Magnetism
It is believed that around 2000 BC the Chinese had already discovered the magnetic property of natural iron ore. In a part of Turkey formerly called Magnesia, there are deposits of an iron one which has the property of attracting iron. As this ore was discovered by Greeks they called it magnetite. This natural magnet was used by mariners for navigation and was called "Lodestone".
Properties of Magnet
When a bar magnet is suspended freely, it always comes to rest in a north-south direction. This property is used in the construction of a mariner's compass.
Pole of a Magnet
The magnetic force appears to be concentrated at or near the ends of the magnet. These areas are called the magnetic poles.
Types of Magnets
There are two major types of magnets: Permanent magnets and Temporary magnets. Permanent magnets retain their magnetism over a long period of time whereas temporary magnets retain their magnetic properties only under certain conditions like presence of a strong magnet or an electric current in the vicinity.
Laws of Magnetism
To find out about laws of magnetism you can perform the following simple experiment.
Magnetic Induction
The nail retains its magnetism only as long as the magnet is held near it or in contact with it. It will even pick up several other nails. But is loses its magnetism as soon as the magnet is removed. Magnetism produced in this manner due to the presence of a magnet is called induced magnetism.
Magnetic properties of Steel and Iron
Iron gets magnetized faster but loses its magnetism as soon as the inducing magnet is removed. Hence soft iron is said to have high susceptibility but low retentivity. This property of soft iron is very useful in making temporary electromagnets where we need strong but temporary magnets. If the magnets used in these devices were to retain their magnetism for a longer period, the devices would not function properly.
Magnetization
Place the soft iron piece AB flat on a table and stroke it with a bar magnet from A to B with one end of a bar magnet as shown in figure. When the magnet reaches B it is lifted, and brought back to A to repeat the stroke. After a few strokes you will notice that A will possess the same polarity as the pole being rubbed with it.
Demagnetization
The best method to demagnetize is by keeping the magnet in a coil through which an alternating current is passed. When the current starts flowing through the coil, the magnet is pulled out of the coil in east-west direction and kept far away. The magnetic field produced due to alternating current changes its direction continuously. As the magnet is withdrawn far away the magnetization becomes weaker and weaker until finally the magnet is completely demagnetized.
Magnetic Keepers
If two magnets are placed side by side there will be mutual repulsion or attraction. This weakens the strength of the magnet. To prevent this, bar magnets are placed side by side with opposite poles near. A soft iron piece called a keeper is placed across the poles as shown in figure. This soft iron piece provides a path for the magnetic field lines to form a continuous loop. Thus it helps in preserving the magnetic field.
Theory of Magnetism
A completely satisfactory theory of magnetism has still not been formulated. At present it is assumed that magnetism is due to the movement of the electrons. The electrons circulate around the nucleus of the atom.
Magnetic Fields and Field Lines
If a small compass needle is placed at a point near the north pole of a bar magnet, the needle turns round on its pivot and settles in a fixed direction.
Neutral Points in a Magnetic Field
As a result of two magnetic fields acting at the same place, the resultant field has a special feature. At a particular point, if the compass needle does not point in any particular direction, then there is no net magnetic field at the point. Such a point is called Neutral point or the Null point.
The Earth as a Magnet
About AD1600, William Gilbert, who was a court physician of Queen Elizabeth of England, performed certain experiments using spherical lodestones. When he placed small, pivoted magnetic needles at different positions on this sphere, he found that their behavior was quite similar to that of magnetized needles at corresponding places on the earth's surface. Later he concluded that the earth behaves like a huge magnet.
Summary
Angle of dip is the angle made with the horizontal by the earth's total magnetic field.
