Resistance in electrical circuits is of two types - active and reactive. Active is represented by resistors, incandescent lamps, heating spirals, etc. In other words, all the elements in which the flowing current directly performs useful work or, in a special case, causes the desired conductor heating. Reactive, in turn, is a generic term. Under it is understood capacitive and inductive reactance. In the elements of the circuit with reactance, various intermediate energy conversions occur during the passage of an electric current. A capacitor (capacity) accumulates a charge, and then gives it to the circuit. Another example is the inductive resistance of a coil, in which part of the electrical energy is converted into a magnetic field.
In fact, there are no “pure” active or reactive resistances. The opposite is always present. For example, when calculating the wires for power lines of long length, not only active resistance, but also capacitive, are taken into account. And considering the inductive resistance, you need to remember that both the conductors and the power source make their adjustments to the calculations.
Determining the total resistance of the circuit section, it is necessary to add the active and reactive components. Moreover, it is impossible to obtain a direct sum by the usual mathematical action, therefore they use the geometric (vector) addition method. A rectangular triangle is constructed, the two legs of which are active and inductive resistance, and the hypotenuse is complete. The length of the segments corresponds to the current values.
Consider inductance in an alternating current circuit. Imagine a simple circuit consisting of a power source (EMF, E), a resistor (active component, R) and a coil (inductance, L). Since the inductive resistance arises due to the self-induction EMF (E si) in the turns of the coil, it is obvious that it increases with increasing circuit inductance and increasing current flowing along the circuit.
Ohm's law for such a circuit looks like:
E + E si = I * R.
Having determined the derivative of the current from time (I CR), we can calculate the self-induction:
E si = -L * I ex
The “-" sign in the equation indicates that the action of E si is directed against a change in the current value. Lenz’s rule says that with any change in current, EMF of self-induction occurs. And since such changes in alternating current circuits are natural (and constantly occur), E si forms a significant reaction or, which is also true, resistance. In the case of a DC power supply, this dependence is not fulfilled, and when trying to connect a coil (inductance) to such a circuit, a classic short circuit would occur.
To overcome E si, the power source must create such a potential difference at the terminals of the coil that it is sufficient, at least, to compensate for the resistance E si. This implies:
U kat = -E si.
In other words, the voltage across the inductance is numerically equal to the electromotive force of self-induction.
Since with increasing current in the circuit, the magnetic field increases , which in turn generates a vortex field, which causes an increase in the countercurrent in the inductance, we can say that there is a phase shift between voltage and current. One peculiarity follows from this: since the EMF of self-induction prevents any change in the current, when it increases (the first quarter of the period on a sinusoid), the field generates a countercurrent, but when it falls (second quarter), on the contrary, the induced current is co-directed with the main one. That is, if we theoretically allow the existence of an ideal power source without internal resistance and inductance without an active component, then the oscillations of the source-coil energy could occur for an unlimited time.