Magnet

A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but creates a force that pulls on certain metals, such as iron, steel, nickel, and cobalt. Magnets can either attract or repel other magnets, which makes them very useful in everyday life.

One common example of a magnet is the small magnet stuck to a refrigerator door to hold notes or pictures. These are called permanent magnets because they stay magnetized all the time. Permanent magnets are made from special materials, like alnico or ferrite, that keep their magnetic properties even without an external force.

Another type of magnet is called an electromagnet. This kind of magnet is made by running an electric current through a coil of wire. When the current flows, the coil becomes magnetic, but when the current stops, the magnetism disappears. Electromagnets are often used in machines and devices where controllable magnetism is needed, such as in motors and computers.

Discovery and development

Main article: History of electromagnetic theory

See also: Magnetism history

Long ago, people found special rocks called lodestones or magnetite. These rocks could pull iron toward them and were used to make the first magnetic compasses. People in places like Anatolia, India, and China knew about magnets more than 2,500 years ago.

Later, people learned that heating iron and letting it cool in the direction of Earth's magnetic field could make it a magnet. This helped make better compasses for travel. Scientists like Hans Christian Ørsted and Joseph Henry found that electric currents could create magnetic fields, which led to the making of strong electromagnets.

Physics

A magnet is a special material that makes an invisible force called a magnetic field. This force can pull certain metals, like iron, closer to the magnet or push other magnets away. How strong this force is and which way it pushes or pulls depends on the shape of the magnet and what it is made from.

Magnets have two ends called poles — a north pole and a south pole. Opposite poles pull together, while similar poles push apart. The Earth works like a very big magnet, with its magnetic north pole close to the geographic north pole. Some materials, like iron and steel, can become magnets or be pulled toward them, but others, like plastic or wood, do not feel this force at all.

Common uses

Magnets are used in many everyday items. For example, VHS tapes, audio cassettes, floppy disks, and hard disks store information using magnetic tape. Credit, debit, and automatic teller machine cards have a magnetic strip with your account information.

Older televisions and computer monitors used magnets to help show the picture. Magnets are important in sensors, speakers, and microphones. They help change electrical signals into sound. Electric guitars use magnetic pickups to change string vibrations into electrical signals.

In medicine, magnetic resonance imaging uses strong magnets to see inside the body without surgery. Compasses are magnets that point toward Earth's magnetic field to show direction. Magnets are also fun in toys, like the Magnet Space Wheel and Levitron. They can hold notes on refrigerator doors or be used to make jewelry.

Medical issues and safety

Further information: Electromagnetic radiation and health

Most scientists think that magnets do not harm health. But some people wonder if moving magnetic fields could cause health problems.

There are safety concerns with magnets. If someone has a pacemaker—a small device in the chest to help the heart—it can be affected by strong magnetic fields. People with pacemakers should avoid places with strong magnetic fields, like rooms with MRI machines.

Children should not swallow small magnets from toys. If more than one magnet is swallowed, it can be dangerous and harm the body. Rooms with powerful MRI machines keep out metal objects because the strong magnetic fields can move metal around.

Magnetizing ferromagnets

See also: Remanence

Ferromagnetic materials can become magnets in a few simple ways. One way is to heat the material above a special temperature called the Curie temperature. Let it cool inside another magnetic field, and tap it gently. This makes strong permanent magnets.

Another way is to place the object inside a magnetic field. After you take it out, it may stay a little magnetic. Moving it while it’s vibrating can help. You can also magnetize something by stroking it with another magnet, moving the magnet along the object in one direction again and again. Running an electric current through a coil near the object can also make it magnetic.

Demagnetizing ferromagnets

Magnetized materials can lose their magnetism in several ways. One way is by heating them past a special temperature, called the Curie temperature. This makes their magnetic properties disappear.

Another method uses an alternating magnetic field. This can demagnetize tools, erase credit cards, or wipe data from hard disks.

Mechanical shocks, like hammering, can also disrupt the alignment of magnetic domains and reduce magnetization. However, this might damage the object.

Types of permanent magnets

Many materials can act as magnets because of how tiny particles inside them, called electrons, behave. Some metals, like iron ore, cobalt, and nickel, are naturally magnetic. People have used these metals for a long time to study magnetism.

Today, we also make magnets from mixtures of materials. For example, ceramic magnets are made from a mix of iron oxide and other materials. This makes them cheap and easy to shape. There are also special magnets called rare-earth magnets. These are made from elements like samarium–cobalt and neodymium–iron–boron (NIB). They are very strong but cost more. Each type of magnet has its own uses, depending on how strong it needs to be and how much it costs.

Main article: Rare-earth magnet

Main article: Single-molecule magnet

Electromagnets

Main article: Electromagnet

An electromagnet is made by coiling a wire into loops, called a solenoid. When electricity flows through the wire, it creates a magnetic field, like a regular magnet. Wrapping the coil around a special material, like an iron nail, can make the magnetic field stronger.

Electromagnets are used in many important machines, such as particle accelerators, electric motors, junkyard cranes, and magnetic resonance imaging machines. They can also be shaped in special ways for science experiments.

Units and calculations

Magnets have special powers because of an invisible force called a magnetic field. This field can pull or push certain metals like iron. It can also attract or repel other magnets.

Scientists use different ways to measure these magnetic powers. The most common ways are called SI units, but physicists sometimes use other ways too. There are two main types of magnetic fields scientists learn about: one called B and another called H. These fields help us understand how magnets work and how they can move or change other objects.