Every fan of electrical engineering at least once had to hear about an incomprehensible electrical manifestation called surface or "skin effect". For those of them who still do not know what it is, the explanations presented in the article will help to familiarize themselves with this phenomenon. In addition, here you can find out how it is applied in practice.
What is skin effect?
First, you need to understand what the surface effect is, which is defined in the technical literature as a decrease in the amplitude of e / m oscillations when penetrating into a conducting medium. In this case, the moving charges under the influence of the e / m field from the depth of the conductor are displaced closer to its upper layer. As a result, the current becomes inhomogeneous in its structure, which manifests itself as a decrease in the charge density in the central part with respect to the periphery.
The depth to which the charges penetrate can be determined by the following expression:
That is, the specified dependence can be formulated as follows: with an increase in the frequency of the alternating current, the depth of its penetration into the conductor body decreases. With a double increase in frequency, this indicator will decrease to the square root of 2.
Features of current distribution
To visually imagine how the current is distributed in a conductor round in cross section, it is enough to study the photo below.
It is clearly seen from it that at a certain depth closer to the center of the wire there is either no current at all, or its value is so small that it is neglected.
A number of interesting conclusions follow from this, namely:
- when cutting out from the body of the conductor its central part, through which the current practically does not flow, the conditions for the movement of charges do not change;
- if the copper wire is made hollow (without a core), its electrically conductive characteristics will remain the same, and the cost will be significantly reduced;
- the conductivity indicator will remain the same, only its inductance and capacitance will change.
From the results obtained, it follows that the complex resistance of any conductor depends not only on the material from which it is made, but also on the frequency of the flowing current. With large values of this indicator, almost all charges will begin to move along the surface (outer boundary) of the wire, that is, they will have direct contact with the external environment.
Applying skin effect
The dependence of the power of the current in the wire on the frequency of the signal makes it possible to transmit electrical messages of different periodicity through one cable. In this case, high-frequency signals are displaced towards the outer radius, and low-frequency signals flow closer to the center.
The high-frequency communication systems are arranged and operate according to the indicated principle. In them, a current with a frequency of 50 Hertz simultaneously flows through the supply wires and telephone communication between the two parties (attendants and dispatchers, in particular) is realized through them. Due to the presence of the skin effect, monolithic wires of large cross-section are rarely used (most often they are made stranded).