GED Science Practice Test: Magnetic Field
Magnetic fields are areas where an object exhibits or experiences a magnetic influence. In everyday life, magnetic fields are most often encountered as an invisible force created by permanent magnets which pull on materials such as ferromagnetic materials, such as iron, cobalt or nickel; and attract or repel other magnets. A magnet is an object made of ferromagnetic material that is capable of producing a magnetic field. If you bring a magnet close to a stack of paper clips (made up at least partially of iron), the paper clips will move toward the magnet, once they are within the magnetic field of the magnet. There are other examples of magnetic fields as well; the earth, for example, produces its own magnetic fields.
Like electrical current, magnets and magnetic fields have a direction. A magnet has ends called magnetic poles. Magnetic poles are the places on a magnet where the magnetic field lines begin and end. Magnetic field lines are said to start at the North pole and end at the South pole of a magnet. The magnetic field lines for a magnet are pictured below:
Though magnetic field lines are invisible, we know they exist for a couple of reasons. One is that if you place a magnet in a bunch of iron filings, the iron filings will align themselves with the magnetic field of the magnet. The following picture shows iron filings lining up with the magnetic field of a magnet.
Notice that the iron filings form small circles when they are close to the magnet, and larger circles when farther away from the magnet. This is because the magnetic force exerted by a magnetic field is higher the closer the iron filings are to the magnet.
Though iron filings show the magnetic field lines, they do not indicate the direction of the field (the arrows on the diagram above). The piece of evidence we have for the direction of magnetic fields is the behavior of compasses when placed within the magnetic field of a magnet. Compasses are devices that point toward the north pole of the earth and assist in navigation. However, compasses do not selectively point to the earth’s north pole; they will point toward the north pole of any magnet. The following diagram shows the behavior of compasses when placed within the magnetic field of a magnet:
Notice that like in electricity, opposites attract in magnetism as well. The north end of the compass points toward the south end of the magnet, or away from the north end of the magnet.
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It is generally known that a compass needle will point north. It does not point to the geographical North Pole, but to the magnetic north pole. In certain cultures, stories have been told about sailors whose compasses used to point south. For a long time, no one believed these stories. Then scientists discovered that periodically the magnetic poles of Earth reverse themselves.
What assumption made scientists unwilling to believe the stories about these ancient sailors?