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Physics - Field of Action - Electric Field :


This last lesson in physics deals with the nature, origin and action of the different electric, magnetic and electromagnetic fields.



Let us remember the experience of the sphere cited with reference to Figure 10, (by clicking here), we can take the same diagram for another explanation.

It was a sphere that was dropped from the top of a skyscraper and which, under the action of Earth's gravity, followed a strictly vertical course.

Now imagine that we drop an identical sphere from a point very close to the first : it follows a new vertical path, close to the previous one.

Suppose we repeat the same experiment, gradually moving the starting point of the different spheres along one side of the skyscraper. This would determine a set of vertical lines close to each other. The set of these vertical lines would determine what is called the gravity field of action or more simply gravity field.


In another part of this work, it is said that around an electrified body, electrical forces are manifested. These forces act in the surrounding space as the force of Earth's gravity acts on the spheres of the previous experience.


Consider for example two metal plates, one charged with positive electricity and the other with negative electricity, and arranged as shown in Figure 1.

If a particle charged with negative electricity detaches from the upper plate (negative), it falls on the lower plate (positive) after having followed a rectilinear path like the spheres of the previous 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 included between the two plates. If we consider this last force, we will say that the space between the two plates constitutes an electric field of action or more simply, that between the two plates there is an electric field.

Let us examine the different actions that can occur in an electric field (Figure 2). They differ according to the type of charge of the bodies, charge which can be positive and symbolized by the sign +, or negative and symbolized by the sign -.


The Figure 2-a shows the action existing between two charges of the same sign (positive charges in the example, but which could have been negative) : the two electrified bodies repel each other. In other words, a repulsion field is created between the two bodies.

To study the field existing between the two bodies, we must place positively electrified particles at different points in the space surrounding them. Each time the particles are left free to move, starting from a point close to the surface of each body, they travel along paths similar to those plotted in Figure 2.

These paths represent the direction of the forces inside the field and they are generally designated by the name of lines of force.

As we said above and as we have just seen, a repulsion field is created between two charges of the same sign. Our example considered two positive charges (Figure 2-a). If we now take two negative charges, we would see the formation of the same repulsion field, but the arrows of the force lines would be directed in the opposite direction. In other words, the main lines would have the same direction but in opposite directions.

The Figure 2-b shows what happens when the bodies have electric charges of opposite signs. Here, the lines of force from the two bodies meet and strengthen each other. This means that the positive particle subjected to the field is not only repelled by the positive body, but also attracted by the negative body.

Let us return for a moment to the experience of Figure 1. In this one, the path taken by the particle was reversed since it moved away from the negative plate while approaching the positive plate. However, if we replaced the negative particle with another positive one, we would see that the electric field between the two plates is formed by parallel lines of force. A field of this kind, in which the lines of force are parallel and where the electric forces are of equal intensity at the points being equidistant from the two plates, is called uniform field.

The particular constitution of the uniform field in 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, isolated, ie not subjected to the influence of another field. In both cases, the lines of force are in the form of rays moving away from the positive body while converging towards the negative body.

Electrified bodies can be extremely small. We can even consider that the smallest negatively electrified body is the electron that we know, and that in parallel, the smallest positively electrified body is the proton. In these cases, we no longer speak of electrified bodies but of point electrical charges or more simply of positive charge for the proton and negative charge for the electron.

From previous experiences and from what we have just specified, we can generalize :

An electron at rest is surrounded by an electric field. Between two electrons, there is a repulsive force that depends on the distance between them. Similarly, two protons repel each other. A proton attracts an electron like an electron attracts a proton, it is precisely these forces which partly allow the stability of the system constituted by the atom.

We also express this by saying that electricities of opposite signs attract each other, while electricities of the same sign repel each other.

The electric field theoretically propagates to infinity around an electron ; but as its intensity decreases in proportion to the square of the distance, it soon becomes negligible.

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


First define what is meant by magnetic field : it is any point in space where the action of a magnet is felt. This definition naturally leads us to speak of magnets, whether they are, as we will see, natural or artificial.

