Monday, December 3, 2012

CHAPTER 3 MAGNETISM AND ELECTROMAGNETISM



3.1       MAGNETISM




Long ago, in the middle ages, it was found that a mineral called ‘lodestone’, which is in fact an iron ore, attracted small iron objects.  So ‘magnetism’, which is a natural phenomenon, was discovered.  It got its name from a district in Asia Minor called ‘Magnesia’ where lodestone was found.



It was found too that an iron bar or needle, if rubbed with lodestone, could also be made to attract small pieces of iron - that is, the magnetism could be imparted from the lodestone to the iron.  Such a needle, if placed on a wooden raft and left to float in a bowl of water, always tended to lie in a rough North to South direction.  So we had the first primitive compass.


At this stage it never occurred to anybody that there was any connection whatever between magnetism and electricity, and it was left to Faraday to bring these two together in ‘electromagnetism’. 



3.2       ELECTROMAGNETISM




In 1820 Oersted discovered that an electric current flowing in a wire caused a magnetic field around it.  This can easily be detected by placing a small compass near the wire and observing the movement of the needle when current is switched on.  This is shown in Figure 3.1.



Figure 3.1 MAGNETIC FIELD AROUND A CONDUCTOR



The effect can be intensified by bending the wire into a loop.  The magnetic fields from each bit of the wire are brought together inside the loop, where the magnetic field is concentrated and intensified.



                                     WITHOUT IRON                                                        WITH IRON
                     

                           Figure 3.2 MAGNETIC FIELD AROUND A COILED CONDUCTOR

If now the wire is bent into several loops, or a ‘helix’, as shown in Figure 3.2, the magnetic fields of each ‘turn’ are superimposed, and the field down the middle is still further intensified.  The result is a ‘coil’ which, when current flows in it, produces an artificial magnet, called an ‘electromagnet’.  Unlike a natural magnet, whose magnetism is always present, an electromagnet can be switched on or off at will.

If iron is introduced inside the coil, the magnetic strength is still further increased, and ‘permanent’ magnets can be made this way.  Very powerful electromagnets can be built, which are widely used: they can actuate solenoids or valves directly; they can drive any device needing a fore-and-aft motion; and they are used with cranes in scrap-yards for picking up large weights of scrap-iron.  On a smaller scale they are used to operate relays and switching devices.

Although it may not at first seem so, solenoids and other electromagnets operate just as well with alternating as with direct current.



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