MAGNETISM AND MATTER

INTRODUCTION: - In 800 B.C. shepherds on the island of Magnetia in Greece observed that the nails in their shoes were glued to the ground due to presence of magnetic ores in the area. The word magnet owes its origin to this island of Magnetia. Since 400 B.C., Chinese had been using magnetic needles to determine direction while sailing. Caravans used magnetic needles to navigate across the Gobi desert. In 1600 A.D. William Gilbert, a physician with Queen Elizabeth-I, wrote a book “De Magnet”. Some facts of magnetism given in the books are: (1) Earth behaves as a giant magnet with its magnetic field along north-south direction. (2) A bar magnet hung to freely rotate horizontally aligns itself along the north-south direction. The end pointing towards north is called its magnetic North Pole and the end pointing towards the south is called its magnetic South Pole. (3) Like magnetic poles repel each other, while unlike poles attract. (4) When a magnet is cut into pieces, each piece behaves as a magnet having both types of magnetic poles (dipoles). However, in an electric dipole, positive and negative charges can be separated and each is called an electric monopole. Magnetic monopole does not exist. (5) Magnets can be prepared using iron and its alloys.

MAGNETIC AND ELECTRIC FIELD LINES: (1) The magnetic field lines in a bar magnet or a solenoid form closed loops from north pole to south pole outside the magnet or solenoid and from south pole to north pole inside it. Electric field lines are not closed loops. They start from the positive charge and end on the negative charge in an electric dipole. (2) Tangent to a field line at a point indicates the direction of the field at that point. (3) The magnitude of field in a region is represented by the number of field lines per unit area in that region. (4) Field lines do not intersect one another.

EQUIVALENCE BETWEEN A BAR MAGNET AND SOLENOID: - The magnetic field produced by a solenoid is due to passage of electric current through it. How a magnetic field is produced by a bar magnet without any apparent observable current can be explained as under. A bar magnet is made up of atoms in which definite number of electrons move in various possible orbits. This constitutes electric current around a closed path. This current and the spin of electrons result in magnetic dipole moment. If the vector sum of magnetic dipole moments of all electrons is zero, then such a substance will NOT act as a bar magnet. Despite the fact that the atoms of iron possess magnetic dipole moment, an ordinary piece of iron does not behave as a magnet. It can be converted into a magnet by keeping in a strong magnetic field for some time and reducing the magnetic field slowly to zero. When a piece of iron is kept in a strong magnetic field, the elemental atomic currents get redistributed in the iron piece and do not return to the original current distribution on removal of the externally applied magnetic field.