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Magnetism
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Chapter FourMagnetism
4.1. Permanent Magnets and Magnetic Fields
1. Magnetic material has been known for a long time and used in navigational compasses by
the Chinese at least by the 11th century. Magnetic material was first found in Magnesia in
Asia Minor and was named after the place where it was found. Most magnets have iron in
them and are called ferromagnetic (from the Latin ferrum for iron).
2. Each magnet has two poles, North and South. The names were given because a permanent
magnet will always have its north pole point to magnetic north (which is actually
somewhere in the middle of Hudson bay). Magnetic poles always come in pairs. No
monopoles, (like single electric charges). If you cut a magnet, you still get a north and
south. There is no "Coulomb's Law" for magnetic fields.
3. Opposite poles attract and like poles repel. So the north pole of a compass points to
magnetic north of the earth (which is really a south magnetic pole). The north pole of a
compass should be called the "north seeking" pole.
4. Surrounding a magnet is a magnetic field. We can draw field lines always going from north
to south. At each point in space, the field points in the direction that the north pole of a
little compass would point. The direction of the magnetic field is tangent to the field lines.
Fields have the capability of producing forces on certain particles.
5. Magnetic field lines have no beginning and no end. They are always continuous loops. This
is because there are no monopoles, (single north or south poles). Remember that electric
field lines always started and ended on charged objects. This happens because there are
electrically charged objects with a single charge. Because there are no magnetic objects
with a single pole, there is no place for the magnetic field to start or end, so magnetic field
lines are always continuous.
6. The strength of the field is proportional to the number of lines per unit area (the density
of lines).
4.2. Geomagnetism
,The earth has a magnetic field. Magnetic north pole is like the south pole of a magnet since
"north seeking poles" point north. Lines go at an angle here in DelMarVa. Also, our compasses
point about 3°W of true geographic north. (Not because of lines at an angle, but because of
"Hudson Bay.") The earth's magnetic field is set up, not because of permanent magnets, but by
rotation of the iron core.
4.3. 5.3 Electric Currents Produce Magnetism
We have talked briefly about permanent magnets, and now we will talk about another way to
make a magnetic field: by moving charges. Actually, all magnetic fields we know about (if there
are no monopoles) are made from moving charges. Even permanent magnets are made because
subatomic charged particles are moving in atoms.
, 4.3.1. Magnetic Field of a Wire (Hans Christian Oersted – 1820)
The field goes in circles around the wire. The direction is given by the right hand rule. Thumb is
in direction of current. The fingers curl in the direction of the magnetic field. What is its
magnitude? A drawing of the field lines shows they go in circles around the wire and are denser
near the wire. Where the field lines are denser, the magnetic field is stronger. It is stronger near
the wire.
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