The ends of a magnet.

What do the poles do?
When a magnet is held from a string:
 The north pole points north
 The south pole points… can you
guess?
How do we know this?
We can measure it with a compass!
On school magnets, the RED pole is the
north pole and the BLUE pole is the
south.
But what happens if you put two
magnets together?

Talk to your partner- what do you
think?
So let’s try it.
First of all, north to south…
south to north…
north to north…
south to south…
What did we find out?


So now we know that “like” poles repel each
other…
and that “opposite” poles attract each other.
They do this because there is a FORCE
between them.
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.
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.
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.
Figure: Origin of magnetic dipoles: (a) The spin of
the electron produces a magnetic field with a
direction dependent on the quantum number
(b) Electrons orbiting around the nucleus create a
magnetic field around the atom.
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
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).
S
N
Note: Some theorists believe that magnetic monopoles may
have been made in the early Universe. So far, none have been
detected.
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!
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).
Scientists Can Be Famous, Too!
Tesla
Famous, continued
Gauss
The Concept of “Fields”
Michael Faraday
realized that ...
A magnet has a
‘magnetic field’
distributed throughout
the surrounding space
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.
Field Lines Around a Magnet
Field Lines Around a Doughnut Magnet
Field Lines Around a Bar Magnet
Field Lines Around a Magnetic Sphere
Field Lines of Repelling Bars
Field Lines of Attracting Bars