As specified at the end of the previous lesson entitled "Electromagnetic Induction", we will now analyze a new type of current totally different from the one considered so far. However, you certainly know all this current, at least by name, since it is alternating current.
1. - ALTERNATIVE CURRENT :
We have always examined the circuits traversed by the current supplied by one or more batteries, current called direct current because it always has the same direction of circulation. The current flowing in the electrical circuit of Figure 1-a is a direct current.
The current flowing through this circuit is always directed, in the conventional sense, from the positive pole to the negative pole of the battery : it enters the resistance by the end marked with the letter A and leaves by the one identified at the using the letter B. In such a circuit, the voltage generating the direct current is called continuous voltage. There are other types of generators, which deliver a current, which by its characteristics is called alternating current.
To understand the difference between these two types of current, it is necessary first to refer to Figure 1-b. This represents the same electrical circuit as Figure 1-a, with the difference that it is powered by an alternating current generator which we can note in passing the graphic symbol.
In this figure 1-b, the polarities of the alternating current generator, indicated by the signs "+" and "-" are identical to the polarities appearing in the circuit of Figure 1-a. Consequently, in these two cases, the current flows in the same direction crossing the resistance of A towards B.
However, in the case of an alternating generator, the current flows in one direction only during a very short time, after which it reverses. We are then in the presence of the figure 1-c where the polarities of the generator are reversed and where the current crosses the resistance of B towards A. Even in this new direction of circulation, the current persists only during a very short time for then come back in the case of Figure 1-b and so on.
We can say that the current periodically changes its direction of circulation, that is, it traverses the resistance alternately from A to B and from B to A during very short periods of time. from this explanation, we understand the origin of the name of the alternating current.
The intensity of an alternating current varies constantly, in the case of Figure 1-b it increases from zero to a maximum value determined by the generator and the resistance, then decreases to return to zero. At the instant when the intensity is zero, the generator reverses its polarities, we are in the case of Figure 1-c, the intensity increases again to the same maximum as before and then down to zero. At this moment, a change of polarities is reproduced and the cycle begins again.
As long as the resistance is fixed, the variations in intensity of the current I can only be due to analogous variations of the voltage supplied by the generator. This voltage has the same characteristics as the current it provides and is called alternating voltage.
There are therefore two basic types of electric current that are :
The direct current symbolized by the acronym C.C. and the alternating current symbolized by the acronym C.A.
It is good to remember that AC power is much more common than DC since it is used in industry and homes. The alternating current is produced by means of generators called alternators and installed in power plants.
1. 1. - ALTERNATING CURRENT GENERATION
To understand how the alternating current can periodically change its direction of flow and vary its intensity, we must consider the operating principle of an alternating current generator.
The operation of such a generator is based on the phenomenon of electromagnetic induction analyzed in the previous theoretical lesson. Indeed, this generator comprises an inductor circuit supplied with direct current to produce the necessary induction flux, and an induced circuit in which is precisely induced the desired alternating current.
Figure 2 are shown in a very simplified way these two circuits.
The inductor circuit is formed of two windings connected in series and powered by a battery. Between these two windings is arranged the induced circuit shown in Figure 2 by a single turn. The ends of this turn constitute the poles of the generator and are connected to a resistor which represents the circuit outside the generator. With this arrangement, the turn of the induced circuit is traversed by the induction lines of the flux produced by the inductor circuit.
The variation of the inductive flux necessary for the creation of a current induced in the turn is obtained in our case by a rotation of the complete inductive circuit around the turn. In Figure 2, the arrows represent the direction of rotation while the point 0 materializes the center of the movement. It should be noted that the rotation takes place at constant speed.
Figure 3 are shown eight different positions taken by the induction flux during a complete revolution of the inductor circuit (circuit which is not shown for the purpose of not overloading the figures).
The inductive circuit is supposed to take eight seconds to complete one revolution and so takes a second to move from one position to the next.
By following the examples in Figure 3, we immediately see why the generator reverses at a certain time its polarities and consequently reverses the flow direction of its current.
