Magnetism is the force exerted by magnets when they attract or repel each other. Magnetism is caused by the motion of electric charges.
Every substance is made up of tiny units called atoms. Each atom has electrons, particles that carry electric charges. Spinning like tops, the electrons circle the nucleus, or core, of an atom. Their movement generates an electric current and causes each electron to act like a microscopic magnet.
In most substances, equal numbers of electrons spin in opposite directions, which cancels out their magnetism. That is why materials such as cloth or paper are said to be weakly magnetic. In substances such as iron, cobalt, and nickel, most of the electrons spin in the same direction. This makes the atoms in these substances strongly magnetic—but they are not yet magnets.
To become magnetized, another strongly magnetic substance must enter the magnetic field of an existing magnet. The magnetic field is the area around a magnet that has magnetic force.
All magnets have north and south poles. Opposite poles are attracted to each other, while the same poles repel each other. When you rub a piece of iron along a magnet, the north-seeking poles of the atoms in the iron line up in the same direction. The force generated by the aligned atoms creates a magnetic field. The piece of iron has become a magnet.
Some substances can be magnetized by an electric current. When electricity runs through a coil of wire, it produces a magnetic field. The field around the coil will disappear, however, as soon as the electric current is turned off.
Earth is a magnet. Scientists do not fully understand why, but they think the movement of molten metal in Earth’s outer core generates electric currents. The currents create a magnetic field with invisible lines of force flowing between Earth’s magnetic poles.
The geomagnetic poles are not the same as the North and South Poles. Earth’s magnetic poles often move, due to activity far beneath Earth’s surface. The shifting locations of the geomagnetic poles are recorded in rocks that form when molten material called magma wells up through Earth’s crust and pours out as lava. As lava cools and becomes solid rock, strongly magnetic particles within the rock become magnetized by Earth’s magnetic field. The particles line up along the lines of force in Earth’s field. In this way, rocks lock in a record of the position of Earth’s geomagnetic poles at that time.
Strangely, the magnetic records of rocks formed at the same time seem to point to different locations for the poles. According to the theory of plate tectonics, the rocky plates that make up Earth’s hard shell are constantly moving. Thus, the plates on which the rocks solidified have moved since the rocks recorded the position of the geomagnetic poles. These magnetic records also show that the geomagnetic poles have reversed—changed into the opposite kind of pole—hundreds of times since Earth formed.
Earth’s magnetic field does not move quickly or reverse often. Therefore, it can be a useful tool for helping people find their way around. For hundreds of years, people have used magnetic compasses to navigate using Earth’s magnetic field. The magnetic needle of a compass lines up with Earth’s magnetic poles. The north end of a magnet points toward the North Magnetic Pole, which holds a south magnetic charge.
Earth’s magnetic field dominates a region called the magnetosphere, which wraps around the planet and its atmosphere. Solar wind, charged particles from the sun, presses the magnetosphere against Earth on the side facing the sun and stretches it into a teardrop shape on the shadow side.
The magnetosphere protects Earth from most of the particles, but some leak through it and become trapped. When particles from the solar wind hit atoms of gas in the upper atmosphere around the geomagnetic poles, they produce light displays called auroras. These auroras appear over places like the U.S. state of Alaska, Canada and Scandinavia, where they are sometimes called “Northern Lights.” The “Southern Lights” can be seen in Antarctica and New Zealand.