Magnetism

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.

Geomagnetic Poles

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.

Fast Fact

Animal Magnetism
Some animals, such as pigeons, bees, and salmon, can detect Earth's magnetic field and use it to navigate. Scientists aren't sure how they do this, but these creatures seem to have magnetic material in their bodies that acts like a compass.

Fast Fact

Historic Directions
The ancient Greeks and Chinese knew about naturally magnetic stones called "lodestones." These chunks of iron-rich minerals may have been magnetized by lightning. The Chinese discovered that they could make a needle magnetic by stroking it against a lodestone, and that the needle would point north-south.

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