Magnetic
Field,
The
term magnetism comes from the region of Magnesia, a city in Western Turkey,
where Greeks found lodestones, which attracted iron pieces across the
space. It is also observed that,
magnets attract as well as repel. We
can explain this dual nature of magnetic force by proposing that each magnet
has two poles, north pole (N) and south pole (S). You will observe two things during the activities:
1)
When
two magnets are brought near each other, like poles repel; opposite poles
attract.
2)
When
a magnet is brought near a piece of iron, the iron also gets attracted to the
magnet, and it acquires the same ability to attract other pieces of iron.
We
like to represent this force effect of a magnet on iron-like objects with a
concept called magnetic field. The
concept of field can be best understood if we remember the gravitational force
of Earth on object near it. We say that
the mere presence of Earth sets up gravitational field in the surrounding
space, and that we can represent this gravitational force effect with lines
starting from Earth and radially diverging away to infinity.
Moon
is caught in Earth’s field. Likewise,
the Astronaut in space walk is feeling the Earth’s gravity. Space shuttle is also in the Earth’s field. The
reason why they don’t fall is beyond the scope of this course, but I will
explain for completeness. None of them
fall towards Earth because they all have enough horizontal speed to make around
the Earth. If you were able to
horizontally throw a baseball at 18,000 mi/h, I would also make around the
Earth and return to you. Therefore, we
represent the Earth’s attractive gravitational force with field lines. The direction of field lines represent the
direction of force a body would experience around Earth, and the density of
field lines (how closely they are separated) represents the strength of the
force. For example, closer you are to
the Earth, stronger the force.
Similarly,
a magnet sets up a magnetic field in its surrounding space in which it magnetically
affects any other magnetic material.
The strength is represented by the density of the magnetic field
lines. Magnetic field lines are closed
curves leaving from North pole and entering the South pole when you follow them
on the outside the magnet.
A
compass, which is a small magnet itself, lines up parallel to the magnetic
field lines at the point it is placed.
The tip of the arrow is the North magnetic pole, and its end is the
South magnetic pole.
The
building blocks of magnets are atoms, which are small tiny magnets. As far as the magnetism is concerned, we can
view an atom as if it is a tiny compass/magnet, pointing to the north
direction. We will see later that, the
motion of electrons (moving electric charge) is the fundamental reason of
magnetism. For practical purposes we
can focus on a cluster of atoms, called magnetic domains that are
aligned in a specific direction. Each
domain may consist of billions of aligned atoms. Under normal conditions, a magnetic material like iron doesn’t
behave like a magnet because the domains don’t have a preferred direction of
alignment. On the other hand, the
domains of a magnet (or a magnetized iron) are all aligned in s specific
direction. Domains are separated from
the adjacent domains by domain walls.
In general, alignment within a domain is the same for all atoms of that
domain. However, the atoms of one
domain are aligned in a different direction than the atoms of another
domain. This situation is sketched
below for a magnetic material, a magnetized material, and for a nonmagnetic
material. A nonmagnetic material
doesn’t have any domain structure.
Domains
can be induced into alignment.
Consider a common iron nail. Its
domains are randomly oriented, like the first picture above. If you bring a magnet is brought nearby, the
domains of the iron nail will align in such a way that, the north pole of iron
domains will face the south pole of the magnet, and visa versa.
When you remove the magnet, the nail becomes permanent
magnet for a while. The thermal motion
(remember the higher the temperature, the faster the atoms move) of atoms
eventually may cause most of the atoms to return to random orientation. Also, by dropping a magnet, not only will
you break it, but you will also destroy the domain alignments.
Another
way of making a permanent magnet is to stroke a piece of iron (or iron shaving
which you will do as an activity) with a magnet. Iron shaving behave like tiny
magnets.
Electromagnet:
A
wire coils like the one shown in the picture below, can also produce magnetic
field similar to that of a magnet. If
the inside if the coils is filled with a iron core, the magnetic field even
gets stronger due to the additional magnetism from the iron.