Graph between flux density \(B\) and magnetising force \(H\)

The shape of the \(B-H\) curve is non-linear

Indicates \(\mu_r = B/\mu_0 H\) is not constant but it varies

\(\mu_r\) largely depends upon the \(B\)

Non-magnetic material \(\rightarrow\) straight line \(\rightarrow\) \(B=\mu_0H\) \(\rightarrow\) \(B \propto H\)

Phenomenon of flux density \(B\) lagging behind the magnetising force \(H\)

Hysteresis is the term derived from the Greek word hysterein meaning to lag behind

When \(H\) is increased gradually by increasing \(I\) in the solenoid (by decreasing the value of \(R\)), the \(B\) also increases until saturation point a is reached and curve so obtained is oa.

If now \(H\) is gradually reduced to zero by decreasing \(I\) in the solenoid to zero, the \(B\) does not become zero and the curve so obtained is ab

When \(H\) is zero, the \(B\) still has value ob

Residual Magnetism and Retentivity

\(B\) ob retained is called residual magnetism and the power of retaining this residual magnetism is called retentivity of the material

To demagnetise \(H\) reversed the direction of \(I\) in the solenoid.

achieved by changing the position of double pole, double throw switch (i.e., position ‘2’).

When H is increased in reverse direction, the \(B\) starts decreasing and becomes zero and curve follows the path bc.

Thus residual magnetism is wiped off by applying \(H\) oc in opposite direction

Coercive Force

This value of \(H\) oc required to wipe off the residual magnetism is called coercive force

To complete the loop, the \(H\) is increased further in reverse direction till saturation reaches (point ‘d’) and the curve follows the path cd.

Again \(H\) is reduced to zero and the curve follows the path de.

Where oe represents the residual magnetism.

Then H is increased in the positive direction by changing the position of reversible switch to position ‘1’ and increasing the flow of current in the solenoid.

The curve follows the path of efa and the loop is completed.

Again of is the magnetising force utilised to wipe off the residual magnetism oe.

Hence, cf is the total coercive force required in one cycle of magnetisation to wipe off the residual magnetism.

loop (abcdefa) so obtained is called hysteresis loop.