Which statement is true about magnetism

Which of the following statements is true about magnets?

Magnets always attract other magnets.

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All metals are attracted by a magnet.

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A horseshoe magnet can be used to make a compass.

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Magnetic poles exert maximum attraction.

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Solution

The correct option is D Magnetic poles exert maximum attraction.

magnetism

Magnetism is the force exerted by magnets when they attract or repel each other

Subjects

Earth Science, Geographic Information Systems (GIS), Geography, Geology

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

The Earth is a magnet. Scientists do not fully understand why, but they think the movement of molten metal in the Earth’s outer core generates electric currents. The currents create a magnetic field with invisible lines of force flowing between the 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 the Earth’s surface. The shifting locations of the geomagnetic poles are recorded in rocks that form when molten material called magma wells up through the Earth’s crust and pours out as lava. As lava cools and becomes solid rock, strongly magnetic particles within the rock become magnetized by the Earth’s magnetic field. The particles line up along the lines of force in the Earth’s field. In this way, rocks lock in a record of the position of the 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 the 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 the 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 magnetic north pole.

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 the Earth on the side facing the sun and stretches it into a teardrop shape on the shadow side.

The magnetosphere protects the 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 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 the 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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Writers

Kim Rutledge

Melissa McDaniel

Santani Teng

Hilary Hall

Tara Ramroop

Erin Sprout

Jeff Hunt

Diane Boudreau

Hilary Costa

Illustrators

Mary Crooks, National Geographic Society

Tim Gunther

Editors

Jeannie Evers, Emdash Editing, Emdash Editing

Kara West

Educator Reviewer

Nancy Wynne

Producer

National Geographic Society

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