Properties of Magnetic Field Lines
- They are closed and continuous curves
- They are directed from the N-pole towards the S-pole of the magnet(out side the magnet)
- The tangent at any point on a field line gives the direction of magnetic field at that point
- They never intersect one another
- They are crowded near the poles of the magnet where the magnetic field is strong and are far separated near the middle of the magnet
Types of Magnets
Bar magnets .Horseshoe magnets bring the N and S poles close together, and this gives a strong magnetic field between the poles. They are used to lift heavy weights. With older magnetic materials it is necessary to place a ‘keeper’ across the poles when the magnet is stored to prevent the loss of magnetism.
The magnet used in a loudspeaker is very similar to the horseshoe magnet. The central pillar is made of one pole, and on the other side of a small gap, the opposite pole surrounds it. The coil of the loudspeaker moves in this gap.
Electromagnets are made out of a coil of wire (often called a solenoid). When an electric current is passed through the coil, a magnet is formed with the N- pole at one end of the coil and the S-pole at the other end. If the coil is wrapped around a magnetically soft core, then when the .coil is magnetized, it magnetizes the core as well, and a very much stronger magnetic field is made. When the current is switched off, the coil loses its magnetism, so the core does as well.
Methods of Magnetization
Permanent magnets are usually magnetised by putting them into a coil of wire and passing a large direct current of electricity through the coil for a moment. Very high currents are used, and special equipment must be used to make the method safe.
With some modern alloys, the magnetic field is applied to the molten alloy at very high temperature. and then the alloy is allowed to solidify while remaining in the magnetic field.
Steel can be magnetized to a certain degree by placing the bar N-S and hammering it. Ships become magnetised in this way during manufacture, as the hammering allows the Earth’s field to magnetise them slightly.
Methods of Demagnetization
The only method of demagnetisation that is guaranteed to work is to heat the magnet. All magnets have a temperature (called the Curie temperature) at which they lose their magnetism. This temperature ranges from less than 100 C to over 500 C. When the: magnet is cooled down,some of the magnetism may return.
Many magnets can be demagnetised by placing them in a coil of wire connected to an alternating electric current (a.c.) source. The current is switched on and while the a.c. current is flowing the object is slowly taken out of the coil. In this way the object is magnetised in the opposite direction each time the current reverses. but as it is removed from the coil the amount of magnetisation is reduced each time. Hammering or even dropping a powerful magnet may cause the loss of some of its magnetization.