Light and mechanical effects of current (8 grades, on the day of physics)

Types of electric shock

Flowing through the human body, electric current causes thermal, electrochemical and biological effects.
The thermal effect of the current manifests itself in heating and burns of individual parts of the body; electrochemical in the decomposition of blood and other organic liquids; the biological effect of the current is associated with irritation and excitation of living tissues of the body, which is accompanied by involuntary convulsive contractions of muscles, including the muscles of the lungs and heart muscles, and can cause cessation of the activity of the circulatory and respiratory organs.

These effects of current can lead to two types of damage: electrical injury and electrical shock.

Electrical injuries include electrical burns, electrical marks, electroplating of the skin, electroophthalmia, and mechanical injuries.

The cause of electrical burns can be the action of an electric arc (arc burn) or the passage of current through the human body as a result of contact with a live part (electric burn). An electric burn is, as a rule, a burn of the skin at the point of contact of the body with a live part due to the conversion of electrical energy into thermal energy. Since human skin has many times more resistance than other body tissues, it generates most of the heat. Electrical burns occur in electrical installations, mainly with voltages up to 1000 V.

An arc burn is caused by exposure to an electric arc on the body, which is created during a discharge when a person approaches live parts energized above 1000 V, or during short circuits in electrical installations

voltage up to 1000 V. High temperature electric arc can cause extensive burns to the body and lead to death.

Electrical marks, also called shock marks or electrical marks, are dead spots on the skin of a person who has been exposed to an electrical current. In most cases, electrical signs are painless and treatable.

Electrometallization of the skin is caused by the penetration of tiny metal particles into its upper layers, melted under the action of an electric arc. Subsequently, the damaged area is restored and takes on a normal appearance, and the pain disappears. Cases of eye damage can be very dangerous, often leading to loss of vision. Therefore, work in which such cases are possible must be performed with protective glasses. At the same time, the worker’s clothing must be buttoned with all buttons, the collar must be closed, and the sleeves must be lowered and buttoned at the wrists.

Often, simultaneously with metallization of the skin, an electric arc burn is possible.

Electrophthalmia is an inflammation of the outer membranes of the eyes that occurs as a result of exposure to a stream of ultraviolet rays. Such irradiation is possible when an electric arc occurs, for example, during short circuits, which is a source of intense radiation not only of visible light, but also of ultraviolet and infrared rays.

Prevention of electroophthalmia when servicing electrical installations is ensured by the use of special safety glasses, which simultaneously protect the eyes from splashes of molten metal.

Mechanical damage occurs as a result of sharp involuntary convulsive muscle contractions under the influence of current. This can lead to falls from heights, joint dislocations, fractures, etc.

Electric shocks refer to the type of injuries that occur when exposed to low currents (of the order of several hundred milliamps) and voltages up to 1000 V. With electric shocks, the outcome of the impact of current on a person can vary from a slight, barely noticeable convulsive contraction of the muscles of the fingers to a fatal injury, associated with cessation of cardiac or respiratory function.

The degree of electric shock from electric shocks is characterized by its threshold value. The following currents are characteristic: threshold palpable, threshold non-releasing, threshold fibrillation.

Threshold perceptible current is the smallest value of perceptible current that causes perceptible irritation when passing through the human body.

Threshold non-releasing current is the smallest value of non-releasing current that, when passing through a person, causes irresistible convulsive contractions of the muscles of the arm in which the conductor is clamped.

Threshold fibrillation current is the lowest value of fibrillation current that causes cardiac fibrillation when passing through the body.

As will be shown below, the current flowing through a person fluctuates widely and depends on numerous physical and physiological phenomena that are difficult to account for. Unlike previous years, the prevailing opinion in electrical safety engineering is that it is inappropriate to standardize dangerous and safe threshold values ​​for voltage and current in industry and in everyday life.

Table 1. The nature of the effect of electric current on the human body

The main factors influencing the outcome of electric shock to a person are as follows.

