InterviewSolution
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InterviewSolution
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What do you understand by "hysterisis"? How does this property infulence the choice of materials used in different appliances where electromagnets are used ? |
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Answer» Solution :Cycle of magnetisation : When a ferromagnetic SPECIMEN is slowly magnetised, the intensity of magnetisation varies with magnetic field through a cycle is called cycle of magnetisation. (2) Hysterisis : The LAGGING of intensity of magnetisation (I) and magnetic induction (B) behind magnetic field intensity (H) when a magnetic specimen is subjected to a cycle of magnetisation is called hysterisis. (3) Retentivity : The value of 1 for which H = 0 is called retentivity or residual magnetism. (4) Coercivity : The valueof magnetising force required to reduce I is zero in reverse direction of H is called coercive force or coercivity. (5) Hysterisis curve : The curve represents the relation between B or I of a ferromagnetic material with magnetising force or magnetic intensity H is known as Hysterisis curve. (6) Explanation of hysterisis loop or curve : (a) In fig, a closed curve ABCDEFA in H - I plane, called hysteris loop is shown in fig. (b) When ferromagnetic specimen is slowly magnetised, I increases with H. (c) Part OA of the curve shows that I increases with H. (d) At point A, the value I becomes constant is called saturation value. (e) At B, I has some value while H is zero. (f) In fig. BO represents retentivity and OC represents coercivity. (7) Uses : The properties of hysterisis curve, i.e., saturation, retentivity, coercivity and hysterisis loss help us to choose the material for specific purpose. (i) Permanent magnets : A permanent magnet should have both large retentivity and large coercivity. Permanent magnets are used in galvanometers, voltmeres, ammeters, etc. (ii) An electromagnet core : The electromagnet core material should have maximum induction field B even small fields H, low hysterisis loss and high initial permeability. (iii) TRANSFORMER cores, Dynamocore, Chokes, Telephone DIAPHRAGMS : The core material should have high initial permeability, low hysterisis loss and high specific resistance to reduce eddy currents. Soft IRON is the best suited material. 
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