Saturday, August 28, 2021

THE BASIS OF ELECTROMAGNETISM IN ELECTRICITY

 ELECTROMAGNET

A coil of wire in the form of a solenoid has, while it is carrying current, a magnetic field similar to that of a bar magnet, and that it will magnetize a bar of iron placed within it. A bar of iron surrounded by a magnetizing coil in this way forms an electromagnet and is represented in it simplest form below:


 

The question arises as to which end of the iron becomes a north pole and which south. This can be worked out by considering the direction of the field produced by the turns of the coil, but there is a simpler way. Look at one end of the magnet. If the direction of the current in the coil is then the same as that of the hands of a clock, the end being examined is the South Pole. If it is not the same, the end is the North Pole. For example, the left-hand end is the north pole, and the right-hand end is the south. Note that the direction in which the current progresses along the length of the magnet in passing round successive turns is immaterial. All that matters is the direction in which the current moves around the magnet.



Electromagnets are often made of “horseshoe” shape. The poles are thereby brought fairly close together, and instead of being spread over long paths in the air, most of the lines of force are concentrated between the poles. An armature arranged to bridge the poles is very strongly attracted, and magnets of this type can be made to support heavy weights. Note that the two limbs of the horseshoe magnet appear at first sight to be wound in opposite directions. If, however, the magnet is imagined to be straightened out into the bar form, it will be seen that this is not really the case.




USES OF ELECTROMAGNET

Since electromagnets are so familiar, we shall mention only a few examples of their uses. In the electric trembler bell, the attraction of an armature by the magnet is arranged to open a contact in the magnet circuit, so that the armature falls back to its original position. The contact is thus reclosed and the armature again attracted, the operation continuing in this manner so long as the external circuit remains closed.

Electromagnets are used in many kinds of signaling apparatus, and in relays for opening or closing one circuit when current is received over another.

In the telephone receiver, variations of the current in a magnet, winding are made to vibrate a thin iron diaphragm and thus reproduce the vibrations at a distant transmitter which caused the variations in current.

Large electromagnets are employed in magnetic clutches for coupling one rotating shaft to another at will, while both electromagnets and permanent magnets are used in magnetic chucks for holding magnetic material while it is being operated upon by machine tools.

THE MAGNETIC CIRCUIT

Consider the case of an iron ring wound as shown below. A winding of this kind is known as a toroidal coil. When current flows in the winding, a magnetic flux is set up in the iron core, and the closed path taken by the flux is termed the magnetic circuit.

Since lines of force assume the form of closed loops, there is a similar magnetic circuit in other cases. In a straight bar magnet, for example, part of the circuit consists of the magnet itself, and part of the surrounding space.

The properties of the magnetic circuit can be described by comparing it to an electric circuit, but it is important to remember that in the magnetic circuit there is nothing in the nature of a flow of current. In an electric circuit, we have, according to ohm’s law,

Current = Electromotive force/Resistance

In the magnetic circuit, we have flux instead of current and a property called reluctance instead of resistance. The magnetic quantity corresponding to electromotive force is called magnetomotive force or m.m.f., and we may write:

Flux = Magnetomotive force/Reluctance

Earlier units of magnetic flux have been replaced by the weber, named after the German physicist Willhelm Eduard Weber. An equally important quantity for many purposes is the flux density; thus in the toroidal coil, we should be interested not only in the total flux but also in its density over the cross-section of the core. The unit of flux density is the tesla, which is I weber per square meter.




The magnetomotive force is produced by the magnetizing winding. It is proportional to the number of turns and to the current flowing around them, i.e., to the number of ampere-turns. It does not matter, for example, whether we have I ampere flowing around 1000 turns or half an ampere flowing around 2000 turns; in both cases, there are 1000 ampere-turns and the magnetic effect is the same.

The reluctance is proportional to the cross-sectional area of the magnetic path, just as electrical resistance is proportional to the length and inversely proportional to the cross-sectional area of a conductor. Like electrical resistance, also, reluctance depends upon the material of which the path is made.

When we say that one material is a better electrical conductor than another, we mean that it has a lower resistivity. It would, however, be equally correct to say that it had a higher conductivity. In the magnetic case, this course is taken, and the term permeability is used for the magnetic equivalent of conductivity.

