Magnets and Magnetic Fields

Magnets (or current) may be used to make other materials into magnets. A material's ability to be magnetized is called its permeability. (We can magnetize a material by stroking it with a permanent magnet and aligning the domains, or by placing an magnetized object near a strong permanent or electromagnet. Sometimes we do this unintentionally!) Many metals make good magnets, such as iron and nickel. But other metals, such as aluminum, cannot be magnetized.

The ancients (and the moderns) wondered at the properties of lodestones - naturally occurring rocks that could attract metal objects and could even transfer its properties to the objects if they were rubbed by the stone. Galileo studied them extensively, and through the ages they have been associated with all manner of charms and magic powers. We will use them as our introduction to magnetic fields and magnetism. We have seen in the classroom that a wire with a current running through it can influence a compass. The direction of the current flow through the wire influences the magnetic field around the wire and the direction the compass needle actually points. This was first demonstrated by the Danish physicist Hans Christian Oersted. We've seen iron filings suspended on a magnet provide a visual image of magnetic lines of field. But what is magnetism and how does it
work?

We will think of magnetism as being similar to electricity, and indeed the two are closely related. Magnetism is caused by the movement of electrons within an atom. The spin of an electron as well as its movement in orbit can generate a small current which in turn generates a small magnetic field. In some materials all of these "mini-magnets", or domains, align themselves and contribute to a larger magnetic field. Metals with good magnetic properties are called ferromagnetic. While all materials display some form of magnetism, in most the magnetic field is too small to measure with anything except the most sensitive instruments. A magnetic field is an area where a magnetic force can be detected. We will see in other lessons that a current in a wire will generate a magnetic field, and passing a wire loop through a magnetic field can generate a current. What are some other ways that electricity and magnetism are related? One similar characteristic of a magnet to electricity is that opposites attract and likes repel. Just as positive charges are attracted to negative charges, the north and south poles of a magnet will attract each other. Like poles will repel each other. Which raises an interesting observation - the Earth has a magnetic field and we say that its magnetic north pole is located slightly off- axis from the geographic north pole. But if the north end of the needle of a compass points to the magnetic north pole, then what is the true nature of the pole?

One major difference between the way electric charge behaves and the way a magnet behaves concerns the
polarity. An item can have an induced charge such that all its positive charges move to one end and its negative charges move to the other. If the item could be cut in half without disturbing the charge, then there would be two pieces, each of different charge (one positive, one negative.) With a magnet, one end has a north pole and one end has a south pole. But if we cut a bar magnet in half, instead of ending up with a "north half" and a "south
half", we end up with two smaller magnets, each with a north and south poles. Magnets cannot be separated into just a single pole.

Field Lines

Field lines in magnets behave much the same way as they do with electric fields. Our field lines will extend perpendicular from a surface of the magnet, and lines of field will not cross. The stronger a magnet's field is, the denser the lines will be. The direction of the magnetic field will be outward from the north pole and inward at the south pole.

Click here for a simulation of magnetic field lines from Florida State University

For a great explanation of magnetism from NASA, click here.
For more information on magnetic field lines, also from NASA, try here.

www.school-for-champions.com/science/magnetism.htm This site is good for a simple overview of basic magnetism.
And for an ineresting applet, try:

http://web.mit.edu/jbelcher/www/java/part_biot/part_biot.html