Thermal effect of current

Thanks to this action of current, we can illuminate rooms using incandescent lamps. We also use various heating electrical appliances - convectors, electric stoves, irons (Fig. 1).

Rice. 1. These electrical appliances convert electrical energy into heat

Using a meter-long piece of nickel wire (Fig. 2), you can demonstrate the heating of a conductor when an electric current flows through it. To noticeably sag the heated wire due to a thermal increase in length and observe its reddish glow, a current of 2 - 3 Amperes will be sufficient.

Rice. 2. The conductor heats up under the influence of current

A piece of wire heats up when electric current flows through it. The more current in the conductor, the more it will heat up. The length of the heated conductor increases.

You can read more about the amount of heat released in the article about the Joule-Lenz law (link).

Note: Nichrome, nickel, constantan are metal alloys with high resistivity (link). Wires made from such alloys are used in various heating electrical appliances.

The path of current in the human body.

The path of current in the human body affects the lesion in different ways. For some time now, great importance has been attached to this issue, since the analysis of accidents has made it possible to establish their dependence on the type of so-called current loop, that is, on the path of the current through the human body. The most common four loops are: right arm leg, left arm leg, arm arm, leg leg. In most cases, the current circuit occurs along the path of the right hand and foot. The most common and usually accompanied by severe damage is the current path (current loop) hand hand, where the current passes through vital organs, in particular the heart.

Accident analyzes show that approximately 55% of all electrical shocks occur along two main paths: from the hand or hands to the feet and from one hand to the other hand. However, fatal injuries account for half of the reported number of accidents.

The danger is determined not by whether current flows or does not flow through the heart area, but by what part of the body a person touches live parts. The most vulnerable places of the human body are the back of the hand, neck, temple; front of leg, shoulder. The formation of an electrical circuit through vulnerable points leads to fatalities even at very low currents and voltages.

Chemical effect of current

An electric current passing through solutions of certain acids, alkalis or salts causes the release of a substance from them. This substance is deposited on electrodes - plates dipped into the solution and connected to a current source.

This effect of current is used in electroplating - metal coating of certain surfaces. Nickel plating, copper plating, chrome plating, as well as silver and gilding of surfaces are used.

Using a solution of copper sulfate, you can demonstrate the release of a substance under the influence of current. An aqueous solution of this salt has a bluish tint. By passing an electric current (link) through the solution, you can detect copper precipitation on one of the electrodes (Fig. 3).

Rice. 3. Copper is released from a solution of copper sulfate when current flows, depositing on one of the electrodes

At which electrode will copper be deposited?

Copper in the vitriol solution is present in the form of positive ions. Bodies with opposite charges attract each other. Therefore, copper ions will be attracted to the plate, which has a charge with a minus sign. That is, a plate connected to the negative terminal of the current source. This plate is called a negative electrode, or cathode.

The second plate connected to the positive terminal of the battery is called the anode.

Note: Copper sulfate can be found at a hardware store. Its chemical formula is \(\large CuSO_{4}\). It is used in agriculture for spraying the foliage of fruit trees, shrubs and vegetable crops - for example, tomatoes, potatoes. Included in various solutions used in the fight against plant diseases and insect pests.

Application of the chemical action of current in medicine

The chemical effect of current is used not only in electroplating.

Passing an electric current through solutions causes the movement of charged particles of the substance in them - positive and negative ions. The human body contains fluids in which certain substances are dissolved. This means that such liquids contain ions.

By applying special electrodes moistened with drug solutions to individual areas of the body and passing small currents through them, it is possible to introduce certain medications into the body (Fig. 4).

Rice. 4. Electrophoresis is based on the chemical action of current

This administration of drugs is called electrophoresis and is used in physiotherapeutic rooms of clinics and sanatoriums.

Electrical resistance of the human body.

