Magnetising force which is necessary to neutralise completely the magnetism is an electromagnet after the value of magnetising force becomes zero. The given statement is true for:

Magnetising force which is necessary to neutralise completely the magnetism is an electromagnet after the value of magnetising force becomes zero. The given statement is true for: Correct Answer coercive force

Hysteresis Loop:

Consider an un-magnetised iron bar AB wound with N turns as shown in Fig. below.

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The magnetising force H (= NI/l) produced by this solenoid can be changed by varying the current through the coil.

The double-pole, double-throw switch (DPDT) is used to reverse the direction of current through the coil.

We shall see that when the iron piece is subjected to a cycle of magnetisation, the resultant B-H curve traces a loop abcdefa called a hysteresis loop.

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Step 1:

• When the current in the solenoid is zero,  then, B, H = 0.
• As H is increased,  the flux density (+ B) also increases until the point of maximum flux density (+ B_max) is reached.
• The material is saturated and beyond this point, the flux density will not increase regardless of any increase in current or magnetising force.
• Note that the B-H curve of the iron follows the path oa.
   

Step 2:

• If now H is gradually reduced, it is found that the flux density B does not decrease along the same line by which it had increased but follows the path ab.
• At point b, the magnetising force H is zero but flux density in the material has a  finite value + Br (= ob) called residual flux density.
• The power of retaining residual magnetism is called retentivity of the material.
   

Step 3:

• To demagnetise the iron piece (i.e. to remove the residual magnetism ob), the magnetising force H is reversed by reversing the current through the coil.
• When H is gradually increased in the reverse direction, the B-H curve follows the path bc so that when (H = oc), the residual magnetism is zero.
• The value of H (= oc) required to wipe out residual magnetism is known as coercive force (Hc).