Natural magnets are fragments of certain so-called magnetic iron oxides which are found in abundance in certain countries, such as SWEDEN.

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

It was not until the 12th century that we were able to make artificial magnets using iron bars that we left for a while in an intense magnetic field.

A magnet attracts iron, steel, and to a much lesser degree, nickel and the colbat. Generally speaking, magnetic substances are all those which are attracted to a magnet.

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

If it is a straight magnetic bar, the poles are at both ends and the neutral line is the midline (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 far from any artificial magnetic field, it is oriented so that one of the poles, always the same, is directed towards magnetic north (slightly different from geographic north). This pole is called north pole, the other pole being the south pole. This is the principle of the magnetic compass, constituted simply by a light magnetic needle, placed on a pivot.

Using a straight magnet and a compass, it is easy to observe that the poles of the same name repel each other and that the poles of opposite names attract each other (Figure 4).


The Figure 5 shows the lines of force between two poles of the same name, that is to say when there is repulsion (Figure 5-a), and two poles of opposite names, that is to say when there is attraction Figure 5-b).


It is by convention that a distinction is made between the North Pole and the South Pole. We can even see a kind of contradiction in the fact that the north pole of a magnet is precisely the one that is attracted to geographic north.

We know that a magnet always has at least two poles and a neutral line. If we break or cut a magnet along the neutral line A-B Figure 6-a, we immediately make appear 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 each having two poles and a neutral line.


If we heat a magnet, we find that its magnetic properties decrease. If the heating is weak, we observe that the magnet resumes by itself its primitive magnetization. On the other hand, if the magnetic bar is heated to red, it definitely loses all magnetization. A piece of red-hot iron is not attracted to a magnet.

A piece of a magnetic substance put in the vicinity of a pole of magnet becomes magnet in its turn. It is said to be magnetized by influence. After removal, certain bodies, such as cast iron and steel, retain a certain residual or remanent magnetization and have in turn become magnets.

We have learned that one of the inseparable manifestations of electric current is the presence of a magnetic field, that is to say a force field capable of acting on magnets. The magnet also creates a magnetic field. Experiments, a thousand times repeated and a thousand times varied, have shown that the magnetic field of the magnet and that of the electric current are strictly of the same nature. It would be strange if the causes were not the same.

We know that, in any metal, like copper and iron, we can consider that the electrons, constituent elements of matter, are animated by circular or elliptical movements around the nucleus of the atom. These movements are obviously elementary electric currents, since, by definition, an electric current is a displacement of electrons. However, with each displacement of an electron necessarily corresponds the appearance of a magnetic field.

But outside of a piece of copper, there is no magnetic field because the electronic trajectories are oriented in all directions. The external magnetic field is the result of the elementary magnetic fields, oriented at random, necessarily have a zero result ; we naturally find as many elementary fields in one direction as in the opposite direction ; therefore, their effects cancel out outside the atom and the result is zero.

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

When we approach a soft iron rod (pure iron) of a magnet, we cause a change of orientation of the electronic trajectories of the atoms. These tilt so as to be placed all in parallel planes : the iron bar becomes magnetized by influence. As soon as it is moved away from the magnet, the trajectories begin to distribute themselves again at random and the magnetization disappears. However, in certain metals such as steel, the trajectories at least partially retain the orientation that was given to them for the first time ; they have the property of becoming permanent magnets.

The magnetic field is therefore the place where the magnetic force is exerted. This obviously has a magnitude or intensity and a direction.

A magnetic needle protected against any other magnetic action is oriented in the direction of the magnetic field. It can thus be used to measure its effects.

At a given location, the direction of the magnetic field is defined by a line of force. It goes without saying that these lines have no real existence and that at every point of a magnetic field, it crosses a line of force. On the outside of a magnet, we admit that the magnetic current goes from the north pole to the south pole. This is only a view of the mind, because in reality, it does not circulate anything in a magnetic field. More precisely, one could say that the magnetic field at a point is a modification of the properties of space at this point.

