Magnetism - Magnetism is a force of attraction or replusion that acts at a distance. It is due to a magnetic field, which is caused by moving electrically charged particles or is inherent in magnetic objects such as a magnet.

Magnetism - A magnet is an object that exhibits a strong magnetic field and will attract materials like iron to it. Magnets have two poles, called the north (N) and south (S) poles. Two magnets will be attacted by their opposite poles, and each will repel the like pole of the other magnet. Magnetism has many uses in modern life.

Types of magnets

There are permanent magnets, temporary magnets and electromagnets.

Permanent magnets

A permanent magnet is one that will hold its magnetic properties over a long period of time.

Magnetite- Magnetite is a magnetic material found in nature. It is a permanent magnet, but it is relatively weak.

Alloys- Most permanent magnets we use are manufactured and are a combination or alloy of iron, nickel and cobalt. Rare-earth permanent magnets are a special type of magnet that can have extreme strength.

Temporary magnets

A temporary magnet is one that will lose its magnetism. For example, soft iron can be made into a temporary magnet, but it will lose its magnetic power in a short while.

By wrapping a wire around an iron or steel core and running an electrical current through the wire, you can magnetize the metal and make an electromagnet. If the core is soft iron, the magnetism will diminish as soon as the current is turned off. This feature makes electromagnets good for picking up and dropping objects. Typically DC electricity is used, but AC current will also result in an electromagnet.

Cutting a magnet- An interesting characteristic of magnets is that when you cut a magnet into parts, each part will have both N and S poles.

Attraction and repulsion- Magnets strongly attract iron, nickel and cobalt, as well as combinations or alloys of these metals.

Also, unlike poles of two magnets will attract, but like poles will repel. Thus, N and S attract, while S and S will repel each other.

Magnetic Force- The magnetic field of an object can create a magnetic force on other objects with magnetic fields. That force is what we call magnetism.

When a magnetic field is applied to a moving electric charge, such as a moving proton or the electrical current in a wire, the force on the charge is called a Lorentz force.

Factors Determining Magnetic Properties - There are three main factors that determine the magnetic property of a material.

  • The most basic factor is the configuration of the electrons in the material's atoms.
  • At the next level, the ability of the atoms or molecules in the material to align magnetically is important in determining whether the material responds to a magnetic field.
  • A final factor is the alignment of domains or sections in a solid object.

Magnetic Lines of Force

  • Magnetic lines of force are continuous and will always form closed loops.
  • Magnetic lines of force will never cross one another.
  • Parallel magnetic lines of force traveling in the same direction repel one another. Parallel magnetic lines of force traveling in opposite directions tend to unite with each other and form into single lines traveling in a direction determined by the magnetic poles creating the lines of force.
  • Magnetic lines of force tend to shorten themselves. Therefore, the magnetic lines of force existing between two unlike poles cause the poles to be pulled together.
  • Magnetic lines of force pass through all materials, both magnetic and nonmagnetic.
  • Magnetic lines of force always enter or leave a magnetic material at right angles to the surface.

Types of Magnetic Materials- On the basis of magnetic property, magnetic materials can be classified into following three types-

Ferromagnetic Materials-  Ferromagnetic and ferrimagnetic materials are the ones normally thought of as magnetic; they are attracted to a magnet strongly enough that the attraction can be felt. These materials are the only ones that can retain magnetization and become magnets; a common example is a traditional refrigerator magnet.

Paramagnetic Materials- Paramagnetic substances, such as platinum, aluminum, and oxygen, are weakly attracted to either pole of a magnet. This attraction is hundreds of thousands of times weaker than that of ferromagnetic materials, so it can only be detected by using sensitive instruments or using extremely strong magnets. Magnetic ferrofluids, although they are made of tiny ferromagnetic particles suspended in liquid, are sometimes considered paramagnetic since they cannot be magnetized.

Diamagnetic Materials- Diamagnetic means repelled by both poles. Compared to paramagnetic and ferromagnetic substances, diamagnetic substances, such as carbon, copper, water, and plastic, are even more weakly repelled by a magnet.

