What are Magnets?
A permanent magnet has a magnetic field surrounding it. A magnet field can be envisioned to consist of lines of force that radiate from the north pole (N) to the south pole (S) and back to the north pole through the magnetic material.
A permanent magnet, such as the bar magnet has a magnetic field surrounding it that consists of lines of force, or flux lines. For clarity, only a few lines of force. Imagine, however, that many lines surround the magnet in three dimensions. The lines shrink to the smallest posible size and blend together, although they do not touch. This effectively forms a continuous magnetic field surrounding the magnetic.
Attraction and Repulsion of Magnetic Poles:
When unlike poles of two permanent magnets are placed close together, an attractive force is produced by the magnetic field, as indicated. When two like poles are brought close together, the repel each other.
Altering a Magnetic Field:
When a nonmagnetic material such as paper, glass, glass, wood, or plastic is placed in a magnetic field, the lines of force are unaltered.However, when a magnetic material such as iron in a magnetic field, the lines of force tend to change course and pass though the iron rather than through its surrounding air. They do so because the iron provides a magnetic path is more easily established than that of air. Illustrated this principle. The fact that magnetic lines of force follow a path through iron or other materials is a consideration in the design of shields that prevent stray magnetic fields from affecting sensitive circuits.
Magnetic Flux (Φ):
The group of force lines going from the north pole to the south pole of a magnet is called the magnetic flux, symbolized by Φ (the Greek letter phi). The number of lines of force in a magnetic field determines the value of the flux. The more lines of force, the greater the flux and the stronger the magnetic field.
The unit of magnetic flux in the Weber (Wb). One whole equal 108 lines. The Weber is a very large unit; thus, in most practical practical situations, the microweber (μWb) is used. One microweber equals 100 lines of magnetic flux.
Magnetic Flux Density (B):
The magnetic flux density is the amount of flux per unit area perpendicular to the magnetic field. Its symbol is B, and its SI unit is the tesla (T). One tesla equals one weber per square meter (WB/m2).The following formula expresses the flux density:
Where Φ is the flux and A is the cross-sectional area in square meters (m2) of the magnetic field.
Although the tesla (T) is the SI unit for unit for flux density, another unit is called the gauss,from the CGS (centimeter-gram-second) system, is sometimes used (104 gauss = 1 T). In fact, the instrument used to measure flux density is the gaussmeter.
How Materials Become Magnetized:
Ferromagnetic materials such as iron, nickel, and cobalt become magnetized when placed in the magnetic filed of a magnet. W have all seen a permanent magnet pick up things like paper clips, nails, and iron filings. In theses cases, the object becomes magnetized (that is, it actually becomes a magnet itself) under the influence of the permanent magnetic field and becomes attracted to the magnet. When removed from the magnetic field, the object tends to lose its magnetism.
Ferromagnetic materials have minute magnetic domains created within their atomic structure. These domains can be viewed as very small bar magnets with north and south poles. When the material is not exposed to an external magnetic field, the magnetic domains are randomly oriented. When a material is placed in a magnetic field, the domains align themselves. Thus, the object itself effectively becomes a magnet.
Magnetic Properties of Solid:
From the study of magnetic fields produced by bar magnets and moving charges, i.e., currents, it is posible to trace the origin of the magnetic properties of the material. It is observed that the field of a long bar magnet is like the filed produced by a long solenoid carrying current and the field of the short bar magnet resembles that of a single loop. This similarity between the fields produced by magnets and current urges an enquiring mind to think that all magnetic effects may be due to circulating currents (i.e., moving charges); a view first held by Ampere. The idea was not considered very favourably in Ampere’s time because the structure of atom was not known at that time. Taking into consideration, the internal structure of atom, discovered thereafter, the Ampere’s view appears to be basically correct.
The magnetism produced by electrons within an atom can arise from two motions. First, each each electron orbiting the nucleus behaves like an atomic sized loop of current that generates a small magnetic field; the situation is similar to the field created created by the current loop, each electron possesses a spin that also gives rise to a magnetic field. The net magnetic created by the electrons within an atom is due to the combined field created by their orbital and spin motions. Since there are a number of electrons in an atom, there current of spins may be so oriented of aligned as to cancel the magnetic effects mutually or strengthen the effects of each other.An atom in which there is a resultant magnetic filed, behaves like a tiny magnet and is called magnetic dipole. The magnetic fields of the atoms are responsible for the magnetic behaviour of the substance made up of these atoms. Magnetism is, therefore, due to the spin and orbital motion of the electrons surrounding the nucleus and is thus a property of all substance. It may be mentioned that the charged nucleus itself spins giving rise to a magnetic filed. However, it is much weaker than that of the orbital electrons. Thus the source of magnetism of an atom is the electrons. Accepting this view of magnetism it is conclude that it is impossible to obtain an isolated north pole. The north-pole is merely one side of a current loop. The other side will always e present as a south pole and these cannot be separated. This is an experimental reality.
