FORM 2-PHYSICS-TOPIC 3-MAGNETISM
MAGNETISM
The Concept of Magnetisation and Demagnetisation
Explain the concept of magnetisation and demagnetisation
Magnetisation:Is a process of making a magnet from a magnetic substance.
Demagnetisation:Is the process by which a magnet is made to lose its magnetism.
Demonstrating Magnetisation and Demagnetisation
Demonstrate magnetisation and demagnetisation
Methods of magnetisation
There are various ways, including;
- Induction
- Stroking
- Electrical method
Induction method.
This is done by placing a piece of unmagnetised steel bar near or in contact with a pole of a magnet and then removing it.
In this case an iron nail placed near a bar magnet will be induced with magnetism.
Stroking method
This is done by stroking a bar magnet into an unmagnetised steel bar.There are two stroking methods, namely;
- Single touch: A magnetised bar magnet is formed by a single stroke.-A steel bar is stroked repeatedly by a very strong bar magnetism the same direction with the north pole i.e from A to B. The bar magnet is lifted at B and then returned at A. After several strokes the steel bar will be magnetised with north pole at A and south pole at B.
- Double touch: Two bar magnets are used to magnetise a single steel bar. The steel bar is magnetised by two bar magnets from its center to its ends using left and right hands simultaneously for several times. Between each stroke the two bar magnets are lifted up high and returned to the center for another stroking.
In this case the steel bar will be magnetized with south pole at A and north pole at B.
Note:
- In both single and double touch methods, the magnetising magnet none of their strengths.
- Between successive strokes, the pole is lifted high above bar, otherwise the magnetism already induced in it will tend to be weakened.3.Consequent poles will be formed at the center of the steel bar when the two bar magnets placed at the center are of like poles. Same poles will be obtained on both ends.
Electrical method
A cylindrical coil wound with many of insulated copper wire is connected in series with a battery. A steel bar is placed inside the solenoid and the current switched on and off.When the steel bar is removed and tested, it is found to be magnetised.
Note:If the current is switched off for so long, the bar will not be magnetized.The poles of the bar magnet depends on the direction of flow of current. The end at which the direction of the current is in clockwise direction will be south pole and if anticlockwise it will be north pole.
Methods of demagnetization
Electrical method
The magnet is placed inside a solenoid through which an a. c is flowing.The magnet is withdrawn from the solenoid while the current is flowing pointing in the W-E direction. When the magnet is held in W-E direction, it doesn’t remain with residue magnetism due to induction from earths magnetic field.
Other methods of demagnetisation include;
- Heating a magnet.
- Hammering while pointing E-W direction.
Methods of Storing Magnets
Design methods of storing magnets
Magnets are stored in magnetic keepers.They are stored in pairs,with unlike poles together and with pieces of iron(magnetic keepers) across both ends.The keepers are magnetised by induction.
