The world around us is filled with various electrical devices and an inquisitive person willingly or unwillingly interested in those physical processes that occur during their functioning. At the same time, in the conversations of specialists, the term self-induction often flashes and I want to understand what phenomenon lies behind this term.
Self-induction, as the name immediately implies, has a common nature with conventional induction, which, in turn, describes the relationship between the electric current of a conductor placed in an alternating magnetic field. It is known that the flow of current is caused by an EMF source. Accordingly, the phenomenon of self-induction is the appearance of an EMF, which, as a vector, is oriented in the direction opposite to the source. De facto, the presence of this phenomenon prevents too rapid rise or fall of the current in the conductor, which occurs according to the exponential law, Figure 1.
Influence of self-induction on the speed and direction of current change
Self-induction as a phenomenon is characteristic of inductive elements. A good example of this is an inductor. It is a winding of insulated wire and in most cases is additionally clad on an iron core to increase the inductance.
The value of the EMF of self-induction is proportional to the product of the inductance of the coil by the rate of change of the current through it, and with the opposite sign. Therefore, if, with an increase in the current strength, the EMF of self-induction is directed against the EMF of the source and prevents an increase in current, then when the current falls, it, on the contrary, adds up with it and, thereby, prevents a decrease in current. In fact, the presence of inductance in the circuit due to self-induction provides effective smoothing of current surges.
As a distant mechanical analogy of self-induction, one can refer to Newton's third law: for the action on the body of the force arises a counteracting force, which is directed exactly in the opposite direction.
Examples of useful and harmful manifestations of self-induction
The phenomenon of self-induction is often purposefully used in practice. For example, in a fluorescent lamp in the current flow circuit, a choke is introduced, turned on as shown in Figure 2. After the circuit is broken by the starter, the choke, due to self-induction, holds the voltage for some time their clamps, i.e. on the lamp, which is enough to ignite it and switch to normal mode glow.
The second example is the ignition system of a gasoline engine. It also includes a choke. After opening the current flow circuit with a slider or a semiconductor key element, a sharp increase occurs EMF of self-induction, the voltage value of which is sufficient for electrical breakdown of the spark gap of the candle, figure 3.
An example of the harmful consequences of self-induction is burning of contacts of sockets, automatic machines and similar switching devices, Figure 4. In this case, the skipping spark leads to electrical corrosion of the working surface, which is accompanied by its accelerated aging.