What Is Magnetism

The knowledge of electricity and Magnetism has existed for around 2000 years. Yet, the realization of the existence and relation came in the year 1820. While lecturing in the university, Hans Christian Oersted , a Danish physicist noticed that a current in a straight wire caused a noticeable deflection in a nearby magnetic compass needle. He investigated this phenomenon. Oersted saw a weird alignment of the needle which was tangential to an imaginary circle which has the straight wire as its center and has its plane perpendicular to the wire. This is  detectable when the current is high while the needle is sufficiently close to the wire so that the earth’s magnetic field may be ignored. Overturning the flow of the current reverses the orientation of the needle. The deflection increases on increasing the current or bringing the needle closer to the wire. Iron filings dispersed around the wire arrange themselves in concentric circles with the wire as the center. Oersted concluded that moving charges or currents produced a magnetic field in the surrounding space.

The science of magnetism is old. Specific properties of magnetic materials such as pointing in the north-south direction, repulsion when like magnetic poles meet and attraction with unlike poles; and cutting a bar magnet in two leads to two smaller magnets were very much present in prehistoric era. Even our ancestors knew that magnetic poles cannot be isolated. When a bar magnet of dipole moment m is placed in a consistent magnetic field B, (a) the force on it is zero, (b) the torque on it is m ~ B, (c) its potential energy is .m.B, where we choose the zero of energy at the orientation when m is perpendicular to B.  Gauss fs law for magnetism states that the net magnetic flux through any closed surface is zero.

The earth’s magnetic field resembles that of a (hypothetical) magnetic dipole situated at earth’s centre. The north magnetic pole corresponds to the pole near to earth’s geographic North Pole. Similarly, the south magnetic pole  is the pole near to earth’s geographic South Pole. This dipole is aligned making a tiny angle with the rotation axis of the earth. The magnitude of the field on the earth’s surface (4 x10 inverse 5 T) is to specify earth’s magnetic field of on its surface, three quantities are required such as the magnetic declination, horizontal component, and the magnetic dip. These are known as the elements of the earth’s magnetic field. Magnetic materials are broadly classified as: diamagnetic, paramagnetic, and ferromagnetic. For diamagnetic materials ÷ is negative and small and for paramagnetic materials it is positive and small. Ferromagnetic materials have large ÷ and are characterized by non-linear relation between B and H. They show the property of hysteresis. Substances, which at room temperature, retain their ferromagnetic property for a long period of time are called permanent magnets.

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