Magnetic Fields

The Magnetic Field

Magnets are fun to play with. Magnets attract paper clips but not aluminum washers. Why is that? Magnets attract other magnets -- sometimes. If they are held in a certain way two magnets will repel each other or, perhaps, repel each other until one spins around and then jumps back to the other. Magnets make great toys but they have more serious uses, too. Magnets can tell directions because compasses are just bar magnets. Magnets are used in motors -- from electric starter motors in our automobiles to linear induction motors that propel modern rapid transit trains.

Magnets have two kinds of poles -- somewhat like the two kinds of electric charges.

Like poles repel each other.

Unlike poles attract each other.

Unlike electric charges, the magnetic poles can not be isolated.

If a magnet is broken into two pieces, each piece will still have two poles.


There is a magnetic field around a magnet which looks somewhat like the electric field around a "dipole", a pair of positive and negative electric charges

We can see a magnetic field easier than we can see an electric field because we can use a series of compass needles or iron filings that will align themselves with the magnetic field.

We can also "see" or "find" or "determine" a magnetic field by the force it exerts on a moving charge.


Force on a Charge

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(c) Doug Davis, 2002; all rights reserved