>> Self-induction. Inductance

§ 15 SELF-INDUCTION. INDUCTANCE

self induction. If an alternating current flows through the coil, then the magnetic flux penetrating the coil changes. Therefore, in the same conductor through which the alternating current flows, an induction emf arises. This phenomenon is called self-induction.

In self-induction, the conducting circuit performs a dual role: an alternating current in the conductor causes the appearance magnetic flux through a surface bounded by a contour. And since the magnetic flux changes with time, induction emf appears. According to the Lenz rule, at the moment of current increase, the intensity of the eddy electric field directed against the current. Therefore, at this moment, the vortex field prevents the current from rising. On the contrary, at the moment the current decreases, the vortex field supports it.

The phenomenon of self-induction can be observed in simple experiments. Figure 2.13 shows a parallel connection of two identical lamps. One of them is connected to the source through a resistor R, and the other is connected in series with the coil L, equipped with an iron core.

When the key is closed, the first lamp flashes almost immediately, and the second - with a noticeable delay. The self-induction emf in the circuit of this lamp is large, and the current strength does not immediately reach its maximum value (Fig. 2.14).

The appearance of the EMF of self-induction during opening can be observed in the experiment with the circuit shown schematically in Figure 2.15. When the key is opened, an EMF of self-induction arises in the coil L, which maintains the initial current. as a result, at the moment of opening, a current flows through the galvanometer (colored arrow), directed against the initial current before opening (black arrow). The strength of the current when the circuit is opened may exceed the strength of the current passing through the galvanometer when the key is closed. This means that the EMF of self-induction is greater than the EMF of the cell battery.

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It is well known that a train departing from a station cannot immediately reach the desired speed.

The required speed is reached only after a certain period of time. During this interval, a significant part of the energy of the locomotive is spent on overcoming the inertia of the train, i.e., on the formation of a reserve of kinetic energy, and a very small part - on overcoming friction.

Due to the fact that a moving train has a reserve of kinetic energy, it cannot stop instantly and will move by inertia for some more time, i.e., until the entire supply of kinetic energy imparted to it by the locomotive at the beginning of the movement is used up for friction.

Similar phenomena take place in a closed electrical circuit when the current is turned on and off.

At the moment of switching on direct current(Figure 1) around the conductor is formed magnetic force field.

Picture 1. Inertia of electric current. When the current is turned on, a magnetic field appears around the conductor.

In the first moments after turning on the current, a significant part of the energy of the current source is spent on creating this magnetic field and only a small part is spent on overcoming the resistance of the conductor, or rather, on heating the conductor with current. Therefore, at the moment of closing the circuit the current does not immediately reach its limiting value . The limiting current strength is set in the circuit only after the end of the process of formation of a magnetic field around the conductor (Figure 2).

Figure 2. When the current source is turned on, the current in the circuit is not established immediately.

If, without breaking the closed circuit, turn off the current source from it, then the current in the circuit will not stop immediately, but will flow in it, decreasing gradually for some time (Figure 3) until the magnetic field around the conductor disappears, i.e. That is, until the entire supply of energy contained in the magnetic field is used up.

Figure 2. Influence of the EMF of self-induction on the current in the circuit. When the current source is turned off, the current in the circuit does not stop immediately.

So, the magnetic field is the carrier of energy. It accumulates energy in itself when the DC source is turned on and gives it back to the circuit after the current source is turned off. The energy of a magnetic field thus has much in common with the kinetic energy of a moving object. The magnetic field causes the "inertia" of the electric current.

We know that whenever there is a change in the magnetic flux penetrating the area bounded by a closed electrical circuit, there appears in this circuit EMF induction .

In addition, we know that any change in the current strength in the circuit entails a change the number of magnetic lines of force covered by this chain. If a closed circuit is immovable, then the number of magnetic lines of force penetrating a given area can only change when new lines enter outside this area or when existing lines go beyond this area. In both cases, the magnetic lines of force must cross the conductor during their movement. Crossing the conductor, the magnetic lines of force induce an EMF of induction in it. But since in this case the conductor induces an EMF in itself, this EMF is called EMF self-induction.

