Magnetism

Discovery, legend

Legend has it that rocks in the province of magnesia in Greece pulled the nails from a shepherd boy's shoes, the boy's name was Magnes. We call these rocks magnets.

Compasses

The Chinese discovered that if a magnetic rock was carved into a long shape and hung from a thread or floated on water the shape would always align itself in a north-south direction. Since south was an important direction in the religion of Geomancy these devices became "South Pointers".

When these devices came to Europe they were named compasses.

The end of the compass that points to the north geographic pole (Ocean, polar bears and Santa Claus) was called the north seeking pole. This was shortened to north pole.

Magnetic poles and forces.

Magnets have north poles, N, and south poles, S.

These always come in pairs, no north poles have ever been found alone.

A north pole attracts a south pole.

A north pole repels north poles.

A south pole repels south poles.

This leads to a table like the one for electricity.

 N S N R A S A R

The earth is a magnet

The reason that compasses point north is that the earth is a magnet. Unfortunately, due to the way the magnetic poles were named, north poles of magnets point to the north geographic pole of the earth because it attracts them and therefor must be a south pole of a magnet.

The north geographic pole of the earth is the south pole of a magnet.

(Alas geographers name this pole the "North magnetic Pole" of the earth.)

Perhaps the name of the magnetic poles will be corrected when the United States adopts the metric system.

The earth is not a permanent magnet it is an electromagnet. Its magnetism is made by electric currents flowing inside the liquid metal core of the earth.

The magnetic poles of the earth flip over every few hundred thousand years or so. So the name will be corrected by nature at some point in the future. These changes are named "magnetic reversals."

When a rock cools it freezes in the direction of the magnetic field through the rock. When lava cools at the mid ocean spreading ridges it records the direction of the earth's magnetic field. The resulting bands of north and south magnetized rock provided crucial evidence for continental drift.

Magnets and electric charges

Magnets do not exert forces on stationary electric charges. Many students think that a magnetic north pole will attract a negative charge. This is an incorrect belief.

The table of magnetic force on a stationary charge.

 N S + 0 0 - 0 0

Magnetic forces on moving charges.

Magnets exert forces on moving electric charges.

The direction of the force is given by the right hand rule.

To find the direction of the force F on a positive charge q, in a magnetic field B.

Point the extended thumb of your right hand in the direction of motion of the electric charge. (Remember that you have one thumb and that there is one moving charge.

Then move your hand around until the fingers point in the direction of the magnetic field. (Remember that you have many fingers and that there are many magnetic field lines.)

Also remember that outside of a magnet the field lines point from the north pole toward the south pole.

Your palm will then be oriented to push in the direction of the magnetic force on the moving charge.

Magnets exert strong attractive forces on iron. these forces are called ferromagnetic forces.

Both the north and the south poles of a magnet attract iron.

(cobalt and nickel are also strongly attracted to magnets.

 Scientific Explorations with Paul Doherty © 2005 10 October 2005