Why is residual magnetism is necessary?

Answer 1

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Answer 2

'Residual magnetism' isn't something that's 'necessary'; rather, it's something you're stuck with, whether you want it or not! Residual magnetism is due to a phenomenon called 'hysteresis', which is derived from a Greek word, meaning 'to lag'.

A bit of background first. If we were to wind an insulated coil around the sample of ferromagnetic material, and pass a current through that wire, we would create and apply magnetic field strength (symbol: H), expressed in amperes per metre, to that sample. This results in a magnetic field being set up within the sample, the intensity of which we call its flux density (symbol: B) expressed in teslas.

If we gradually increase the magnetic field strength, the resulting flux density would also increase until a point, called 'saturation' is reached -at this point any further increase in magnetic field strength will NOT increase the flux density. If we were to graph this behaviour, then the result would look something like an elongated 'S', rather than a straight line. This graph is known as a B-H curve.

Now, if we were to reduce the magnetic field strength to zero, the magnetic flux density would also reduce towards zero (following a slightly-different curve) but would not reach zero when the magnetic field strength reaches zero -in other words, when we remove the magnetic field strength, the sample 'retains' some flux density -and we call this 'residual magnetism' or, more accurately, 'residual flux density' or 'remanance'. This is what we mean by 'hysteresis' -i.e. changes in magnetic flux density lag behind changes in magnetic field strength.

To remove this residual flux density, we would actually need to reverse the direction of the magnetic field strength (by reversing the direction of the current through the coil) until the flux density falls to zero.

Different ferromagnetic materials have different values of residual flux density. For example materials that make good permanent magnets have very high values of residual flux density while others, such as metals used to make transformers, electromagnets, etc., have very low values of residual flux density.

To summarise, residual magnetism is something that occurs naturally and the amount of residual magnetism depends on the type of magnetic material involved. It's not a matter of being 'necessary', it's simply a characteristic of ALL magnetic materials.