The electrical resistance of the circuit through which current passes through the human body consists of the electrical resistance of the active and inductive wires; electrical resistance of machines, devices or devices that are connected in series with the human body; electrical resistance of the transition contact between live parts of the equipment touched by a person; the human body's own electrical resistance.

The resistance of the human body is a complex complex of biophysical, biochemical and other phenomena. It is usually divided into two parts: skin and blood vessel resistance and nerve resistance. The top layer of skin has a noticeable resistance compared to the resistance of the internal organs. The presence of sweat glands in the skin greatly changes its electrical resistance. There is very little nerve resistance. It is this component of the total resistance that plays the most significant role in current conduction, and therefore in the outcome of electrical injury. The electrical resistance of a living organism is influenced by a large number of factors. The condition of the skin is of significant importance: damage to the stratum corneum (pores, scratches, abrasions and other microtraumas); hydration with water or sweat; contamination with various substances, especially those that conduct electricity well (metal or coal dust, scale, etc.).

The resistance of the human body, i.e. the resistance between two electrodes applied to the surface of the body, can be conventionally considered to consist of three resistances connected in series: two resistances of the outer (horny) layer of the skin and one, called the internal resistance of the body, which includes the resistance of the inner layer of the skin and resistance of internal body tissues. In general, these resistances have active and capacitive components.

In practical calculations, it is necessary to know and evaluate the numerical values ​​of the resistance of a human electrical circuit between two electrodes applied to the body. Type of current and voltage. Research (see Table 1) and the practice of operating electrical installations show that direct current, compared to alternating current of the same values, is less dangerous for humans. This is explained primarily by the fact that due to the presence of a capacitive component in the electrical resistance of the human body, the current density, and therefore the field strength in the tissues, will be greater at equal voltages in the case of alternating current damage than in the case of direct current damage. Another significant circumstance is that with alternating current the damaging amplitude voltage can be 1.4 times greater than the effective voltage. And finally, the probability of the formation of an electrical circuit through vulnerable points with alternating current is greater than with direct current, because alternating current networks cover an incomparably larger number of installations, moreover, the most diverse ones, while direct current networks have more limited and specialized applications.

What has been said about the relative danger of injury from direct and alternating currents is true only for small voltages of the order of 250 - 300 V. At higher voltages, direct current is more dangerous than alternating current with a frequency of 50 Hz, due to the possibility of throwing the victim away from live parts under high voltage , which is extremely rarely observed with similar alternating current injuries. The thrown person may receive mechanical injury, which may result in death (for example, in a fall).

In general, it should be noted that the question of the comparative danger to humans of alternating and direct current needs further study, which will expand our understanding of the biophysics of electrical injury.

Voltage applied to an electrical circuit leads to the transformation of electrical phenomena into other phenomena, the impact of which on the human body directly causes one or another outcome of the lesion. There is an opinion that the outcome of electric shock depends on the network voltage: the higher the voltage, the more dangerous the consequences of electrical injury. In statistical reports, electrical injuries are recorded and subdivided by network voltage values. Based on the same criteria, data are analyzed and electrical injuries are classified, research and experiments are conducted. Meanwhile, such a study of electrical trauma does not always give a correct idea of ​​this damaging factor.

Our current Rules divide all installations into voltages below and above 1000 V. In installations with voltages above 1000 V, the main cause of fatal injuries is burns caused by the passage of electric current. In installations below 1000 V, the main cause of damage is due to the direct effect of current. Statistics show that fatal electrical injuries occur predominantly in installations up to 1000 V.

Fatal injuries also occur at low voltages (65, 36, 24, 12 V). Their analysis shows that they are caused not only by fibrillation current, which cannot be obtained at these voltages. Damages from 12 to 65 V can lead to death only under special circumstances, for example, if the electrical circuit occurs through places vulnerable to current, if environmental conditions are unfavorable. There are also other possible causes of death that have not yet been sufficiently studied.

