Electricity and Magnetism

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Transcript Electricity and 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.
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.
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.
William Gilbert, an English
physician, first proposed in 1600
that the earth itself is a magnet,
and he predicted that the Earth
would be found to have magnetic
poles.
What is Magnetism?
Magnetism is the force of
attraction or repulsion of
a magnetic material due
to the arrangement of its
atoms, particularly its
electrons.
Magnets have been known for centuries.
The Chinese and Greeks knew about the
“magical” properties of magnets. The
ancient Greeks used a stone substance
called “magnetite.” They discovered that
the stone always pointed in the same
direction. Later, stones of magnetite called
“lodestones” were used in navigation.
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.
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
All of us are familiar with magnets. In a magnet we have
magnetic poles – the north and the south pole.
 A North seeking pole is called the North Pole.
 A South seeking pole is called the South Pole.
Like magnetic poles repel and unlike magnetic poles
attract.
A bar magnet cut into halves always makes new, complete
magnets with both a north and a south pole. The poles
always come in pairs. You can not separate a pair into
single poles.
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.
At the poles of a magnet, the magnetic field lines are
closer together.
Unlike poles of magnets attract
each other and like poles of
magnets repel.
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.
 Magnetic Fields:
 A magnet that is moved in space near a second magnet
experiences a magnetic field.
 A magnetic field can be represented by field lines.
 The strength of the magnetic field is greater where the lines
are closer together and weaker where they are farther
apart.
Characteristics of Magnetic Field
Lines
The spacing of the lines indicates the relative strength of
the force.
Outside a magnet, the lines are concentrated at the
poles. They are closest within the magnet itself.
By convention, the lines proceed from S to N inside a
magnet and from N to S outside a magnet, forming
closed loops.
The lines do not cross one another.
The earth is like a giant magnet!
The nickel iron core of the earth gives the earth a
magnetic field much like a bar magnet.
The Concept of “Fields”
Michael Faraday
realized that ...
A magnet has a
‘magnetic field’
distributed throughout
the surrounding space
Field Lines Around a Magnet
These lines are a map of the magnetic field around a bar
magnet. The needle of a magnetic compass will follow the
lines, with the north end showing the direction of the field.
Our Earth is a big magnet.
• The Earth’s magnetic field is thought
to originate with moving charges.
• The core is probably composed of iron
and nickel, which flows as the Earth
rotates, creating electrical currents
that result in the Earth’s magnetic
field.
The earth's magnetic
field. Note that the
magnetic north pole and
the geographic North Pole
are not in the same place.
Note also that the
magnetic north pole acts
as if the south pole of a
huge bar magnet were
inside the earth. You know
that it must be a magnetic
south pole since the north
end of a magnetic
compass is attracted to it
and opposite poles attract.
How to break a magnet:
1. Drop it
2. Heat it
Sometimes, the
Earth’s magnetic
poles flip. This
happens every
half-million years
or so.
Magnetic North Pole
Magnetic South Pole
The solar wind is constantly bombarding
the Earth’s magnetic field. Sometimes
these charged particles penetrate that
field. These particles are found in two
large regions known as the Van Allen
Belts.
The Earth’s magnetic field extends far into
space. It is called the “magnetosphere.”
When the magnetic particles from the sun, called
“solar wind”, strike this magnetosphere, we see a
phenomenon called…
The Aurora Borealis in the Northern Hemisphere
And the Aurora Australis in the Southern Hemisphere