Electromagnets
Electromagnets are made of soft iron because area of hysteresis loop for soft iron is small. Therefore, energy loss is small for a cycle of magnetisation and demagnetisation.
( Permanent magnets are made by the materials such as steel, for which residual magnetism as well as coercivity should be high. Electromagnets are made by the materials such as soft iron for which residual magnetism is high, coercivity is low and hysteresis loss is low).
Important Points
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Magnetic length = 5 / 6 * geometric length of magnet. �
About 90 % of magnetic moment is due to spin motion of electrons and
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remaining 10 % of magnetic moment is due to the orbital motion of electrons. �
When a magnet having magnetic moment M is cut into two equal parts�
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( i) Parallel to its length |
M’ = m / 2 * l = M / 2
( ii) Perpendicular to its length M’ = m * l / 2 = M / 2
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When a magnet of length I, pole strength m and of magnetic moment M is turned into a semi-circular arc then it new magnetic moment�
M’ = m * 2R = m * 2 * 1 / π( πR = I) = 2M / π( M = m * l)