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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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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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Earth's Magnetosphere
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