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ELECTOMAGNETISM
In the late 1700s and early 1800s many scientists
and philosophers believed that all forces of nature
had the same source. This was especially easy to believe
about electricity (see Electricity)
and magnetism (see Magnetism).
Both phenomena seemed to have two kinds of something
- a positive and a negative charge or a north and a
south pole. In addition, like charges and poles repel
and unlike charges and poles attract. It was noted
that compass needles could be affected by lightening
strikes and Benjamin Franklin had reported that he
had magnetized needles with a stored electric current.
However, no firm evidence existed that linked electricity
and magnetism until Hans Christian Oersted performed
a critical experiment during a lecture in 1820. It
is unclear whether  this
was an accident or a carefully constructed experiment.
Oersted himself had believed that electricity and magnetism
were linked for at least 12 years. Whatever his intent,
he placed a wire above the compass needle and connected
both ends across a battery and the needle spun until
it was at right angles to the wire.
In further experiments, using instruments
similar to the one pictured below, he was able to determine
that the magnetic influence surrounded the wire in
a circle.
 Courtesy
of Gabinete de Fisica of the University of Coimbra.
A magnetic needle balances on the central rod. The
two end posts support a metal wire. Each end of the
wire extends down through the wooden posts and is connected
to a small metal post in the base. When one metal post
was connected to the positive pole of a battery and
the other metal post was connected to the negative
pole of a battery, current would flow in the wire.
The needle would then swing until it was at right angles
to the wire.
The rule that has emerged from Oersted's
work is as follows: If you hold a wire in the palm
of your right hand so that the thumb points in the
direction of the current, your fingers circle in the
same direction as the magnetic field.
 This
drawing illustrates this rule. The red line is a segment
of a wire and the arrow designates the direction of
the current, I. The blue circles represent the magnetic
field lines, B, and the arrow heads signify the direction
of the magnetic
field (remember, physicists agree that the magnetic
field goes from north to south).
 Oersted's
discovery that a current creates a magnetic field was
very important. Michael Faraday, a research physicist
and lecturer at the Royal Institution in England, read
Oersted's paper describing his discovery and his conclusions.
Since Oersted proved that an electric current could
create a magnetic field, Faraday became determined
to use a magnetic field to create an electric current.
In 1831 he succeeded in creating an electric current
with a changing magnetic field. The emphasis is on
the changing magnetic field. For example, if a magnetic
is moved in the region of a current carrier connected
in a closed circuit or vice versa, current will begin
to flow in the carrier. If the magnet remains still,
no current will flow. This is the basic principle of
an electric generator. Create a cylindrical coil of
wire with a hollow middle, connect the coil to a meter
that measures current, and move a strong bar magnet
in and out of the coil. Current will flow one way when
you move the magnet in and the other way when you pull
the magnet out. No current will flow when the magnet
is still.
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Website Link to see an animation
Alternately, you could turn a cylindrical
coil of wire between opposite poles of stationary magnets.
The force turning the coils of wire could be your hand,
or the wind or falling water. This electric generator
could be small and light a bulb or large and light
a city. The connection between electricity and magnetism
was made complete with Faraday's work. An electric
current - any moving charge, really - creates a magnetic
field. A changing magnetic field causes current to
flow in a conductor in a closed circuit. (More precisely,
a changing magnetic field creates an electric field
and an electric field causes charged particles to move.)
Now imagine, for a moment, that a wire
carrying a current is placed in a magnetic field caused
by a strong magnet in such a way that the magnetic
field is perpendicular to the current. In the drawing
below the vertical arrows represent the magnetic field,
the thick red line represents the wire and the red
arrow shows the direction of the current.
 Experiment
shows that the wire experiences a force that is perpendicular
to both the external magnetic field and to the wire.
(This force can be large enough to cause a wire to
leap out of the U of a horseshoe magnet!) In the case
pictured above the force would always push the wire
toward you - out of your computer screen. A helpful
tool is an extension of the right hand rule discussed
above. Open your right hand flat and allow your thumb
to point in the direction of the current, and cause
your fingers to point in the direction of the external
magnetic field. Your palm would then push the wire
in the direction of the force.
Remember that a current is simply a movement
of charge. Imagine that there is a small, positively
charged particle  (red
circle) moving in a magnetic field (blue lines). The
effect of a magnetic field on a charged particle can
be described by that on a current carrying wire.
Looking at this from above, we would
see the magnetic field lines pointing toward us. The
convention for drawing lines is to show the point of
the arrow when the line is coming toward you and an
x when the line is going away.
The magnetic field lines come out of
the screen and the charged particle is moving right,
and the force is perpendicular to the movement. The
Force perpendicular to the motion will cause the charge
to move in a circular path. (use a pop-up for "Centripetal
Force" and also use "Circular
Path from Magnetic Field") Negatively charged
particles circle in the opposite direction. The size
of the magnetic field and the size of the charge, mass
and velocity of the particle determine the curvature
of the circle.
There are very many important applications
of this principle, from aiming a beam of charged particles
in a television picture tube, to particle accelerators
in atomic physics to instruments flown on satellites
to identify charged particles.
One very interesting and important application
of this relationship is the self-excited dynamo machine.
With a very weak upward magnetic field
(solid arrows), the electrons in a rotating disk of
conducting material will move inward (if the circuit
is complete) because of the magnetic field. The inward
movement of electrons is equivalent to a current (dotted
line) directed outward toward the edge of the disk.
A conductor that touches the disk and then winds around
the shaft counter-clockwise completes the circuit.
As current flows (dotted lines) counter-clockwise around
the wire, it generates a magnetic field that is directed
upward everywhere inside the loop. (Convince yourself
of this by grasping a hoop in your right hand with
your thumb pointing in a counter-clockwise direction
tangent to the hoop. Your fingers will be curled upwards
inside the hoop.) The magnetic field due to the current
in the loop adds to the original upward weak field
to make it stronger. A stronger upward magnetic field
makes the current stronger which makes the magnetic
field even stronger.
This is one of the proposed mechanisms
for the Earth's magnetic field. In the 20th century
scientists determined that the outer portion of the
Earth's core was mostly molten iron. Convection creates
flows of molten metal. Replace the rotating disk in
our model with the molten outer core of the Earth.
The presence of any weak magnetic field, even one from
the Sun or the moving molten iron in the Earth, can
create a ring of current in the molten outer core.
This would increase the strength of the weak magnetic
field. As the field gets stronger, the current gets
greater making the field even stronger. This process
accounts for about 90% of the Earth's magnetic field.
The dynamo process explains why the Earth's magnetic
field approximates a bar magnetic through the center
of the Earth. (For more detail visit The
Dynamo Process and Origin
of the Earth's Magnetism.)
The connection between electricity and
magnetism proved to have profound implications -- and
applications. In addition to the ideas already discussed,
Faraday was able to develop the connection into a theory
of electromagnetism and link it to visible light. Other
scientists have broadened his work until electromagnetism
has become the cornerstone of physics that it is today.
More of this can be explored in The
Electromagnetic Spectrum, a critical tool in astronomy.More
on the Earth's magnetic field is explored in A
Magnet in Space and The Earth's Magnetosphere.
In addition, the electromagnetic theory proved essential
to understanding the dynamic processes of the Sun,
which is developed in Why
Do Sunspots and CME's Occur? and The Sun's Magnet.
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