From Figure 3-a in which the turn is completely traversed by the inductor flux and consider what happens during the second during which the flow moves to reach the position of Figure 3-b. During this second, the flow has moved at an angle of 45° or an eighth of a turn in the direction indicated by the arrow. The consequence of this rotation is that, as shown in Figure 3-b, the turn is no longer traversed by the entire induction flow since some of the lines of this flow are external to the turn.
Due to the decrease in the flux, a current I is induced in the turn, the direction of flow of which is such that it produces in turn an induction flow directed in the same direction as that of the inducing flux and this according to the law of LENZ. We therefore know the direction of the induction lines of the induced flux in the turn and applying the rule of the corkscrew, we deduce the flow direction of the current I in the turn. In the circuit outside the generator, in other words in the resistor, the current flows from A to B. Because according to its conventional sense, the current is directed from the positive pole of the generator to its negative pole, we can indicate the polarities appearing at the terminals of the spire.
The end of the turn connected to the point A is of positive polarity while that of the point B is of negative polarity.
Continuing its rotation, the flux embraced by the turn decreases and vanishes completely after 2 seconds when it reaches the position of Figure 3-c. During this second second, the inductive flux turned a new eighth turn which is a quarter turn in relation to Figure 3-a is an angle of 90°. At the moment when the flux passing through the turn vanishes, the induced current I flows in the same direction as before and this always to produce a flow directed from left to right in order to counteract the cancellation of the flow. kissed by the spire. From the position of Figure 3-c, the flow embraced by the coil starts to increase given the rotation. When this stream has completed another eighth of a turn during the third second, it describes from its starting position an angle of 135°. A number of the induction lines of this stream crosses again the coil. Due to the rotation of the flow, its induction lines are now directed from right to left. The inductive flux increases: the induced current generated in the turn to counteract this increase must produce a flow of opposite direction, thus directed from left to right.
This orientation is the same as in the case of Figures 3-b and 3-c, therefore, the induced current I always flows in the same direction.
The induced current continues to flow in this direction until 1 second later, the inductor flow reaches the position of Figure 3-e having completed a half-turn (the flow is completely embraced by the turn). In this figure, it is no longer represented induced current for the reason that we will see later. A second later, the flow again turned one-eighth of a turn and is in the position of Figure 3-f (rotation angle of 225°). The flux passing through the turn has again decreased and the induced current I, to counteract this decrease, must create an induction flux directed in the same direction as the inducing flux, from right to left, and this, again according to the law of LENZ.
(To facilitate the understanding of Figure 3 above, we report the same figure).
Knowing, therefore, the direction of the induction lines of the induced flux in the turn and applying the rule of the corkscrew, we deduce the flow direction of the current I in the turn.
In the circuit outside the generator, in other words in the resistor, the current flows from B to A. Since according to its conventional sense, the current is directed from the positive pole of the generator to its negative pole, we can indicate figure 3-f the new polarities appearing at the terminals of the turn.
The end of the turn connected to the point A is of negative polarity while the one connected to the point B is of positive polarity.
We thus see that in correspondence of the reversal of the flow direction of the current, the polarities of the generator are also reversed.
The direction of the induced current is reversed as soon as the flow has exceeded the position of Figure 3-e and continues to flow in this new direction until the flow, after passing the positions of Figures 3-g (angle 170°) and 3-h (angle 325°) returns to its initial position which is that of Figure 3-a having thus completed a complete turn (360°).
When the flow reaches this position, the direction of the induced current is reversed again and the cycle starts again if, of course, the rotary motion applied to the inductor circuit is maintained. In conclusion, we can say that the current flows in one direction during a half-turn of the induction flux and in the opposite direction during the following half-turn, this reversal of direction is carried out when the induction lines of the induction flux are horizontal, case of figures 3-a and 3-e.
Since any current results from a displacement of electrons, its inversion materializes thus by a reversal of the direction of displacement of the electrons which constitute it. For this to happen, the electrons must first stop their movement in one direction before returning to the other. There is therefore a moment during which the electrons are immobile.
This immobility of the electrons results in the absence of current I in Figures 3-a and 3-e. In these two figures, the intensity of the current I through the resistor is zero.
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