Magnetic field    
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Created it, 05/10/15

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PHYSICS     “2nd PART”

This last lesson of physics milked of nature, the origin and the action of the various fields electric, magnetic and electromagnetic.



Let us remember the experiment of the sphere referred to of figure 10, one can take the same diagram for another explanation.

It was about a sphere which one dropped from the top of a skyscraper and which, under the action of terrestrial gravity, followed a rigorously vertical course.

Let us imagine now that we drop an identical sphere from a point very close to the first: it follows a new vertical course, close to the precedent.

Let us suppose that we repeated the same experiment by gradually moving the starting point of the various spheres throughout a side of the skyscraper. That would determine a whole of vertical lines close from/to each other. The whole of these vertical lines would determine what one calls the sphere of activity of gravity or more simply field of gravity.


In another part of this work, it is known as that around an electrified body, it appears electric forces. These forces act in surrounding space as the terrestrial force of gravity acts on the spheres of the preceding experiment.


Let us examine for example two metal plates, one being in charge of positive electricity and the other of electricity negative, and laid out as figure 1 indicates it.

If a particle in charge of negative electricity is detached from the higher plate (negative), it falls on the lower plate (positive) after having followed a rectilinear course like the spheres of the preceding experiment.

The displacement of this particle is partly due to the force of gravity but especially to the electric force which acts in the space ranging between the two plates. If this last force is considered, we will say that the space ranging between the two plates constitutes an electric sphere of activity or more simply, that between the two plates exists an electric field.

Let us examine the various actions being able to occur in an electric field (figure 2). They differ according to the type of load from the bodies, charges which can positive and be symbolized by the sign +, or negative and symbolized by the sign -.


The figure 2-a shows the action existing between two of the same loads signs (positive loads in the example, but which could have been negative): the two electrified bodies are pushed back. In other words, it creates for itself between the two bodies a field of repulsion.

To study the existing field between the two bodies, it is necessary for us to place in various points of the space which surrounds them of the positively electrified particles. Each time the particles are left free be driven, on the basis of a point close to the surface of each body, they traverse ways similar to those traced on figure 2.

These ways represent the direction of the forces inside the field and one generally indicates them under the name of tension fields.

As we mentioned it above and as we have just seen it, a field of repulsion is created between two of the same loads signs. Our example considered two positive loads (figure 2-a). If we take two negative charges now, we would note the formation of the same field of repulsion, but the arrows of the tension fields would be directed in opposite direction. In other words, the tension fields would have the same direction but in opposite direction.

The figure 2-b shows what occurs when the bodies have electric charges of contrary signs. Here, the tension fields which leave the two bodies join together and is reinforced mutually. That means that the positive particle subjected to the field is not only pushed back by the positive body, but also attracted by the negative body.

Let us return one moment to the experiment of figure 1. In this one, the course carried out by the particle was reversed since it moved away from the negative plate while approaching the positive plate. However, if one replaced the negative particle by another but positive, one would note that the electric field between the two plates is made of parallel tension fields. A field of this kind, in which the tension fields are parallel and where the electric forces are of intensity equal to the points being at equal distance from the two plates, is called uniform field.

The particular constitution of the uniform field of figure 1 is due to the linearity of the plates and their parallelism.

The figure 2-c shows radial fields, in other words the fields created by positive or negative bodies, insulated, i.e. not subjected to the influence of another field. In both cases, the tension fields are appeared as rays moving away from the positive body while converging towards the negative body.

The electrified bodies can be extremely small. One can even consider that the smallest negatively electrified body is the electron which we know, and who in parallel, the smallest positively electrified body is the proton. In these cases, one does not speak any more electrified bodies but of specific electric charges or more simply of positive load for the proton and negative charge for the electron.

Preceding experiments and of what we have just specified, we can generalize :

An electron at rest is surrounded by an electric field. Between two electrons, it appears a repelling power which depends on the distance which separates them. In the same way, two protons are pushed back. A proton attracts an electron as an electron attracts a proton, these are precisely the forces which partly allow the stability of the system consisted the atom.

One expresses as that by saying as electricities of contrary signs attract each other, whereas of the same electricities sign are pushed back.

The electric field is propagated theoretically ad infinitum around an electron; but as its intensity decreases proportionally with the square of the distance, it is not long in becoming negligible.

The electric field appears in the two situations which can occupy an electric charge: rest or movement. It is not the same magnetic field, which is another mode of influence of the electron.


