Notes
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Topics to be Learn:
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Magnetism :
- The concept that the earth itself is a gigantic magnet has been put forward by William Gilbert for the first time.
- Names given to the two ends of a magnet are North pole and South pole. Because the ends of the magnet always indicate the north and south directions respectively.
- Unlike magnetic poles attract each other That means north pole of one magnet attracted towards the south pole of the other magnet.
Methods of producing magnet : Magnetism in an steel bar can be produced by single-touch method and double-touch method.
Single-touch method : In this method ‘N’ pole of one bar magnet is rubbed over a steel bar from the end A to the end B. On repeating this procedure 15-20 again and again magnetism is developed in the steel bar. This method is called the single-touch method. Double-touch method : Taking two bar magnets and placing their two opposite poles at the centre of the steel bar, both the magnets are rubbed over the steel bar from one end to the other such as south pole of one of magnet is at the ‘A’ end and the north pole of the another bar magnet is at the ‘B’ end. On repeating this procedure 15 to 20 times again and again, magnetism is developed in the steel bar. This method is called the double-touch method.
Characteristics of magnets :
- A freely suspended magnet always settles in the north-south direction.
- Objects made up of iron, nickel and cobalt are attracted towards the magnet.
- There is repulsion between the like poles of the magnet and attraction between the unlike poles of the magnet.
Metals used for making magnets : Alloys of iron. cobalt and nickel :
Magnetic Needle :A magnetized needle that can freely rotate in a horizontal plane is used to indicate the north and south direction.
Magnetic Compass : Magnetic needle is used in the magnetic compass to indicate the direction.
Magnetic Field : Michael Faradey proposed the idea of magnetic lines of force to explain the magnetic field.
- The region or space around the magnet in which the magnetic force acts on an object, is called a magnetic field. The magnetism of a magnet acts upto a certain distance from the magnet.
Magnetic Lines of Force : The invisible lines going from one to the other end of the magnetic strip are called magnetic lines of force.
Properties of Magnetic Lines of Force : The British Scientist Michael Faradey proposed the idea of magnetic lines of force for the first time. The properties of these lines of force are as follows : 
Intensity of Magnetic Field:
Penetrating ability of the magnetic field : Magnetic field can be passed through a cardboard, water as well as a plastic bottle. This property is called penetrating ability of the magnetic field,
Electromagnet : A temporary magnetism is produced in an iron when an electric current in passed through a soft iron strip. If the current is stopped, the electromagnet loses magnetism, This type of magnet is called an electromagnet.
Metal Detectors and their uses :
Q. What is the difference between gravitational force and magnetic force?
Ans. The gravitational force acts on any object which has mass. There is only attraction, no repulsion in case of the gravitational force. The magnetic force does not act on an object. It acts between magnets as well as magnets and magnetic substances. Both attraction and repulsion take place in case of the magnetic force.
Q. Why is repulsion the real test for identifying a magnet?
Ans. If there is an attraction between two objects, there is possibility that one of them can be magnet and other object can be substance like an iron. We cannot say surely both are magnets. However, if there is attraction between the ends of these objects and after keeping an end of one object unchanged, the end of the other object is changed, and if repulsion take place between these two ends of the objects, we can surely say that both the objects are magnets. Because, there is repulsion between like poles of the magnet and attraction between unlike poles of the magnet. Thus, repulsion is the real test for identifying a magnet,