Two cases arise which have to be distinguished. In the first case, the orbits and the spin axes of the electrons in an atom are so oriented that their fields support each other and the atom behaves like a tiny magnet. Substance which such atoms are called para-magnetic substances. In second type of atoms there is no resultant field as the magnetic fields produced by both orbital and spin motions of the electrons might added upto zero. These are called diamagnetic substances, for example the atoms of water, copper, bismuth and antimony.
However, there are some solid substances e.g., Fe, Co, Ni, Chromium dioxide, and Alnico (an iron aluminium – nickel – cobalt alloy) in which the atoms co-operate with each other in such a way so as to exhibit a strong magnetic effect. They are called ferromagnetic substances. Ferromagnetic materials are of great interest for electrical engineers. Recent studies of ferromagnetism have shown that there exists in ferromagnetic substance small regions called ‘domains’. The domains are of microscopic size of the order of millimeters or less but large enough to contain 1012 to 1016 atoms. With each domain the magnetic fields of all the spinning electrons are parallel to one another i.e., each domain is magnetized to saturation. Each domain behaves as a small magnet with its own north and south poles. In ummagnetised iron the domains are oriented in a disorderly fashion, so that the net magnetic effect of a sizeable specimen is zero, When the specimen is placed in an external magnetic field as that of a solenoid, the domains line up parallel of lines of external magnetic filed and the entire specimen becomes saturated. The combination of a solenoid and a specimen of iron inside it thus makes a powerful magnet and it called an electromagnet.
Iron is a soft magnetic material. Its domains are easily oriented on applying as external field and also readily return to random positions when the field is removed. This is desirable in an electromagnet and also in transformers. Domains in steel, on the other hand, are not so easily oriented to order. They require very strong external fields, but ones oriented, retain the alignment. Thus steel makes a good and another such material is a special alloy Alnico V.
Finally, it must be mentioned that thermal variations tend to disturb the orderliness of the domains. Ferromagnetic materials preserve the orderliness at ordinary temperatures. When heated, the begin to lose their orderliness due to the increased thermal motion. This process begins to occur at a particular temperature (different for different materials) called Curie temperature. Above the Curie temperature iron is pare-magnetic but not ferromagnetic. The Curie temperature for iron is about 750°C.
To investigate a ferromagnetic material, a bar of that material such as iron is placed in an alternating current solenoid. When the alternating current is at its positive peak value, its fully magnetises the specimen in one direction and when the current is at its negative peak, it fully magnetises it in opposite direction. Thus as the alternating current changes from its positive peak value to its negative peak value and then back to its positive peak value, the specimen undergoes a complete cycle of magnetization. The flux density versus the magnetization of the specimen for the various value of magnetizing current of the solenoid is plotted a CRO.
Its main features are follows:
The portion of OA of the curve is obtained when the magnetizing current I is increased and AR is the portion when the current is decreased. It may be noted that the value of flux density of any value of current is always greater when the current is decreasing than when it is increasing, i.e., magnetism lags behind the magnetizing current. This phenomenon is known as hysteresis.
The magnetic flux density increases from zero and reaches a maximum value. At this stage the material is said to be magnetically saturated.
3. Remanence or Retantivity:
When the current is reduced to zero, the material still remains strongly magnetized represented by point R on the curve. It is due to the tendency of domains to stay partly in line, once they have been aligned.
To demagnetize the material, the magnetizing current is reversed and increased to reduce the magnetization to zero. This is known as coercive current represented by C on the curve. The coercivity of steel, is more than that of iron as more current is needed to demagnetize it. Once the material is magnetized, its magnetization curve never passes through the origin. Instead, it forms the closed loop ACDC’A, which is called hysteresis loop.
5. Area of the Loop:
The area of the loop is a measure of the energy needed to magnetize and demagnetize each cycle. This is the energy required to do work against internal friction of the domains. This work, like all work that is done against friction, is dissipated as heat. It is called hysteresis loss.
Hard magnetic materials like steel can not be easily magnetized or demagnetized, so they have large loop area as compared to soft magnetic material such as iron which can easily be magnetized. The energy dissipated per cycle, thus, for iron is less than for steel.
Suitability of magnetic materials for different purposes can be studied by taking the specimen through a complete cycle and drawing the hysteresis loop. A material with high retentivity and large coercive force would be most suitable to make a permanent magnet. The cores of electromagnets used for alternating currents where are specimen repeatedly undergoes magnetization and demagnetization should have narrow hysteresis curves of small area to minimize the waste of energy.