Let us define initially what one understands by magnetic field : it is any point of space where is felt the action of a magnet. This definition quite naturally leads us to speak about the magnets, which they are, as we will see it, natural or artificial.

The natural magnets are fragments of certain iron oxides known as magnetic which one finds in abundance in certain countries, like SWEDEN.

The Greeks knew already the stones of magnet and collected some around a city of named MINOR ASIA MAGNESIA, from where the name “magnetite” given to this stone.

It is only at the XII ičme century which one could carry out of the artificial magnets using iron bars that one left during a certain time in an intense magnetic field.

A magnet attracts iron, steel, and with a degree much less marked, nickel and the colbat. Generally, one names magnetic substances all those which are attracted by a magnet.

A magnet has always at least two poles, which are two points in the vicinity of which the magnetic action is most intense. Between two poles, one finds a line neutral along which there is no magnetic property.

If it is about a bar magnetized right, the poles are at the two ends and the neutral line is the line of centers (figure 3).


It is easy to observe that the two poles of a magnet have different properties.

If a magnet is suspended by its center of gravity, in a place distant from any artificial magnetic field, it directs so that one of the poles, always the same one, is directed towards the magnetic north (slightly different from the true north). This pole is known as North Pole, the other pole being the South Pole. It is the principle of the magnetic compass, simply made up by a light magnetized needle, posed on a pivot.

Using a right magnet and of a compass, it is easy to observe that the of the same poles name are pushed back and that the poles of contrary names attract each other (figure 4).


Figure 5 shows the tension fields between two of the same poles name, i.e. when there is repulsion (figure 5-a), and two poles of contrary names, i.e. when there is attraction appears 5-b).


It is by convention that one distinguishes North Pole and South Pole. One can even see a kind of contradiction in the fact that the North Pole of a magnet is precisely that which is attracted towards the true north.

We know that a magnet has always at least two poles and a neutral line. If we break or cut a magnet according to neutral line A-B appears 6-a, we immediately reveal two new poles, on both sides of the cut. And if we make the same operation on each of the two fragments (figure 6-b), we will constitute four magnets having each one two poles and a neutral line.


If we heat a magnet, we note that its magnetic properties decrease. If the heating is weak, it is observed that the magnet takes again itself its primitive magnetization. On the other hand, if the bar magnet is heated with the red, it loses any magnetization definitively. A piece of red-hot iron is not attracted by a magnet.

A piece of a magnetic substance put in the vicinity of a pole of magnet becomes loving in its turn. It is said that it magnetizes by influence. After distance, certain bodies, like the cast iron and steel, preserve a certain residual or remanent magnetization and became magnets in their turn.

We learned that one of the demonstrations inseparable from the electrical current was the presence of a magnetic field, i.e. from a field of force able to operate the magnets. The magnet creates, him also, a magnetic field. Repeated experiments, thousand times and thousand varied times, showed that the magnetic field of the magnet and that of the electrical current were rigorously of comparable nature. It would be strange that the causes are not identical.

We know that, in an unspecified metal, as copper and iron, one can consider that the electrons, elements constituent of the matter, are animated circular motions or elliptic around the core of the atom. These movements are obviously elementary electrical currents, since, by definition even, an electrical current is a displacement of electrons. However, to each displacement of an electron necessarily the appearance of a magnetic field corresponds.

But outside a piece of copper, there is no magnetic field because the electronic trajectories are directed in all the directions. The external magnetic field is the resultant of elementary magnetic fields, randomly directed, have a null resultant inevitably; one quite naturally finds as many elementary fields in a direction in the opposite direction; consequently, their effects cancel yourself outside the atom and the result is null.

A magnet is a body in which the electronic trajectories are parallel. Consequently, their effects are added and result outside in a magnetic field. This extremely simple explanation, makes it possible to explain all the properties of the magnets: the presence of the poles, the neutral line, magnetization by influence, the action of the shocks on certain magnets…

When we approach a soft iron bar (pure iron) of a magnet, we cause a change of orientation of the electronic trajectories of the atoms. Those rock in order to place all in parallel plans: the iron bar becomes magnetized by influence. As soon as one moves away it from the magnet, the trajectories again start to be distributed randomly and magnetization disappears. But, in certain metals like steel, the trajectories preserve at least partially, the orientation that them first once was given; they have the property to become permanent magnets.

The magnetic field is thus the place where is exerted the magnetic force. This one has obviously a size or intensity and a direction.

A magnetized needle protected from any other magnetic action is directed in the direction of the magnetic field. It can be thus used to measure the effects of them.

