Name: Magnetic Susceptibility and Relative Magnetic Permeability
Uploaded: 2019-07-19
Duration: 7 min 13 s
Description: Magnetic susceptibility and relative magnetic permeability sound like complicated terms, but they're not. In this video, we will learn what they are and the...

Question

Draw a neat sketch of a cycle of magnetisation ( $B - H$ loop/hysteresis loop) and explain the term: residual magnetisation (retentivity).

Accepted Answer

Hysteresis Curve : When a ferromagnetic substance is placed in a magnetic field, it is magnetised by induction. If we vary the magnetic intensity $H$ of the magnetising field, the intensity of magnetisation $M$ and the magnetic flux density $B$ in the (ferromagnetic) substance do not vary linearly with $H .$ In other words, the susceptibility $X_{m} (= \frac{M}{H})$ and the permeability $μ (= \frac{B}{H})$ of the substance are not constants, but vary with $H$ and also depend upon the past history of the substance.

The variation in $M$ with variation in $H$ is shown in Fig. 13. The point $O$ represents the initial demagnetised state of the substance $(M = 0)$ and a zero magnetic intensity $(H = 0)$ .

As $H$ is increased, $M$ increases non-uniformly along OA. As the substance acquires a state of magnetic saturation then any further increase in $H$ does not produce any increase in $M$ .

If now the magnetising field $H$ is decreased, the magnetisation $M$ of the substance also decreases following a new path $AB$ (not the original path $AO$ ). Thus, $M$ lags behind $H$ . When $H$ becomes zero, $M$ still has a value equal to $OB .$

The magnetisation remaining in the substance when the magnetising field is reduced to zero is called the “residual magnetism”. The power of retaining this magnetism is called the “retentivity” or the “remanence” of the substance. Thus, the retentivity of a substance is a measure of the magnetisation remaining in the substance when the magnetising field is removed. In Fig. 13, OB represents the retentivity of the substance.

If now the magnetising field $H$ is increased in the reverse direction, the magnetisation $M$ decreases along $BC,$ still lagging behind $H$ , until it becomes zero at $C$ where $H$ equals $OC .$ . The value $OC$ of the magnetising field is called the “coercive force” or “coercivity” of the substance. Thus, the coercivity of a substance is a measure of the reverse magnetising field required to destroy the residual magnetism of the substance.

As $H$ is increased beyond $OC$ , the substance is increasingly magnetised in the opposite direction along $CD .$ At $D$ the substance is again magnetically saturated.

By taking $H$ back from its maximum negative value (through zero) to its original maximum positive value, a symmetrical curve $DEFA$ is obtained. At points $B$ and $E$ where the substance is magnetised in the absence of any external magnetising field, it is said to be a “permanent magnet”.

It is thus found that the magnetisation $M$ (and also the magnetic flux density $B$ ) always lags behind the magnetising field $H$ . The lagging of $M$ (or of $B$ ) behind $H$ is called “hysteresis.

The closed curve $ABCDEFA$ which represents a cycle of magnetisation of the substance is known as the “hysteresis curve (or loop)” of the substance. On repeating the process, the same closed curve is traced again but the portion $OA$ is never obtained.

Draw a neat sketch of a cycle of magnetisation ( $B - H$ loop/hysteresis loop) and explain the term: residual magnetisation (retentivity).

Important Questions on Magnetic Classification of Substances

Learn and Prepare for any exam you want !

Draw a neat sketch of a cycle of magnetisation (B-H loop/hysteresis loop) and explain the term: residual magnetisation (retentivity).

Important Questions on Magnetic Classification of Substances

Learn and Prepare for any exam you want !

Draw a neat sketch of a cycle of magnetisation ( $B - H$ loop/hysteresis loop) and explain the term: residual magnetisation (retentivity).