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.