When a direct current source is connected to any closed circuit, the area bounded by this circuit begins to penetrate magnetic lines of force from the outside. Each magnetic line of force coming from the outside, crossing the conductor, induces in it EMF self-induction.

The electromotive force of self-induction, acting against the EMF of the current source, delays the increase in current in the circuit. After a few moments, when the increase in the magnetic flux around the circuit stops, the self-induction EMF will disappear and the current strength is set in the circuit, determined by Ohm's law:

I=U/R

When the current source is turned off from the closed circuit, the magnetic lines of force must disappear from the space bounded by the conductor. Each outgoing magnetic line of force, when crossing the conductor, induces in it an EMF of self-induction, which has the same direction as the EMF of the current source; therefore, the current in the circuit will not stop immediately, but will flow in the same direction, gradually decreasing until the magnetic flux inside the circuit completely disappears. The current flowing through the circuit after the current source is turned off is called self-induction current.

If the circuit breaks when the source is turned off, then the self-induction current manifests itself in the form of a spark at the point where the circuit opens.

Self-induction is the appearance in the conductor of an electromotive force (EMF) directed in opposite side relative to the power supply voltage when current flows. In this case, it occurs at the moment when the current strength in the circuit changes. A changing electric current generates a changing magnetic field, which in turn induces an EMF in the conductor.

This is similar to the formulation of Faraday's law of electromagnetic induction, where it says:

When a magnetic flux passes through a conductor, an emf arises in the latter. It is proportional to the rate of change of the magnetic flux (math. time derivative).

E=dФ/dt,

Where E is the EMF of self-induction, measured in volts, F is the magnetic flux, the unit of measurement is Wb (weber, it is also equal to V / s)

Inductance

We have already said that self-induction is inherent in inductive circuits, so we will consider the phenomenon of self-induction using the example of an inductor.

An inductor is an element that is a coil of insulated conductor. To increase the inductance, the number of turns is increased or a core made of a magnetically soft or other material is placed inside the coil.

The unit of inductance is Henry (H). Inductance measures how strongly a conductor opposes electric current. Since around each conductor through which current flows, a magnetic field is formed, and if you place the conductor in an alternating field, a current will appear in it. In its turn magnetic fields each turn of the coil add up. Then around the coil through which the current flows, a strong magnetic field will arise. When its strength changes in the coil, the magnetic flux around it will also change.

According to Faraday's law of electromagnetic induction, if a coil is pierced by an alternating magnetic flux, then a current and an EMF of self-induction will appear in it. They will impede the current that flowed in the inductor from the power supply to the load. They are also called extracurrent EMF self-induction.

The formula for the EMF of self-induction on inductance is:

That is, the greater the inductance, and the more and faster the current changes, the stronger the surge in EMF will be.

With an increase in current in the coil, an EMF of self-induction occurs, which is directed against the voltage of the power source, respectively, the increase in current will slow down. The same happens when decreasing - self-induction will lead to the appearance of an EMF, which will maintain the current in the coil in the same direction as before. It follows that the voltage at the coil terminals will be opposite to the polarity of the power source.

In the figure below, you can see that when the inductive circuit is turned on / off, the current does not appear abruptly, but changes gradually. The laws of commutation also speak of this.

Another definition of inductance sounds like this: the magnetic flux is proportional to the current, but in its formula, the inductance acts as a proportionality factor.

Transformer and Mutual Inductance

If you place two coils in close proximity, for example, on the same core, then the phenomenon of mutual induction will be observed. Let us pass an alternating current through the first one, then its alternating current will pierce the turns of the second one and an EMF will appear on its terminals.

This EMF will depend on the length of the wire, respectively, the number of turns, and also on the magnitude of the magnetic permeability of the medium. If they are placed just next to each other, the EMF will be low, and if you take a core made of magnetically soft steel, the EMF will be much higher. Actually, this is how the transformer works.

Interesting: this mutual influence of the coils on each other is called inductive coupling.

Benefit and harm

If you understand the theoretical part, it is worth considering where the phenomenon of self-induction is applied in practice. Consider the examples of what we see in everyday life and technology. One of most useful applications- this is a transformer, we have already considered the principle of its operation. Now they are less and less common, but previously fluorescent tubular lamps were used daily in lamps. The principle of their work is based on the phenomenon of self-induction. You can see her diagrams below.

