Answer to Question #211487 in Electricity and Magnetism for Anita

Question

Identify the statement that is not true of ferromagnetic materials.

Answer 1

Which statement is not true of ferromagnetic materials. Energy loss is proportional to the area of hysteresis loop. They have fixed value of μr. They have large magnetic susceptibility.

Magnetic Hysteresis:-

The lag or delay of a magnetic material known commonly as Magnetic Hysteresis, relates to the magnetisation properties of a material by which it firstly becomes magnetised and then de-magnetised

We know that the magnetic flux generated by an electromagnetic coil is the amount of magnetic field or lines of force produced within a given area and that it is more commonly called “Flux Density”. Given the symbol B with the unit of flux density being the Tesla, T.

We also know from the previous tutorials that the magnetic strength of an electromagnet depends upon the number of turns of the coil, the current flowing through the coil or the type of core material being used, and if we increase either the current or the number of turns we can increase the magnetic field strength, symbol H.

Previously, the relative permeability, symbol $\mu_r$ was defined as the ratio of the absolute permeability μ and the permeability of free space $\mu_0$ (a vacuum) and this was given as a constant. However, the relationship between the flux density, B and the magnetic field strength, H can be defined by the fact that the relative permeability, μr is not a constant but a function of the magnetic field intensity thereby giving magnetic flux density as: B = μ₀H.

Then the magnetic flux density in the material will be increased by a larger factor as a result of its relative permeability for the material compared to the magnetic flux density in vacuum, μoH and for an air-cored coil this relationship is given as:

magnetising force equation

So for ferromagnetic materials the ratio of flux density to field strength ( B/H ) is not constant but varies with flux density. However, for air cored coils or any non-magnetic medium core such as woods or plastics, this ratio can be considered as a constant and this constant is known as μo, the permeability of free space, ( μo = $4\pi \times10^{-7}$ ).

By plotting values of flux density, ( B ) against the field strength, ( H ) we can produce a set of curves called Magnetisation Curves, Magnetic Hysteresis Curves or more commonly B-H Curves for each type of core material used as shown below.