How Everything Works
How Everything Works How Everything Works

Question 512

How does a magnet work and is there a way that I can determine which end of the magnet is north and which end is south?
The magnetic fields that are responsible for the interesting behaviors of magnets can be created either by (1) moving electric charge or (2) changing electric fields. We can ignore the second process because it has very little to do with permanent magnets. Instead, let's focus our attention on the first process: moving electric charge producing magnetic fields. Whenever electric charges flow through a wire, a phenomenon that we call an electric current, they create magnetism. Many appliances use electricity and electric currents to create magnetism, notably televisions, motors, and audio speakers. But a permanent magnet doesn't use an obvious electric current to create its magnetic field. Instead, it uses the spinning character of the electrons inside the material from which that magnet is made. Electrons are electrically charged and they have an intrinsic spinning character. A simplistic view of an electron is as a spinning, electrically charged ball. Since its charge is in motion, an electron acts as a magnet and has both a north pole and a south pole. In most materials, the magnetic electrons are turned in opposite directions, canceling out one another's magnetism so that the overall material is non-magnetic. But in a few special materials, including most steels, the cancellation is imperfect and some magnetism remains. In a permanent magnet, this remaining magnetism is particularly apparent. The material is, in effect, a big collection of magnetic electrons that all work together to create a large magnet.

To determine which end of a permanent magnet is its north pole and which is its south, take a compass and hold it a reasonable distance from one end of the magnet. If the north end (often the red end) of the compass needle points toward this end of the magnet, you know that this end of the magnet is a south pole! That's because opposite poles attract and the "north" end of the compass needle, a north pole, is attracted to south poles. Interestingly enough, the magnetic pole near the earth's geographic north pole is actually a south magnetic pole. That's why the north pole of the compass needle points toward the earth's north geographic pole. When you use a compass to detect which pole of the magnet is north, be careful not to bring the compass needle too close to the permanent magnet. A strong permanent magnet can remagnetize the compass needle and reverse its poles. To make sure that this hasn't occurred, check to see whether the compass still points toward the north pole after you bring it near strong permanent magnets.


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