Summarizing what has been said regarding the absence of a direct relationship between the outcome of the injury and voltage, current, we state that it is impossible to standardize with high accuracy in industry (and in everyday life) dangerous and safe threshold values ​​of current and voltage.

Magnetic effect of current

Copper by itself is not attracted to a magnet. This can be verified using a small magnet and a piece of copper wire (Fig. 5a).

In Figure 5, a piece of copper wire is suspended from two stands using thin threads that do not conduct electricity.

However, during the flow of electric current, the copper conductor begins to interact with the magnet - to be attracted to or repelled from it (Fig. 5b).

Rice. 5. A magnetic field arises around the current-carrying conductor, due to which the conductor interacts with the magnet

It is not the copper conductor itself that interacts with the magnet, but the current flowing through this conductor.

Why does a current-carrying wire interact with a magnet?

An electric current is a large number of electrons running through a wire from one end to the other. Electrons have a charge.

A magnetic field arises around moving charges. Thanks to this, the current-carrying wire turns into a small magnet. And it begins to interact with the magnet, being attracted to it or repelled from it.

At the same time, the wire, as a lighter object, will move. And the magnet will continue to remain in place. Due to the fact that its mass is significantly greater than the mass of a piece of wire.

The direction of movement of the wire depends on the polarity of its connection to the battery and on how the magnet poles are positioned.

The action of an electromagnet is based on the magnetic action of current.

Homemade electromagnet

It is easily made from a piece of flexible insulated copper wire and an iron nail.

The nail must be wrapped in a piece of paper - a sleeve (Fig. 6). Then you need to wind 200 - 300 turns of thin copper wire in insulation onto the sleeve. You need to connect a battery from a flashlight to the terminals of the resulting coil.

Rice. 6. You can make a homemade electromagnet from scrap materials

During the flow of current, various small iron objects are attracted to the nail - paper clips, buttons, nails, iron filings, sawdust, etc.

Having disconnected the battery, we will see that as soon as the current stops, the nail stops attracting iron objects.

The duration of the existence of an electrical circuit through the human body.

The outcome of electric shock is related to the time factor. When analyzing accidents, much attention is paid to this parameter, especially considering the presence of contradictions in the assessment of the dangerous (and safe) time of current passing through a person. On the one hand, there are injuries with a severe outcome even with small currents and a very short duration of current passing through a person (fractions of a second), on the other hand, cases with a favorable outcome (excluding burns) with a duration of injury of several seconds or more.

Due to the above contradictions, it is not possible to strictly substantiate the dependence of the outcome of the lesion on the duration of the existence of the electrical circuit.

Basic Concepts

Electric current is the ordered movement of charged particles, due to which an electromagnetic field can be generated. Charged particles include the following: electrons, protons, neutrons, holes and ions. In the scientific literature, a neutron has no charge, but participates in the formation of an electromagnetic field.

In addition, some people do not know why electric current is a vector quantity . This statement follows from its definition, since it has a direction. In some sources you can find the following definition: electric current is the speed at which the charges of elementary particles change at a certain point in time. Current is characterized by strength and voltage (potential difference). Properties that electric current has: thermal, mechanical, chemical and the creation of an electromagnetic field.

Current strength and type

Current strength is the number of charged particles passing through a conductor per unit of time equal to one second. Conductivity materials are divided into three groups: conductors, semiconductors and dielectrics. Conductors are substances that are capable of conducting current because they contain free electrons. Their presence can be determined from the table of D.I. Mendeleev, using the electronic configuration of the chemical element.

Semiconductors can conduct a stream of charged particles under certain conditions. A simple example is a semiconductor diode, which conducts current in only one direction. Charge carriers are electrons and holes. There are no charge carriers in dielectrics at all, therefore, this fact excludes the conduction of electricity at all.

Current strength is designated by the letter I and is measured in amperes (A). 1 A is a unit of measurement of the strength of a constant current that passes through two conductors of infinite length and a very small cross-sectional area, which are parallel to each other and located in vacuum space at a distance of one meter from each other, and each meter of such a conductor can cause an interaction force, equal to 2*10^(-7) N.