If the permeability of a perfectly non-magnetic substance is taken as a unity that of a paramagnetic substance is taken is very slightly greater than unity and that of a diamagnetic substance very slightly less. We may, however, ignore these small differences.

Ferromagnetic substances, on the other hand, may have permeability several thousand times as great as that of a non-magnetic substance. If we say, for example, that the sample of iron forming the core of the toroidal coil we have been considering has a relative permeability of1000, we mean that under the prevailing conditions the flux produced is 1000 times as great as it would be with an air core.

There is this important difference between magnetic permeability and electrical conductivity: whereas, apart from temperature variations, electrical conductivity is constant for a given material, the permeability of magnetic materials varies according to the strength of the magnetic field.

MAGNETIC LEAKAGE

We have seen that an appreciable magnetic field can be set up in a non-magnetic substance, and the same is true even of a vacuum. There is no magnetic “insulator’ by which we can confine the flux to well-defined paths. Magnetic calculations are therefore complicated by uncertainty as to how much flux will take the path provided and how much will spread where it is not wanted. Owing to this leakage factor, calculations are usually less exact for magnetic than for electric circuits.

The lack of a magnetic insulator is felt also when it is desired to keep magnetic flux away from a particular area. It is often possible, however, to provide a path of low reluctance which will take most of the flux, and this principle is used in magnetic screens for protecting delicate apparatus from unwanted fields.

The dotted line in the diagram below represents the increase in flux in an iron core as the current in a magnetizing winding is gradually increased. Vertical heights represent flux, and horizontal distances represent ampere-turns. \if, when point a is reached, the current is gradually reduced again to zero, the flux, instead of also returning to zero, follows the full-line curve to b, thus showing that after the magnetizing force has been removed there is still some residual magnetism left in the iron. The power of retaining magnetism in this way is known as retentivity or remanence and is related to the height ob in the diagram.

 


 

If the connexions of the magnetizing coil are now reversed and the current is again gradually increased, it tends to magnetize the core in the opposite direction. At length the residual magnetism is overcome, and at point c the iron is no longer magnetized. The distance oc is a measure of the force necessary to destroy the residual magnetism. The power of resisting demagnetization in this manner varies greatly with the material and is termed coercivity.

If the reversed current is further increased until a point d is reached, and then gradually reduced to zero and again increased in the original direction, the complete full-line curve will be obtained. It is evident that the changes in magnetization always lag behind the changes in magnetizing force which produce them, and this tendency is termed hysteresis.

The complete full-line figure is known as the hysteresis loop, and its area is a measure of the energy lost in changing the magnetic state of the material employed. This energy appears as heat in the core. It is greatest when the hysteresis loop is wide, as in the case of soft iron.

High retentivity and high coercivity are clearly desirable in material for a permanent magnet. On the other hand, these properties are not wanted in an electromagnet which is required to lose its magnetism as soon as the current is removed.

A permanent magnet is demagnetized if it is raised to red heat, while mechanical vibration is also detrimental. Even the passage of time results in appreciable weakening, and magnets the strength of which is required to remain constant are artificially ‘aged”.

MAGNETIC MATERIALS

Soft iron has good permeability, low retentivity, and low coercivity, and is, therefore, suitable for electromagnets. The addition of a small proportion of silicon produces an alloy with a lower hysteresis loss and a higher ohmic resistance. By the last quality, we mean a higher resistance when considered as an electrical conductor; and this is a desirable feature in many cases.

Alloys of iron and nickel have some interesting properties. Although both of these are magnetic materials, it is possible to make from them an alloy that is practically non-magnetic. Combined in other proportions they form materials having low hysteresis losses and high permeability, especially at low magnetizing forces. In most cases, special heat treatment is necessary to obtain the desired characteristics.

Hard steels, particularly those containing tungsten, chromium or cobalt, have a high coercivity and are suitable for permanent magnets. Certain alloys of iron, nickel, aluminum, and cobalt are still better.

As a final example of the curious variations in magnetic properties produced by alloying different materials, we may mention that magnetic alloys of copper, aluminum and manganese are known, although all these substances are practically non-magnetic.

 

 

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