The ability of a substance to conduct electric current is called. General information. Substances whose aqueous solutions conduct electric current are called electrolytes

Substances aqueous solutions which are carried out electric current, are called electrolytes. Unlike metals, which have electronic conductivity, and semiconductors, which have electron-hole conductivity, electrolytes have ionic conductivity.

Sometimes conductive solutions themselves are called electrolytes, although a more correct expression is an electrolyte solution.

Water molecules disintegrate to a small extent into ions:

The concentration of hydrogen ions determines the acidity of the solution, and the concentration of hydroxyl ions characterizes the alkalinity of the solution. In pure water, ion concentrations N+ and HE- equal. Clean water dissociates very weakly. B 1 mole water at 22º WITH disintegrates into ions mole.

However, it is very difficult to obtain such water, because There is always carbon dioxide in the air, which, when dissolved in water, increases the concentration of hydrogen ions. Since water has a large dielectric constant () and water molecules have a significant dipole moment ( Kl∙m), then around water molecules at interatomic distances ( nm) there is quite a strong electric field. The latter is the direct cause that weakens the force of electrostatic attraction of ions in the molecules of the dissolved substance. Therefore, in the process of dissolving a salt or alkali, molecules decompose into anions and cations due to thermal collisions. If the molecules of a solute in water do not dissociate into ions, then the solution is not a conductor. For example, aqueous solutions of sugars and glycerin are insulators.

The result of dissociation is the formation of solvates (hydrates), when water molecules “envelop” ions, forming a solvation shell around them (Figure 1).

Figure 1 Solvation shells: a – cation; b – anion

To create an electric current in the electrolyte, it is necessary to lower electrodes made of a conductive material (metal, coal, etc.) into a bath with an electrolyte solution, to which a current source is connected (Figure 2). Such a device is called a galvanic or electrolytic bath.

Figure 2 Electrolytic bath: 1 - bath with solution

copper sulfate; 2 - cathode; 3 – current source; 4 – anode;

I - velocities of positive and negative ions

An ion in an electrolyte is acted upon by two forces: the force from electric field and the force of resistance to movement on the part of the medium. The force acting from the electric field is calculated by the formula:

where is the charge of the ion, Cl; - electric field strength, .

The force due to the interaction of molecules surrounding the ion is proportional to the speed:

where is the coefficient of resistance to the movement of ions in the medium.

When an ion moves in an electrolyte, equilibrium is quickly established between the forces and the movement of the ion between the electrodes can be considered as uniform and linear, therefore:

From (4) it follows:

If we designate , then . Coefficient b called ion mobility. The physical meaning of mobility is that it characterizes the speed of ions in the electrolyte at electric field strength E = 1 .

Since the current in electrolytes represents the ordered movement of ions of both signs, caused by the action of an external electric field, the current density in the electrolyte is determined by the expression:

, (6)

Where n+ and - - concentrations of cations and anions; + and - - are their drift speeds, + and - are their charges.

The redox reactions occurring at the cathode and anode obey Faraday's laws.

First Law: the mass of the substance released on the electrode is proportional to the charge flowing through the electrolyte:

, (7)

where is the electrochemical equivalent; I – current strength, A; t- time, With.

Electrochemical equivalents of a number of elements are given in Table 1.

Table 1 Values ​​of electrochemical equivalents

for some substances

Second Law: electrochemical equivalents of elements are directly proportional to their chemical equivalents:

Where F- Faraday number ( F= 96500 ); M– molar mass of the substance released on the electrode; n- its valency, - chemical equivalent.

Products of electroreduction or electrooxidation of electrolyte ions can enter chemical reactions with the solution near the electrode. Such processes are called secondary reactions.

To understand the phenomenon of electrical conductivity of a substance, you first need to understand what electric current is, since these two phenomena are inextricably linked with each other. Electric current is the ordered movement of charged particles, which can occur under the influence of an electric field.

The main feature of electric current is its wide application in energy. This is the only type of energy that can be freely transferred to long distance without any additional processing.

When considering the transfer of energy, it is necessary to touch upon the topic of conductors through which current is transmitted. Charged particles can be electrons and ions in electrolytes and gases; in semiconductors, electrons and holes become such particles. This feature directly depends on the structure of the substance or body. Moreover, each body has its own electrical conductivity.

What is electrical conductivity?

In simple words, electrical conductivity or electrical conductivity is the ability or property of a substance or body to conduct an electric current that is created under the influence of an electric field.