A simplified version of the formulation is as follows: the electric current strength at which the amount of electricity Q passes through the cross-sectional area of ​​the conductor per unit time t is called an ampere. The definition is written as a formula and has the following form: I = Q / t.

There are auxiliary units of measurement, which include mA (0.001 A), kA (1000 A), etc.

The current value is measured using an ammeter , which is connected in series to the circuit. There are only two types of electric current: direct and alternating. If the current remains constant or changes in magnitude without changing direction, then it is called constant.

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Alternating current changes in amplitude value and direction of flow according to some law. Its main characteristic is frequency. According to the law of amplitude changes, they can be divided into the following types: sinusoidal and non-sinusoidal. The first ones change according to a harmonic law and its graph is a sinusoid. The sinusoidal current formula includes the maximum value of the power characteristic Im, time t and angular frequency w = 2 * 3.1416 * f (frequency of the power source current): i = Im * sin (w * t). Another quantity characterizing electric current is voltage or potential difference.

Potential difference

Any substance consists of atoms, consisting of elementary particles. The nucleus has a positive charge, and electrons with a negative charge revolve around it in their orbits. Atoms are neutral because the number of electrons is equal to the number of protons in the nucleus.

When atoms lose electrons, an electromagnetic field is created by protons as they strive to recapture the missing negatively charged particles. If for some reason there is an excess of electrons, then an electromagnetic field with a negative component is formed. In the first and second cases, positive and negative potentials are formed, respectively. The difference between them is called voltage or potential difference.

The magnitude of the difference is directly proportional to the voltage value : as the difference increases, the voltage value increases. When potentials with different signs are connected, an electric current arises, which tends to eliminate the cause of the difference and return the atom to its original state.

Electric voltage is the work done by an electromagnetic field to move a point charge. The unit of voltage is volt (V), and its value can be measured using a voltmeter. It is connected in parallel to the area or electrical appliance on which the potential difference needs to be measured. 1 V is the potential difference between two points with a charge of 1 C, at which the force of the electromagnetic field does work equal to 1 J.

Effect of frequency

From the above formula for the total resistance of the human body, it follows that as the frequency of alternating current increases, the resistance decreases, which leads to an increase in current and an increase in the danger of injury. However, practice shows that this conclusion is valid only within certain frequencies. For a long time it was believed that in the low frequency region the 50-cycle current is most dangerous. With a further increase in frequency within the range of 50 - 400 Hz, the current maintains approximately the same values. Further increase in frequency reduces the risk of injury. But whether it is harmful or not harmful to the human body, there is no affirmative answer yet.

There is a comparative danger to humans from rectified current. The presence of frequency components in it aggravates the outcome of electrical injury. So far this is a little-studied section of electrical safety.

Content

What phenomena are observed in a circuit in which an electric current exists?

As you already know from the material covered, electric current has various actions:

• During thermal action , the conductor through which current flows heats up. This action is described by the Joule-Lenz law ($Q = I^2Rt$)
• By passing current through certain acid solutions, one can see its chemical effect . It allows you to obtain pure metals from such solutions
• Using magnetic action, you can make a magnet from any iron object. Also, when a frame with current flowing in the winding is placed between the poles of a magnet, it begins to rotate.

The fact is that magnetic action always when there is an electric current.

For example, conductors carrying current interact with each other. How? They say that magnetic forces (Figure 1). Clearly, they lead to deformation of conductors.

Figure 1. Deformation of conductors with electric current due to interaction with each other

So we have listed the magnetic phenomena already known to you. It is these phenomena and the forces that arise during them that will be discussed in this section. Let's start by looking at the very fact of the existence of a magnetic field .

Environmental impact.

The environment can in many cases influence the risk of electric shock to a person. Factors of this influence include atmospheric pressure, temperature, humidity, electric or magnetic fields, etc.

