A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.
A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic). These include the elements iron, nickel and cobalt and their alloys, some alloys of rare-earth metals, and some naturally occurring minerals such as lodestone. Although ferromagnetic (and ferrimagnetic) materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism.
Ferromagnetic materials can be divided into magnetically “soft” materials like annealed iron, which can be magnetized but do not tend to stay magnetized, and magnetically “hard” materials, which do. Permanent magnets are made from “hard” ferromagnetic materials such as alnico and ferrite that are subjected to special processing in a strong magnetic field during manufacture to align their internal microcrystalline structure, making them very hard to demagnetize. To demagnetize a saturated magnet, a certain magnetic field must be applied, and this threshold depends on coercivity of the respective material. “Hard” materials have high coercivity, whereas “soft” materials have low coercivity. The overall strength of a magnet is measured by its magnetic moment or, alternatively, the total magnetic flux it produces. The local strength of magnetism in a material is measured by its magnetization.
An electromagnet is made from a coil of wire that acts as a magnet when an electric current passes through it but stops being a magnet when the current stops. Often, the coil is wrapped around a core of “soft” ferromagnetic material such as mild steel, which greatly enhances the magnetic field produced by the coil.
Magnets have been used by people for a very long time. Hindu scriptures refer to medical applications of magnets as far back as the 40th century B.C.; the ancient Chinese, Greeks, Egyptians and Romans also used magnets with medicine. Magnets have helped ancient and modern explorers navigate, through the use of the compass. With the industrial revolution and then the advent of electricity, magnets came to be used in a wide variety of devices.
Uses of Magnets in Our Daily Life
Magnets is playing an important role in a wide range of devices including simple toys, computers, credit cards, MRI machines and business equipment. Magnets range in size from barely-visible specks to industrial monsters weighing tons. Though some are plainly visible, others are often tucked inside the inner workings of appliances and other household, medical and commercial items, doing their job silently and unseen.
Computers and Electronics
Many computers use magnets to store data on hard drives. Magnets alter the direction of a magnetic material on a hard disk in segments that then represent computer data. Later, computers read the direction of each segment of magnetic material to “read” the data. The small speakers found in computers, televisions and radios also use magnets; inside the speaker, a wire coil and magnet converts electronic signals into sound vibrations.
Electric Power and Other Industries
Magnets offer many benefits to the industrial world. Magnets in electric generators turn mechanical energy into electricity, while some motors use magnets to convert electricity back into mechanical work. In recycling, electrically-powered magnets in cranes grab and move large pieces of metal, some weighing thousands of pounds. Mines use magnetic sorting machines to separate useful metallic ores from crushed rock. In food processing, magnets remove small metal bits from grains and other food. Farmers use magnets to catch pieces of metal that cows eat out in the field. The cow swallows the magnet with its food; as it moves through the animal’s digestive system it traps metal fragments.
Health and Medicine
Magnets are found in some commonly used medical equipment such as and Magnetic Resonance Imaging machines. MRIs use powerful magnetic fields to generate a radar-like radio signal from inside the body, using the signal to create a clear, detailed picture of bones, organs and other tissue. An MRI magnet is very strong – thousands of times more powerful than common kitchen magnets. Another medical use for magnets is for treating cancer. A doctor injects a magnetically-sensitive fluid into the cancer area and uses a powerful magnet to generate heat in the body. The heat kills the cancer cells without harming healthy organs.
In the Home
Though it may not be obvious, most homes contain many magnets. Refrigerator magnets hold papers, bottle openers and other small items to the metal refrigerator door. A pocket compass uses a magnetic needle to show which way is north. The dark magnetic strip on the backside of a credit card stores data in much the same way as a computer’s hard drive does. Vacuum cleaners, blenders and washing machines all have electric motors that work by magnetic principles. You’ll find magnets in phones, door bells, shower curtain weights and children’s toys.