Man has long been using electric, chemical, atomic energy for his needs. For a technical description of any of them, there is a set of concepts that allow characterizing their essence. For example, such signs as power, tension, density, etc., are widely used in the study of not only electric, but also other known types of energy. One such universal concept is the term “resistance”, which is widely used in electricity. In other areas there are its analogues - absorption, scattering, reflection, etc. "Resistance" - this, in fact, is a characteristic of the loss of the energy field. The purpose of science and technology is to determine what the cause of resistance is.
Resistance in electrical circuits has a twofold essence - they say, active and reactive resistance. For a conductor, electrical resistance is the main characteristic and is due to the resistance of the conductor material to the movement of current carriers. The reasons for this opposition may be different, which explains its different name. Resistance is always accompanied by the conversion of one type of energy into another by reducing the energy of the main source. For the case of electrical energy, this transition means the conversion of the energy of the emf source into thermal, magnetic or electrical energy.
Historically, the first in the biography of resistance was the study of active resistance, which is due to the conversion of the source energy into conductor heating. This happens because the charges (and these are electrons) under the influence of the emf field of the source move along the conductor, figuratively speaking, “pushing” crystals or molecules of matter. In this case, the mutual exchange-energy transfer leads to an increase in the temperature of the conductor, i.e. there is a conversion of electrical energy into heat. If the emf source does not change its value U and direction, then the current in circuit I is called constant, and the resistance R of such a circuit is calculated based on Ohm's law: R = U / I.
The resistance of the DC circuit can only be active. The reactance "makes itself felt" only in AC circuits that contain a very specific inductance (coil) or capacitance (capacitor). Strictly speaking, any conductor has some inductance and capacitance, but usually they are so negligible that they are neglected. Inductance and capacitance when electric charges flow through them transform their energy into the magnetic field of a coil or the electric field of a dielectric. The energy stored in this way, when the emf source is changed, returns back in the form of the energy of motion of the charges, hence the name - "reactance".
Inductance in an alternating current circuit “resists” the flowing current through the phenomenon of self-induction: a change in current generated by a change in the emf of the source changes the electromagnetic field so that it tries to maintain the current in the circuit due to the stored energy of the magnetic field. The measure of stored energy is a measure of the inductance of the circuit L, which depends on the frequency f of the alternating current. The reactance of the inductor is determined by the following formula:
XL = 2 * π * f * L.
A capacitor in an alternating current circuit accumulates the energy of an electric field by charging a dielectric. When changing the magnitude and / or direction of the emf of the source, the voltage across the capacitor plates is supported by a decreasing current, the longer, the larger the capacitance C of the capacitor.
The capacitor reactance, also frequency-dependent, is calculated by the formula:
Xc = 1 / (2 * π * f * C).
It can be seen from this expression that with increasing frequency and / or capacitance, the resistance decreases. Thus, for an alternating current circuit, where there is a resistor, an inductor and a capacitor, it is necessary to determine a certain total active and reactive resistance. In general, the formula for calculating the impedance has a “Pythagorean flavor”:
Zv2 = Rv2 + (XL + Xc) v2
* note: the sign “v” should read “Z squared”, etc.
And finally, the impedance formula is as follows:
Z = √ (squarte) Rv2 + (XL + Xc) v2.