This physical quantity, which can be measured, on the basis of which the characteristics of a particular conductor are given. The higher this characteristic, the better the body conducts current.

As already mentioned, free charged particles act as conductors of electric current, which means that electrical conductivity depends on the number of such free particles. The greater the concentration of free charged particles, the better the substance or body will conduct electric current.

Main groups of conductors

Since different bodies have different numbers of free particles, electrical conductivity is different for everyone. According to this indicator, bodies can be divided into three main groups.

The first group includes conductors; they have the highest conductivity. Such bodies are best able to conduct electric current. However, conductors can also be of two types, the difference lies in the characteristics of the current flow.

1. The first type of conductors is metals. They have electronic conductivity, since the current flows due to a large number of free electrons;

2. The second type of conductors with high electrical conductivity are various acids, alkaline solutions and salt. Another name for them is electrolytes. In them, free charged particles are ions. Hence the name - ionic conductivity.

It is worth noting that there are substances of a mixed type, that is, both electrons and ions can be charged particles in them. It could be some gases.

The high electrical conductivity of metals is easily explained when considering their structure. In metal atoms, valence electrons can easily move from one atom to another because they are weakly bound to the nucleus. Thus, an electric current is generated.

Electrical resistance and current flow rate

The electrical conductivity of a body directly depends on the resistance of the substance, and its value will be inversely proportional to the resistance indicator.

Electrical resistance is a property of any conductor; the resistance value is equal to the ratio of the voltage to the strength of the flowing current. We can say that the higher the voltage of the supplied current, the higher the speed of current flow, but the resistance of the conductor reduces this indicator.

It should be added that the electric field, which generates the ordered movement of particles, and, consequently, the electric current, propagates in space at the speed of light. That is, electric current always flows at a speed of 300 thousand kilometers per second.

What then is the peculiarity of the speed of movement? The fact is that the speed of current flow is equal to the speed of light, but individual electrons can move at different speeds - from a few millimeters to several centimeters per second. This, of course, is not very high speed.

Why is there such a difference and how can we explain the speed of current propagation? The current voltage precisely determines the speed of movement of individual electrons - several millimeters or centimeters per second.

The fact is that each individual electron moves in one huge stream of electrons that completely fill the conductor. And each electron constantly interacts with other electrons. This happens during the passage of electric current.

Therefore, an individual electron moves extremely slowly, however, the speed of energy propagation in an electrical conductor will be very high. Accordingly, the greater the number of free particles, the better their interaction will be, which means the higher will be the speed of current propagation and transmission speed electrical energy.

The ability of a substance to conduct electric current is called electrical conductivity.

Based on electrical conductivity, all substances are divided into conductors, dielectrics and semiconductors.

Conductors have high electrical conductivity. There are conductors of the first and second kind. Conductors of the first kind include all metals, some alloys and coal. They have electronic conductivity. Conductors of the second type include electrolytes. They exhibit ionic conductivity.

There is no electrostatic field in conductors (Fig. 1.10b).

If a conductor is placed in an electrostatic field, then under the influence of this field the charges in the conductor move: positive - in the direction of the external field, negative - in the opposite direction (Fig. 1.10a). This separation of charges in a conductor under the influence of an external field is called electrostatic induction . The charges separated inside the conductor create their own electric field directed from positive charges to negative, i.e. against the external field (Fig. 1. 10a).

Obviously, the separation of charges in the conductor will stop when the field strength of the separated charges E

internal will become equal to the external field strength in the conductor Eexternal, i.e. Einternal = Eexternal, and the resulting field

E = Einternal – Eexternal = 0

Thus, the resulting field inside the conductor will become zero

(Fig. 1. 10b). An electrostatic screen works on this principle, protecting part of the space from external electric fields (Fig. 1. 11). To ensure that external electric fields do not affect the accuracy of electrical measurements, meter placed inside a closed conductive shell (screen), in which there is no electrostatic field.

1.4. Electrical conductivity. Dielectrics in an electric field

The electrical conductivity of dielectrics is practically zero due to the very strong connection between electrons and the nucleus of dielectric atoms.

If a dielectric is placed in an electrostatic field, then polarization of atoms will occur in it, i.e. displacement of opposite charges in the atom itself, but not their separation (Fig. 1.12a). A polarized atom can be considered as an electric dipole (Fig. 1.12b), in which the “centers of gravity” are positive and negative charges shift.