Much better, the magnetic field created by a magnet has no limit. Theoretically, it extends infinitely around the poles. But the magnetic attraction decreases as the square of the distance, so that at a relatively small distance from a magnet, the attraction becomes so small that it can be considered as zero.

We can, in a way, materialize the magnetic field of a magnet and its lines of force.

Place a thin sheet of Bristol board on a magnetic bar and drop a light rain of fine iron filings. In the magnetic field, each piece of iron becomes a magnet and is oriented in the direction of the lines of force. One thus obtains a trace called magnetic spectrum which is a true materialized image of the lines of force (Figure 7).


Since a magnetic needle tends to take a determined direction, it is because there exists around the earth a magnetic field. Everything happens more or less as if the globe were a vast magnet whose poles would be located in the vicinity of the geographic poles. It should be noted, however, that the geographic poles do not exactly coincide with the magnetic poles and that, moreover, the latter move from year to year (Figure 8). Also shown in this figure are the lines of force of the gravity field. We notice that they are all directed towards the center of the earth.


Just as an electrical mass represents an amount of electricity, so a magnetic mass represents an amount of magnetism. A magnetic mass is evaluated by measuring the force exerted on it in a magnetic field whose intensity is known. Thus, for example, the two poles of a magnetic bar constitute two equal magnetic masses, but of opposite signs (or names).

Depending on whether they have opposite names or the same name, two magnetic masses m and m' attract or repel each other with a force proportional to the product of their magnitude and inversely proportional to the square of the distance between them.

This law known as COULOMB law for magnetism, is expressed by the formula :

F = m . m' / d2

In which :

  • - F is the force expressed in newton ;

  • - m and m' are magnetic masses expressed in weber ;

  • - d2 is the square of the distance separating the two masses, expressed in meters.

On the other hand, the intensity of a magnetic field existing at a point is measured by the action of this field on a magnet whose characteristics are known.

Examination of the above law allows us to see that the intensity of the magnetic field of a magnet is particularly large in the vicinity of the poles and that it decreases rapidly when we move away from it.

The intensity of the magnetic field, also called magnetic induction, is measured in tesla (symbol T) and is designated by the symbol B. This is how we will say that the intensity of the Earth's magnetic field in PARIS is 0.5 x 10-4 tesla.

Certain magnetic fields have, over a large part of their range, a constant intensity and a uniform direction. We then say that these are uniform magnetic fields ; their magnetic spectrum is represented by parallel lines of force.

When we consider a given surface, we can be led to consider a new quantity which is the magnetic induction flux. This is measured in weber (Wb) and its symbol is Φ (phi). Its value is determined by the relationship :

Φ = BS

In which :

  • - Φ is the magnetic induction flow in weber ;

  • - B is the magnetic induction in tesla ;

  • - S is the surface considered in square meters.

Let's put a piece of soft iron in the field of a magnet and draw the magnetic spectrum. We observe that the lines of force gather in the vicinity of the piece of metal to cross it. Everything happens as if these lines, (imaginary, we repeat), had a certain difficulty in circulating in the air when they can cross the metal much more easily.

The phenomenon is even clearer if it is a uniform magnetic field. In the vicinity of the iron, the lines cease to be parallel to approach each other and penetrate into the metal (Figure 9).


The appearance of the figure suggests that metal is more easily penetrated than air by lines of force. It is this idea that we express by saying that it is more permeable.

The permeability (coefficient µ) of a metal is measured as follows :

The metal is placed in a uniform magnetic field. The magnetic flux is measured through any surface, in the field and then in the metal.

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

This coefficient is equal to 1 in a vacuum. It is greater than 1 in magnetic bodies and smaller than 1 in diamagnetic bodies, but in this case always remains very close to 1. In gases, the permeability is very close to 1 and this figure can be used in l without making any appreciable error.

The permeability of iron, in a low intensity field, is of the order of 2 500 ; under the same conditions, that of gray cast iron is of the order of 800. The induction in iron is therefore much greater than in 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 magnetism which, far from having dealt with all the questions relating to it, is very sufficient for the level fixed to our study.


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