Magnetism
34 Magnets
Learning Objectives
- Describe the difference between the north and south poles of a magnet.
- Explain how magnetic poles attract and repel one another.
Magnets are familiar objects that can attract materials containing iron, such as paper clips or the steel surface of a refrigerator door. They can also exert forces on other magnets, either attracting or repelling them depending on their orientation. Careful experiments show that every magnet has two distinct regions called magnetic poles. If a magnet is free to rotate, one end consistently points toward Earth’s geographic north, and this end is called the north magnetic pole (or more precisely, the north-seeking pole). The opposite end is called the south magnetic pole (the south-seeking pole).
Universal Characteristics of Magnets
All magnets obey two simple rules:
- Like magnetic poles repel.
- Unlike magnetic poles attract.
This behavior resembles electrostatic forces, where like electric charges repel and opposite charges attract. Unlike electric charges, however, magnetic poles cannot be separated from one another.
Misconception Alert: Why Does a Compass Point North?
A compass points toward Earth’s geographic North Pole because the region near the geographic North Pole behaves like a magnetic south pole. Since opposite magnetic poles attract, the north-seeking end of the compass needle is attracted in that direction. Although this terminology can seem confusing, it is based on the direction in which a compass needle points rather than on the actual magnetic polarity of Earth.
The fact that magnetic poles always occur in north–south pairs appears across many different length scales. Large astronomical objects such as Earth and the Sun possess magnetic fields with north and south poles, while individual atoms behave like tiny magnetic dipoles because of the motion of electrons and their intrinsic magnetic properties (spin). Although elementary particles such as electrons, protons, and neutrons possess magnetic dipole moments, no isolated magnetic north or south pole—a magnetic monopole—has ever been experimentally detected.
Explore at Home: Refrigerator Magnets
Use two refrigerator magnets to investigate magnetic interactions.
- Can you find an orientation where the magnets repel each other?
- Why do the magnets stick to a refrigerator door but not to most plastic objects?
- Test different household items, such as a steel spoon, an aluminum can, and a plastic utensil. Which materials are attracted by the magnets? What does this tell you about their composition?
This simple activity illustrates that magnetic forces depend both on the orientation of magnetic poles and on the magnetic properties of different materials.
Although permanent magnets may appear to be simple objects, the concepts introduced in this chapter provide the foundation for many important technologies. Magnetic fields generated by powerful superconducting magnets make magnetic resonance imaging (MRI) possible, while miniature magnetic sensors are widely used in biomedical instrumentation, navigation systems, and scientific research. In the following chapters, you will learn how moving electric charges produce magnetic fields and how electricity and magnetism are fundamentally connected.
Section Summary
- Every permanent magnet has two magnetic poles: a north pole and a south pole.
- If a magnet is free to rotate, its north-seeking pole points approximately toward Earth’s geographic North Pole.
- Like magnetic poles repel each other, whereas unlike magnetic poles attract.
- Unlike electric charges, magnetic poles cannot be isolated. Cutting a magnet into smaller pieces produces smaller magnets, each with its own north and south poles.
- Earth behaves approximately like a giant magnet, producing the magnetic field that causes compass needles to align in the north–south direction.
Conceptual Questions
- Volcanic activity along the Mid-Atlantic Ridge continuously creates new oceanic crust as tectonic plates move apart. Rocks formed at the ridge preserve the direction of Earth’s magnetic field at the time they solidify. Scientists observe that the magnetic orientation of these rocks alternates in a symmetrical pattern on either side of the ridge. What does this tell us about Earth’s magnetic field over time? How could measuring the rate of seafloor spreading help reconstruct the history of Earth’s magnetic field?
Glossary
- north magnetic pole
- The end of a magnet that, when freely suspended, points approximately toward Earth’s geographic North Pole (also called the north-seeking pole).
- south magnetic pole
- The end of a magnet that, when freely suspended, points approximately toward Earth’s geographic South Pole (also called the south-seeking pole).
The end of a magnet that, when freely suspended, points approximately toward Earth's geographic North Pole (also called the north-seeking pole).
The end of a magnet that, when freely suspended, points approximately toward Earth's geographic South Pole (also called the south-seeking pole).
