(EAM) Application of Magnetism Lesson

Application of Magnetism

Magnetism is an interaction that allows certain kinds of objects, which are called 'magnetic' objects, to exert forces on each other without physically touching. A magnetic object is surrounded by a magnetic 'field' that gets weaker as one moves further away from the object. A second object can feel a magnetic force from the first object because it feels the magnetic field of the first object. The further away the objects are the weaker the magnetic force will be.

Humans have known about magnetism for many thousands of years. For example, lodestone is a magnetized form of the iron oxide mineral magnetite. It has the property of attracting iron objects. It is referred to in old European and Asian historical records; from around 800 BCE in Europe and around 2600 BCE in Asia.

A moving charged particle, like an electron, has a magnetic field associated with it. Electrons inside any object are moving and have magnetic fields associated with them. In most materials, these fields point in various directions, so the net magnetic field is zero. For example, in the plastic ball below, the directions of the magnetic fields of the electrons (shown by the arrows) are pointing in different directions and cancel each other out. Therefore the plastic ball is not magnetic and has no magnetic field.

In some materials (e.g. iron), called ferromagnetic materials, there are regions called domains, where the electrons' magnetic fields line up with each other. All the atoms in each domain are grouped together so that the magnetic fields from their electrons point the same way. The picture shows a piece of an iron needle zoomed in to show the domains with the electric fields lined up inside them.

In permanent magnets, many domains are lined up, resulting in a net magnetic field. Objects made from ferromagnetic materials can be magnetized, for example, by rubbing a magnet along the object in one direction. This causes the magnetic fields of most, or all, of the domains to line up in one direction. As a result, the object as a whole will have a net magnetic field. It is magnetic. Once a ferromagnetic object has been magnetized, it can stay magnetic without another magnet being nearby (i.e. without being in another magnetic field). In the picture below, the needle has been magnetized because the magnetic fields in all the domains are pointing in the same direction.

Permanent Magnets

Because the domains in a permanent magnet all line up in a particular direction, the magnet has a pair of opposite poles, called north (usually shortened to N) and south (usually shortened to S). Even if the magnet is cut into tiny pieces, each piece will still have both a N and a S pole. These magnetic poles always occur in pairs. In nature, we never find a north magnetic pole or south magnetic pole on its own.

Like (identical) poles of magnets repel one another whilst unlike (opposite) poles attract. This means that two N poles or two S poles will push away from each other while a N pole and a S pole will be drawn toward each other.

Magnetic fields can be represented using magnetic field lines starting at the North pole and ending at the South pole. Although the magnetic field of a permanent magnet is everywhere surrounding the magnet (in all three dimensions), we draw only some of the field lines to represent the field (usually only a two-dimensional cross-section is shown in drawings).

As already stated, opposite poles of a magnet attract each other, and bringing them together causes their magnetic field lines to converge (come together). Like poles of a magnet repel each other and bringing them together causes their magnetic field lines to diverge (bend out from each other).

You can see magnetic field lines by taking bar magnets and placing them under a piece of paper then sprinkling iron filings on top. You might have done this activity in a previous science course. Here are two pictures that show the magnetic field lines converging and diverging.  

By watching the video below, adapted from NASA, you will learn how scientists study planetary magnetic fields. Animations illustrate how iron filings and a compass can show what a magnetic field looks like around a bar magnet. Find out how a magnetometer can be used like a compass to determine what a planet's magnetic field looks like. Learn how an electromagnet works, and how a magnetometer onboard a satellite measures a planetary magnetic field.

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