Electrotechnics: Basics


Electric current of conduction

As in everyday life, we hear the expression of an electric current, it is primarily meant by this is the conduction current. This is just one of the types of currents for the environment, called conductors.

The nature of current conduction due to the property of a substance called - conductors. Let's see that such conductors as they exist in the electric current and which at the same phenomena occur.

Let's start with the fact that the electric current is the flow of electricity, so is the flow of charges, which are the most electricity in the quantitative measurement in the Coulomb (C).

Elementary electric charge

Perhaps the most difficult and at the same time a simple question. What is electricity? We have one fundamental property of matter - is the electric charge. Fundamental - so basic, basic, inalienable, almost dogmatic, as an axiom in geometry.

Electric charges are a quantitative measurement. Each stack consists of elementary, and therefore the total number of steps formed discretely. Elementary particles, and above all the electrons that interest us most of all - have an electrical charge, which is expressed quantitatively measured in coulombs (C). Whatever the amount of charge it is always a multiple of the elementary charge.

The elementary charge of an electron is equal to:

Elementary charge

In addition, there are more electrons and protons, which have the same amount of charge as the electron, but we now have little interest because they are not free charge carriers in conductors.

Free and non-free charges

The easiest answer to the question. What is electricity? It is the nature and behavior of electric charges. Charges may be free and bound, like everything else in nature, they also are in motion. Here is the movement of electric charges is an electric current, or the flow of electricity.

What is different from the free charge is not free, you have to contact? Free charges under the influence of external electric fields, or other mechanical or other forces that can change the trajectory of their movement, speed, character movement, braked and accelerated, increase and decrease its kinetic energy. In other words, the free charges under the influence of external forces changing the nature of their movement, ie kinematics.

What about non-free charges? Since there are free charges, then there must be non-free. This is related charges which under the influence of external forces, any, or Coulomb, or mechanical or other, can not change the kinetic energy.

These charges do not change the nature of their movement, and the forces acting on them face opposition that has its limits. If we take a crystal of salt, e.g., NaCl (sodium chloride), in its sodium and chlorine atoms are connected each individually atom has its electric charge, but they are connected in the crystal and the like hold each other at the leash. Exposure to external electric field can only turn the crystals, but the growth of the electric field of the two atoms are mutually counteract the Coulomb forces. Such opposition has its limits, its borders, and as soon as the field intensity exceeds a certain threshold, the molecule of the substance NaCl be broken off and each atom becomes a free charge. Such charges in the form of atoms are called ions. Before breaking molecules force fields will do the work, which will lead to heating and melting salt crystals.

Perfect conductor of electricity

Let's imagine a perfect conductor of electricity, where there is only one free charges - electrons. In reality, this guide does not exist, there is only an approximation to it, but imagine a provider of electricity is possible.

What do we have as an ideal conductor? The easiest way to present it as shown in the form of a rectangular bar in which free electrons exist. This bar has a volume of V (m3), length, height, length and depth. Inside the bar are uniformly distributed charges - electrons. On the volume bar V, there is some value of the charge density σ (sigma), which shows how many free of charge per unit volume. In other words it is the value of a multiple of the elementary charge, and shows how many coulombs of electricity per unit volume of 1 m3.

Conductor with free electrons

As in nature everything is moving, the free charges in a perfect conductor will stay in a chaotic movement. Everyone will move seemingly random path of least resistance. If through the bar to hold the section plane S, then at any time the number of charges to slip through the plane in one direction and the other, the right and left of it, will be about the same. In this case we say that the flow of electricity through the plane missing since entered the same amount of charge, and many fled.

Square section S is like a sieve through which the charges. The amount of the charges dq passed in time dt through a sieve and is thus the flow of electricity, which is called an electric current. In fact, it is the statistical rate of electricity through a conductor cross-section.

What will happen if an external electric field? What will happen with the charges in the amount of bar V, when the ends will source an electric field?

There will be the following.

Electric current of conduction

Under the influence of an electric field source of free charge carriers - electrons begin to move, because they will operate the Coulomb force. The action of the force F will lead to an increase in the velocity of the charge, they will start to move with acceleration, and every moment of time their velocity will increase. If our cube was very high, and the electric field is constant, then for a long time, the elementary particles were dispersed up to near the speed of light. This is possible in an ideal conductor, since nothing prevents the set speed electrons. But in the real conductor of the substance, such as copper, it is not possible because the electrons meet on the way an obstacle in the form of a crystal lattice.

Once the electrons will cross section S, we can safely say about the flow of electricity through this section. In this case, the time interval dt, will pass through the section S of a certain amount of electricity dq, and hence the current I will have a value different from zero. This will be the most current conduction which was mentioned in the beginning. For the ideal case the current will increase as the speed of the free charge increases. The current in this case will also increase as the rate of charges, but in the real conductor of this happens because the electrons are braking when a collision with the electric field of the crystal lattice.

On what does the value of the current? On many factors. One of them - a cross-sectional area S, and the practice - a cross-section of the wire. Another factor - the density of free carriers per unit volume, or else, the concentration of charges in the conductor. The third factor - the value of the electric field intensity of the source, which is proportional to the voltage source and its quality. The fourth factor - the various obstacles to the free movement of charges, such as the structure of the crystal lattice of the conductor material.

Real conductor of electricity

Almost all metals are conductors of electricity. This is due to the fact that they are of sufficient free electron concentration (second factor) to be good conductors of electricity. Individual features of the crystal lattice of metals affect the ability to conduct current (fourth factor), and to be more precise let him. Conductor cross-section is proportional to the conductivity improves (first factor). The third factor is not mentioned, as relates to the power supply and not to the conductor.

Electrical conductivity of the material is measured in siemens (S). The value of one of conduction siemens shows how much electricity (coulomb) pass through a section of a conductor in one second when the voltage drops on it in one volt.

The conductivity value all metals is different. The most common material for making electrical circuits to the conductors is copper, but aluminum is also used. The best indicator of of conduction is silver, but gold is inferior to copper. Especially as a guide, you can select a non-metal - carbon (graphite).

Typically, materials are classified as conductors if the conductivity is in the range from 10 to 104-108 siemens (S), all that is above 108 - called superconductors, materials with a conductivity of 10 to 10-8 siemens - is often semiconductors, and everything that has a conductivity of less than 10-8 - called insulators.

Heating conductors during the passage of an electric current

In the real conductor current does not increase indefinitely, as the speeding electrons collide with the crystal lattice of the material and all the stored energy is given in the form of photons. For this reason, the wires are heated by an electric current.

The atoms in the lattice sites are constantly are vibrating, and, the higher the temperature conductor material, the stronger the vibrations of the crystal lattice. With increasing temperature, the conductivity of the material deteriorates and the flow of current leads to the heating wires.

Date: 02.01.2016

© Valentin Grigoryev

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