In a given place, the direction of the magnetic field is defined by a tension field. It goes without saying that these lines do not have any real existence and that in any point of a magnetic field, it passes a tension field. Outside a magnet, one admits that the magnetic current goes from the North Pole towards the South Pole. It is only one fantastic notion, because actually, he circulates nothing in a magnetic field. In a more precise way, one could say that the magnetic field in a point is a modification of the properties of space in this point.

Well better, the magnetic field created by a magnet does not have limit. Theoretically, it extends ad infinitum around the poles. But the magnetic pull decrease like the square of the distance, so that at a relatively weak distance from a magnet, attraction becomes so small that one can regard it as null.

One can, to some extent, materialize the magnetic field of a magnet and his tension fields.

Let us place a thin Bristol-board sheet on a bar magnet and drop a light rain from fine iron filings. In the magnetic field, each iron piece becomes loving and is directed in the direction of the tension fields. One thus obtains a layout called magnetic spectrum which is a true materialized image of the tension fields (figure 7).


Since a magnetized needle tends to take a determined direction, it is that there exists around the ground a magnetic field. All occurs about as if the sphere were a vast magnet whose poles would be located in the vicinity of the geographical poles. It is necessary to notice however, that the geographical poles do not coincide exactly with the magnetic poles and that, moreover, those move year by year (figure 8). On this figure the tension fields of the field of gravity are also represented. We notice that they all are directed towards the center of the ground.


Just as an electric ground a quantity of electricity represents, in the same way a magnetic mass represents a quantity of magnetism. One evaluates a magnetic mass by measuring the force which is exerted on it in a magnetic field which one knows the intensity. Thus, for example, the two poles of a bar magnet constitute two equal magnetic masses, but of signs (or names) contrary.

According to whether they are contrary names or of the same name, two magnetic masses m and me attract each other or are pushed back with a force proportional to the product of their size and inversely proportional to the square of the distance which separates them.

This law known as law of COULOMB for magnetism, results in the formula :

F = m . m' / d2

In which :

In addition, the intensity of an existing magnetic field in a point is measured by the action of this field on a magnet which one knows the characteristics.

The examination of the law above enables us to note that the intensity of the magnetic field of a magnet is particularly large in the vicinity of the poles and that it decrease quickly when one moves away from there.

The intensity of the magnetic field, still called magnetic induction, is measured in Tesla (symbol T) and is indicated by the symbol B. Thus one will say that the intensity of the terrestrial magnetic field in PARIS east of about 0,5 x 10-4 tesla.

Certain magnetic fields have in most of their extent a constant intensity and a uniform direction. It is said whereas they are uniform magnetic fields ; their magnetic spectrum is represented by parallel tension fields.

When a given surface is considered, one can be brought to consider a new size which is the flow of magnetic induction. This one is measured in weber (Wb) and its symbol is Ř (phi). Its value is determined by the relation :

Ř = BS

In which :

Let us place a piece of soft iron in the field of a magnet and trace the magnetic spectrum. We observe that the tension fields gather in the vicinity of the piece of metal to cross it. All occurs like if these lines, (imaginary, we repeat it), tested a certain difficulty of circulating in the air whereas they can cross metal much more easily.

The phenomenon is still much more Net if it is about a uniform magnetic field. In the vicinity of iron, the lines cease being parallel to approach the ones the others and to penetrate in metal (figure 9).


The aspect of the figure suggests that metal is more easily let penetrate than the air by tension fields. It is well this idea that one expresses by saying that it is more permeable.

The permeability (coefficient µ) of a metal is measured in the following way :

Metal is placed in a uniform magnetic field. One measures the magnetic flux through an unspecified surface, in the field then in metal.

The relationship between the two numbers measures the permeability and is called coefficient of permeability. 

This coefficient is equal to 1 in the vacuum. It is larger than 1 in the magnetic bodies and smaller than 1 in the diamagnetic bodies, but always remains in this case very close to 1. In gases, the permeability is very close to 1 and one can use this figure in the air without making appreciable error.

The permeability of iron, in a field of low intensity, is about 2 500 ; under the same conditions, that of grey pig iron is about 800. Induction in iron is thus much larger than in the air and it is given by the relation.

B = Boµ 

in which :

- Bo is induction in the air ;

- µ the relative magnetic permeability of iron.

We finish this chapter devoted to the magnetism which, far from to have treated all the questions referring itself to it, is very sufficient for the level fixed at our study.



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