After applying voltage, the current flows through the circuit: phase - inductor - spiral - starter - spiral - zero.

Or vice versa (phase and zero). After the starter is activated, its contacts open, then (a coil with a large inductance) seeks to maintain the current in the same direction, induces a large self-induction EMF and the lamps are ignited.

Similarly, this phenomenon applies to the ignition circuit of a car or motorcycle that runs on gasoline. In them, a mechanical (breaker) or semiconductor key (transistor in the computer) is installed in the gap between the inductor and the minus (ground). This key, at the moment when a spark should form in the cylinder to ignite the fuel, breaks the power circuit of the coil. Then the energy stored in the core of the coil causes an increase in the EMF of self-induction and the voltage at the electrode of the spark plug increases until the breakdown of the spark gap occurs, or until the coil burns out.

In power supplies and audio equipment, it is often necessary to remove unnecessary ripples, noises or frequencies from the signal. For this, filters of different configurations are used. One option is LC, LR filters. Due to current rise inhibition and AC resistance, respectively, it is possible to achieve the set goals.

Self-induction EMF harms the contacts of switches, circuit breakers, sockets, automata and other things. You may have noticed that when you pull out the plug of a working vacuum cleaner from the outlet, a flash inside it is very often noticeable. This is the resistance to a change in current in the coil (motor winding in this case).

In semiconductor switches, the situation is more critical - even a small inductance in the circuit can lead to their breakdown when peak values ​​Uke or Usi are reached. To protect them, snubber circuits are installed, on which the energy of inductive surges is dissipated.

Conclusion

Let's summarize. The conditions for the occurrence of self-induction EMF are: the presence of inductance in the circuit and a change in current in the load. This can occur both during operation, when changing modes or disturbing influences, and when switching devices. This phenomenon can harm the contacts of relays and starters, as it leads to the opening of inductive circuits, for example, electric motors. To reduce the negative impact, most of the switching equipment is equipped with arc chutes.

For useful purposes, the phenomenon of EMF is used quite often, from the filter for smoothing ripple currents and the frequency filter in audio equipment, to transformers and high-voltage ignition coils in cars.

We hope that now it has become clear to you what self-induction is, how it manifests itself and where it can be used. If you have any questions, ask them in the comments below the article!

materials

When the current in the circuit changes, the flux of magnetic induction through the surface bounded by this circuit changes, the change in the flux of magnetic induction leads to the excitation of the EMF of self-induction. The direction of the EMF turns out to be such that when the current in the circuit increases, the emf prevents the current from increasing, and when the current decreases, it stops decreasing.

The magnitude of the EMF is proportional to the rate of change of the current strength I and loop inductance L :

.

Due to the phenomenon of self-induction in electrical circuit with an EMF source, when the circuit is closed, the current is not established instantly, but after some time. Similar processes also occur when the circuit is opened, while the value of the self-induction emf can significantly exceed the source emf. Most often in ordinary life it is used in the ignition coils of automobiles. Typical self-induction voltage at 12V battery voltage is 7-25kV.


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Physics grade 10-11. SELF-INDUCTION

Each conductor through which electric current flows is in its own magnetic field.

When the current strength changes in the conductor, the m.field changes, i.e. the magnetic flux created by this current changes. A change in the magnetic flux leads to the emergence of a vortex electric field and an induction EMF appears in the circuit.

This phenomenon is called self-induction.
Self-induction - the phenomenon of the occurrence of induction EMF in an electric circuit as a result of a change in current strength.
The resulting emf is called EMF self-induction

Manifestation of the phenomenon of self-induction

Closing the circuit

When a circuit is closed, the current increases, which causes an increase in the magnetic flux in the coil, a vortex electric field arises, directed against the current, i.e. an EMF of self-induction occurs in the coil, which prevents the current from rising in the circuit (the vortex field slows down the electrons).
As a result L1 lights up later, than L2.

Open circuit

When the electric circuit is opened, the current decreases, there is a decrease in the m.flow in the coil, a vortex electric field appears, directed like a current (tending to maintain the same current strength), i.e. A self-inductive emf appears in the coil, which maintains the current in the circuit.