A dipole is a system of two opposite charges located at a small distance from each other in the closed space of an atom or molecule.

An electric dipole is a dielectric atom in which the electron orbit is extended in the direction opposite to the direction of the external field Eext (Fig. 1.12b). Polarized atoms create their own electric field, the intensity of which is directed against the external field. As a result of polarization, the resulting field inside the dielectric is weakened. The polarization intensity of a dielectric depends on its dielectric constant. The larger it is, the more intense the polarization in the dielectric and the weaker the electric field in it.

E = Eexternal – Einternal

If a dielectric is placed in a strong electric field, the intensity of which can be increased, then at a certain value of the intensity a breakdown of the dielectric will occur, with electrons being separated from the atom, i.e. The dielectric is ionized and becomes a conductor. The external field strength at which dielectric breakdown occurs is is called the breakdown voltage of the dielectric. And the voltage at which dielectric breakdown occurs is called breakdown voltage, or dielectric strength.

Electric current. In a substance placed in an electric field, under the influence of field forces, the process of movement of elementary carriers of electricity - electrons or ions - occurs. The movement of these electrically charged particles of matter is called electric current.
The unit of current is the ampere (A). This is a current at which an amount of electricity equal to 1 C passes through the cross-section of the conductor every second. Current strength is sometimes measured in thousandths of an ampere - milliamperes (mA) or millionths of an ampere - microamperes (μA), and for larger values ​​- in thousands of amperes - kiloamperes (kA), in formulas the current is denoted by the letter I (i).
In electrical engineering, both direct and alternating current are widely used. Constant is a current whose value and direction remain unchanged at any time (Fig. 9, a).
Currents whose value and direction do not remain constant are called changing, or variable. Most often, electrical devices use a current that varies according to a sinusoidal law, which is obtained from generators AC and transformers (Fig. 9, b). A pulsating current is received from the rectifiers (Fig. 9, c), unchanged in direction, but varying in magnitude.

Electrical conductivity. The property of a substance to conduct electric current under the influence of an electric field is called electrical conductivity. Electrical conductivity various substances depends on the concentration of free (i.e. not associated with atoms, molecules or crystal structure) electrically charged particles. The greater the concentration of these particles, the greater the electrical conductivity of a given substance. All substances, depending on their electrical conductivity, are divided into three groups: conductors, dielectrics (insulating materials) and semiconductors.
Conductors have very high electrical conductivity. There are two types of conductors, which differ physical nature flow of electric current. Conductors of the first kind include metals. The passage of current through them is caused by the movement of free electrons, as a result of which they are called conductors with electronic conductivity. Conductors of the second type are solutions of acids, alkalis and salts (mostly aqueous), called electrolytes. The passage of current through electrolytes is associated with the movement of electrically charged parts of molecules - positive and negative ions, i.e. electrolytes are conductors with ionic conductivity.
There are also substances with mixed conductivity, in which current is carried by electrons and ions. These include, for example, gases and vapors in an ionized state.
Physical nature of electrical conductivity of metals. The high electrical conductivity of metals is well explained on the basis electron theory. According to this theory, valence electrons are relatively weakly bound to their nuclei. Therefore, they move freely between atoms, moving from the sphere of action of one atom to the sphere of action of another and filling the space between them like a gas. These electrons are usually called free.
Free electrons / are in a state of random motion (Fig. 10, a). However, if you introduce a metal conductor into an electric field, then free electrons, under the influence of field forces, will begin to move towards the positive pole (Fig. 10, b), creating an electric current. Thus, electric current in metal conductors is the ordered (directed) movement of free electrons.

Metalloids have a large number of electrons in their outer shell and they are firmly held near their nuclei. Therefore, metalloids are generally dielectrics.
Speed ​​of current flow. The electric field propagates in space at a tremendous speed - 300,000 km/s, i.e. at the speed of light. The electric current passes through the conductor at the same speed. However, each individual electron moves on average along the conductor at a speed of several millimeters or centimeters per second (this speed depends on the strength of the electric field).
How can we explain such a speed of propagation of electric current? The reason is that each electron is in the general electron flow filling the conductor, and when an electric current passes, it experiences continuous influence from neighboring electrons. Therefore, although the electron itself moves slowly, the speed of transfer of motion from one electron to another (the speed of propagation of electrical energy) will be enormous. For example, when you turn on a switch at a power plant, current almost instantly appears in each section electrical circuit the whole city, despite the insignificant speed of electron movement.