Common Uses of Magnets

  • Magnetic recording media: VHS tapes contain a reel of magnetic tape. Common audio cassettes also rely on magnetic tape. Similarly, in computers, floppy disks and hard disks record data on a thin magnetic coating.
  • Credit, debit, and ATM cards: All of these cards have a magnetic strip on one side.
  • Common televisions and computer monitors: TV and computer screens containing a cathode ray tube employ an electromagnet to guide electrons to the screen.
  • Speakers and microphones: Most speakers employ a permanent magnet and a current-carrying coil to convert electric energy (the signal) into mechanical energy (movement that creates the sound).
  • Electric guitars use magnetic pickups to transduce the vibration of guitar strings into electric current that can then be amplified.
  • Electric motors and generators: Some electric motors rely upon a combination of an electromagnet and a permanent magnet, and, much like loudspeakers, they convert electric energy into mechanical energy. A generator is the reverse: it converts mechanical energy into electric energy by moving a conductor through a magnetic field.
  • Medicine: Hospitals use magnetic resonance imaging to spot problems in a patient's organs without invasive surgery.
  • Chucks are used in the metalworking field to hold objects.
  • Compasses: A compass (or mariner's compass) is a magnetized pointer free to align itself with a magnetic field, most commonly Earth's magnetic field.
  • Art: Vinyl magnet sheets may be attached to paintings, photographs, and other ornamental articles, allowing them to be attached to refrigerators and other metal surfaces. Objects and paint can be applied directly to the magnet surface to create collage pieces of art. Magnetic art is portable, inexpensive and easy to create.
  • Toys: Given their ability to counteract the force of gravity at close range, magnets are often employed in children's toys, such as the Magnet Space Wheel and Levitron, to amusing effect.
  • Magnets can be used to make jewelry. Necklaces and bracelets can have a magnetic clasp, or may be constructed entirely from a linked series of magnets and ferrous beads.
  • Magnets can pick up magnetic items (iron nails, staples, tacks, paper clips) that are either too small, too hard to reach, or too thin for fingers to hold. Some screwdrivers are magnetized for this purpose.
  • Magnets can be used in scrap and salvage operations to separate magnetic metals (iron, cobalt, and nickel) from non-magnetic metals (aluminum, non-ferrous alloys, etc.).    Magnetic levitation transport, or maglev, is a form of transportation that suspends, guides and propels vehicles (especially trains) through electromagnetic force. The maximum recorded speed of a maglev train is 581 kilometers per hour (361 mph).
  • Magnets may be used to serve as a fail-safe device for some cable connections. For example, the power cords of some laptops are magnetic to prevent accidental damage to the port when tripped over. The MagSafe power connection to the Apple MacBook is one such example.

Curie Temperature - The Curie temperature (Tc) is the critical temperature beyond which a previously ferromagnetic material becomes paramagnetic. On the atomic level, below the Curie temperature the magnetic moments, contributed mainly by the electrons, are alligned in their respective domains and even a weak external field results in a net magnetization. As the temperature increases to Tc and above however, fluctuations due to the increase in thermal energy destroy that allignment. Tc for nickel is 631K, while that for iron is 1043K.

Earth's magnetic field –

  • Earth's magnetic field (also known as the geomagnetic field) is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun.
  • Its magnitude at the Earth's surface ranges from 25 to 65 microteslas (0.25 to 0.65 gauss). It is approximately the field of a magnetic dipole tilted at an angle of 11 degrees with respect to the rotational axis—as if there were a bar magnet placed at that angle at the center of the Earth.
  • Earth's field changes over time because it is generated by the motion of molten iron alloys in the Earth's outer core (the geodynamo).
  • The intensity of the field is greatest near the poles and weaker near the Equator.
  • It is often measured in gausses (G) but is generally reported in nanoteslas (nT), with 1 G = 100,000 nT.
  • A nanotesla is also referred to as a gamma (γ).
  • A strong refrigerator magnet has a field of about 100 G.

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