0.21 Chapter 7. magnetic materials.  (Page 3/4)

7.5. Right Hand Palm Rule.

This rule lets us determine the polarity of a solenoid or a coil of current. If the fingers of right hand clasping the solenoid is in the direction of the current flow as shown in Figure 7.3. then magnetic flux line will be in the direction of the thumb.

North Pole of the Solenoid is the end from where magnetic flux lines come out. The end where they enter the solenoid is defined as South Pole.

7.6. Magnetic Materials.

The materials which are spontaneously magnetized or have a susceptibility to magnetization are referred to as magnetic materials. They are classified as:

i.Ferromagnetic- spontaneously magnetized and have strong, positive susceptibility to magnetization. Ferromagnetic material has been demonstrated in Figure 7.4. Up to a temperature known as Curie Temperature, in this case 1000K, spontaneous magnetization is maintained as shown in the upper part of the Figure but this self alignment of the dipoles gets disrupted due to thermal fluctuations above 1000K.

ii. Paramagnetic materials: these have no spontaneous magnetization and have weak, positive susceptibility to magnetization. As shown in Figure 7.5., there is no spontaneous magnetization but application of Magnetic Field does cause a weak magnetization.

iii.Diamagnetic materials: these have no spontaneous magnetization and they move away from a region of magnetic field and have a negative, weak susceptibility to magnetization. Diamagnetism is the property of an object which causes it to create a magnetic field in opposition to an externally applied magnetic field, thus causing a repulsive effect. Specifically, an external magnetic field causes eddy currents in such a way that according to Lenz's law , this opposes the external field. Diamagnets are materials with a magnetic relative permeability less than 1.

Consequently, diamagnetism is a form of magnetism that is only exhibited by a substance in the presence of an externally applied magnetic field. It is generally quite a weak effect in most materials, although superconductors exhibit a strong effect.

Diamagnetic materials cause lines of magnetic flux to curve away from the material, and superconductors can exclude them completely (except for a very thin layer at the surface).

In Figure 7.6. a comparative study of diamagnetic and paramagnetic material has been made. In paramagnetic there are atomic magnetic dipoles but randomly arranged and application of magnetic field can align them to result in a weak magnetization. But in diamagnetic materials there are no atomic magnetic dipoles. These get induced by the application of external magnetic field but opposed to the external magnetic field in accordance with Lenz’s Law.

In Figure 7.7. we show the magnetization curve with respect externally magnetic field.

In Figure 7.8, we show the temperature sensitivity of Ferromagnetic and paramagnetic materials.

Therefore

By rearranging the terms we get:

In Table 7.1. the magnetic susceptibilities of paramagnetic and diamagnetic materials are given.