Magnetic permeability and the b/h curve, Electrical Engineering

Magnetic permeability and the B/H curve

To measure the relationship between field strength H and flux density B for a material, a sample is made into a closed ring and a magnetising coil wound onto it. Current flowing in this coil creates a m.m.f. equal to NI which in turn produces a magnetic field


in the ring. So  H is proportional to the current in the energising coil and may be obtained from direct measurement of I. If a change in the current occurs, the flux density changes and this will induce a transient 'back e.m.f' in a second coil that is also wound onto the ring.


1372_Magnetic permeability and the B-H curve.png


The integral of the back emf is proportional to B. Hence the relationship between B and H may be determined from measurement of I (to give H) and the induced back e.m.f in the coils (to give B).

The result of such measurements on various materials produces two types of curves - straight lines of low

B/H =µr

values and curves for  which the value of B/H =µr   varies. This first type are sub divided into paramagnetic
varies. This first type are sub divided into paramagnetic materials if B/H =µr >1 and diamagnetic if B/H =µr <1. The second type are ferromagnetic and have a more complicated dependence of B on H.  At low H these have B/H =µ>>1 (typically 100 - 100000)

85_Magnetic permeability and  B-H curve.png



It is also found that ferromagnetic materials lose these high µr properties above a well-defined temperature (different for different materials) known as the Curie temperature. This typically  r ranges from 200 - 750 °C and limits the maximum temperature that many electromagnetic devices can operate.

The temperature dependence of the ferromagnetic properties give a clue to the physical reason for the different

permeabilities of the materials. Diamagnetic and paramagnetic properties are attributable to the orbiting electrons within the atoms of the material. Ferromagnetic properties arise due to the orientation of atoms on a macroscopic scale so that the magnetic fields created by the orbiting electrons within each atom reinforce each other. These 'domains' of high magnetic strength may contain ~10-12   atoms each. The domains are initially randomly directed but under the influence of an external magnetic field, re-orientate to point in the same direction. Initially this is along the direction of the nearest crystal axis to the direction of the magnetic field.


Steadily increasing the external field brings them directly into line with the external field direction.If the external field is now removed, the domains relax back to point in the nearest crystal direction, leaving a residual magnetism. Materials that have a strong residual magnetism are used for permanent magnets. Cycling the current over one complete cycle produces a hysteresis B/H curve




1726_hysteresis B-H curve.png

Posted Date: 8/22/2012 5:56:39 AM | Location : United States

Your posts are moderated
Related Questions
Equal volumes of all gases contain the samenumber of molecules when the volumes are measured at the same p and T. Therefore for a given volume at the same p and T, with the same

1. The circuit is: A circuit which lights a LED when a voltage is above 14 volts and flashes the LED (at about 1 Hz) below 10 Volts. The circuit should also continuously sounds

How can I get an oscillator to genarate a sinusoid instead of a triangle wave

Q. Consider the MOSFET circuit with variable voltage, with RD = 2k and VDD = 12 V. The static characteristics of the n-channel enhancement MOSFET are given in Figure. (a) Drawt

Q. What is transconductance? Explain its significance from the transfer characteristics. Transconductance is the transistor gain of the JFET; it indicates the amount of control

Q. An op amp has an open-loop frequency response as shown in Figure. (a) Find the approximate bandwidth of the circuit using this op amp: (i) With a closed-loop voltage gain

What do you understand by DRAM and its refreshing? Dynamic RAM (DRAM) is fundamentally the same as SRAM, but this retains data for only 2 or 4 ms on an internal capacitor. But

Explain Intrinsic Semiconductor. Intrinsic Semiconductor: An extremely pure semiconductor is termed as intrinsic semiconductor. But at absolute zero temperature its valence b

1. The size of the multiplexer used to implement a truth table can be cut in half (e.g. 4 inputs instead of 8) if one of the variables is used as an input instead of being connecte

Consider the following IIR filter. The initial input data sequence is given by x n = [10 15 20 15 8 6 9 0 0 0] Construct a table that shows the corresponding signal values at