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Magnetic Field & Magnetism


Magnetism is a fundamental property of all forms of matter. Some materials are called non-magnetic because they exhibit very weak magnetic property.

Only the metallic elements, iron, nickel and cobalt, and certain of their alloys are strongly magnetic. Such substances are called ferromagnetic. Magnetism is also associated with electricity. A conductor carrying an electric current has a magnetic field around it as a result of the movement of the electric charged electrons that constitute the current. All magnetic effects are caused by movements of the electric charges.

This is true even for the substances that behave as magnets without being connected to an external electricity supply. Their magnetic fields originate from movements of electrons within the atoms or molecules of the substance itself. For this reason, magnetism and electricity are not independent phenomena. Many of the basic concepts of magnetism are similar to those of electrostatics (the study of electric charges).

Magnetic Pole and Magnetic Field:

There are two types of magnetic poles north-seeking (or north) poles, and south seeking (or south) poles. Like poles repel each other, where unlike poles attract each other. The stronger the poles the larger are the repulsive or attractive force between them. The force of magnetism reduces with increase in distance. A magnet produces it strongest effects near its poles, but its influence also extends till roughly out the space surrounding it.

The region in which this influence can be detected is known as a magnetic field. A magnetic field can be represented by field lines, which indicate its strength and direction. Where the lines are close together, the field is stronger. Magnetic Effect of an Electric Current being a flow of charges produces a magnetic field. The larger the current the stronger is the magnetic field it produces. If the current is flowing in a wire, the shape of the magnetic field depends on the configuration of the wire. A single straight wire carrying a current for example, has a cylindrical magnetic field surrounding it. Two straight current carrying wires, side by side attract each other when their currents flow in the same direction and repel each other when their currents flow in opposite directions.

Magnetic Compass:

A magnetic compass is a device consisting of a magnetic needle and a dial slue. Magnetic compass points to the earth's magnetic north pole (rather than to the true, geographical North Pole). The earth itself has a magnetic field. The shape of this field is similar to that which would be produced by an enormous bar magnet with its south pole in the northern hemisphere and its north pole in the southern hemisphere, and with its axis slightly inclined to the earth's axis of rotation.

Magnetic Field of the Earth:

A needle of the compass points north because the Earth's magnetic field is bipolar, it has the shape of a field that would result from a very strong bar magnet situated at the center of the earth (aligned roughly along the axis of the earth). A freely suspended needle would tilt down toward in the Northern Hemisphere, up in the Southern Hemisphere and would remain horizontal pointing north at the equator. The magnetic axis is inclined about ll° to the axis of rotation, so a compass needle points only approximately north at most places. In this way geographic poles are different from magnetic poles. The magnetic north and south poles and equator are all somewhat different from geographic ones.

A magnetic field is a field of force produced by moving electric charges, by electric fields that vary in time, and by elementary particles that possess their own 'intrinsic' magnetic field, a relativistic effect which is usually modeled as a spin of the particle. The Lorentz force equation provides the modern physics definition of the magnetic field as the force field responsible for the velocity-dependent force on a charged particle. Magnetic fields are also generated by magnetic objects, like permanent magnets, where the source of the field can be traced back to one of the three fundamental sources.

The magnetic fields due to and within magnetic materials is described using two separate fields which can be both called a magnetic field: a magnetic B field and a magnetic H field. The magnetic field at any given point is specified by both a direction and a magnitude (or strength); as such it is a vector field. This is usually illustrated using magnetic field lines. These lines are strictly a mathematical concept and do not exist physically. Certain physical phenomena, such as the alignment of iron filings and, more generally, of magnetic dipoles in a magnetic field, produce lines in a similar pattern to the imaginary magnetic field lines of the object.

In electromagnetism, magnetic fields are intimately related to electric fields; a changing magnetic field generates an electric field and a changing electric field produces a magnetic field. The full relationship between the electric and magnetic fields, and the currents and charges that create them, is described by the set of Maxwell's equations. In view of special relativity, electric and magnetic fields are two interrelated aspects of a single object, called the electromagnetic field. A pure electric field in one reference frame is observed as a combination of both an electric field and a magnetic field in a moving reference frame. In quantum physics, this electromagnetic field is understood to be caused by virtual photons.

Most often this quantum description is not needed because the simpler classical theory is sufficient. Magnetic fields have had many uses in ancient and modern society. The Earth produces its own magnetic field, which is important in navigation since the north pole of compass points toward the south pole of Earth's magnetic field, located near the Earth's geographical north. Rotating magnetic fields are utilized in both electric motors and generators. Magnetic forces give information about the charge carriers in a material through the Hall Effect. The interaction of magnetic fields in electric devices such as transformers is studied in the discipline of magnetic circuits.

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