Question:
Published on: 21 June, 2022

What is meant by hysteresis in a magnetic circuit? What is the significance of B-H curve?

Answer:

The magnetization curve is useful to explain the magnetic properties of a ferromagnetic material like iron, cobalt and nikel. The magnetization curve is also called B-H curve or hysteresis loop which is a plot of internal magnetic flux density against external magnetic field.

So, hysteresis is the lagging of intensity of magnetization behind the applied magnetizing field in a complete cycle of magnetization.

Let us consider a magnetized material whose domains are aligned in a random fashion so that the internal magnetization is zero. This situation corresponding to the point O of Fig. 24(a). As a magnetic field increases from O to maximum value $$H_m$$, the magnetic flux density increases along the path OP which is referred as the initial magnetization curve. The flux density reaches at its saturated value $$B_m$$ when the external field is $$H_m$$.

Fig. 24(a)

Fig. 24(b)

When ferromagnetic material such as iron ring is subjected to an external magnetic field H and if the magnetic field increases continuously, the magnetic flux density B also increases. The variation of B with H shown in Fig. 24(b) is known as B-H curve. The ferromagnetic material can be strongly magnetized by applying a high magnetic field. After the removal of magnetic field they retain the magnetization. They lose ferromagnetism completely when heated above certain temperature known as Curie temperature. They exhibit non – linear B-H characteristics and relative permeability $$\mu_r$$ is a function of magnetic flux density, $$B=\mu_0\mu_rH$$, where relative permeability $$\mu_r$$ is varied with the flux density B.

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