Overview of Magnetism

Magnetism is a physical force by which certain materials and moving electric charges attract or repel other materials. It is one of the most familiar forces in everyday life, holding notes to a refrigerator and steering the needle of a compass, yet it comes from the behavior of electrons inside atoms. Magnetism is closely tied to electricity; together they make up the larger subject of electromagnetism.

Every magnet has two poles, called north and south. Like poles repel each other, while opposite poles attract. So far, no isolated single magnetic pole has ever been found in nature.

What causes magnetism

Magnetism comes from the motion and spin of electrons. A moving electric charge creates a magnetic field, and each electron also behaves like a tiny magnet because of its spin. In most atoms, electrons occur in pairs whose magnetic effects cancel out. When an atom has unpaired electrons, it has a net magnetic moment and can respond to a magnetic field. In some materials these atomic magnets line up over large regions called domains, producing a strong overall magnetism. The magnetic force and the way moving charges create a magnetic field are explored in separate lessons.

Magnetic poles and fields

A magnetic field is the region around a magnet where its force can be felt. By convention, field lines point out of the north pole and into the south pole. The field is strongest near the poles. Earth itself acts like a giant magnet, which is why a compass needle aligns north-south. Some surprising details about the planet’s poles are covered in the lesson on confusion about the north magnetic pole, and methods for detecting a magnetic field are explained separately.

Types of magnetic materials

Materials are grouped by how they react to a magnetic field:

• Ferromagnetic materials are strongly attracted and can keep their magnetism after the field is removed. Iron, nickel, and cobalt are the common elements that are ferromagnetic at room temperature. Above a characteristic temperature, called the Curie temperature, they lose this property.
• Paramagnetic materials, such as aluminum, are only weakly attracted because they have unpaired electrons but no aligned domains. The effect disappears when the field is removed.
• Diamagnetic materials, such as copper and bismuth, are weakly repelled by a magnetic field because all their electrons are paired.

The reasons behind these differences are discussed in classifications of magnetic materials and the factors determining magnetic properties. Coils of wire carrying current become electromagnets, linking magnetism back to electricity.