An increase in air temperature affects a person's sweating, as a result of which the electrical resistance of his body drops and the danger of electric shock increases.

Similar phenomena are also associated with high humidity. Here there is a decrease not only in electrical resistance, but also in the body’s overall resistance to electric current.

The influence of these two factors, temperature and humidity, is recorded in regulatory documents.

The third atmospheric factor, ambient air pressure, also affects sensitivity to electric current. As the pressure increases, the risk of injury decreases. For example, statistics show that no fatal or severe electrical injuries have been recorded during underwater electric welding, although cases of contact of divers working underwater with live elements and contacts have been noted repeatedly.

The opposite picture was established for low atmospheric pressure, which is especially significant in connection with the electrification of mountainous areas. It has been experimentally proven that low atmospheric pressure increases the danger of electric current for living organisms.

Basics of Electrical Engineering

Electrical engineering deals with technical devices and installations designed for the production, distribution and use of electricity. Many machines and technical installations use electrical energy to operate because it can be converted into other forms of energy, such as thermal energy or mechanical energy, without large losses. In order to be aware of the dangers when using electrical appliances, as well as to better understand the need to comply with safety requirements (VDE requirements), knowledge of the basics of electrical engineering is necessary.

Basic Concepts

Electric current circuit Electrical energy can only be transmitted in a closed circuit. It is called an electric current circuit. The movement of electrically charged particles in a circuit is called electric current. In metal conductors it consists of a flow of electrons, in conducting liquids (electrolytes) and in gases (plasma) - of ions. Due to their good electrical conductivity, copper and aluminum are used as materials for electrical conductors. Metals have free electrons that are loosely bound to the atoms and therefore can be easily exchanged between them. Poor conductors have fewer free electrons; non-conducting materials (insulating materials, also called dielectrics) have almost no free electrons, such as ceramics or synthetic materials.

To understand the electric current circuit, a simple hydraulic circuit can be used (Fig. 1). In the hydraulic network, the pump creates pressure; The fluid flow drives the hydraulic motor. Similarly, in an electric current circuit, a generator creates voltage, and the flow of electrons drives an electric motor (Fig. 2).

Rice. 1. Hydraulic flow circuit

Rice. 2. Electric current circuit

Electrical voltage (U) The hydraulic pump creates overpressure on one side and underpressure on the other side. The pressure difference causes fluid flow. In the case of a generator, a shortage of electrons is created at one pole (positive pole) and an excess of electrons at the other (negative pole). The resulting difference in electronic pressure is called electrical voltage. Electrical voltage is measured in volts (V).

A measuring device for measuring electrical voltage is called a voltmeter. Voltage meters show the voltage difference between the contacts (Fig. 3).

Rice. 3. Voltage and current measurement

Electric Current (I) Electric current can flow if voltage exists and the circuit is closed. The number of electrons passing through a conductor per unit time is called electric current. Electrical current is measured in amperes (A). A measuring device for measuring electric current is called an ammeter. The current meter must be connected to the electrical circuit in such a way that current flows both through the electrical appliance and through the measuring device (see Fig. 3).

Electrical Resistance (R) All electrical wires and devices create greater or lesser resistance to electrical current. The resistance value and condition of the wires depend on the size of the wire cross-section, as well as on the ambient temperature (Table 1). The resistance value is measured in ohms (Ω - omega).

Creating tension

Creating voltage by separating electrical charges is the basis of electrical energy production. In this case, other types of energy are usually converted into electrical energy.

Voltage due to induction occurs when an electrical conductor (coil) moves in a magnetic field (Fig. 4). This ability to create voltage (induce it) is mainly used in power plant generators and in vehicles (Fig. 5). The creation of voltage due to chemical energy occurs when various metals or materials come into contact with a conductive liquid (electrolyte). This produces a galvanic cell. Many connected galvanic cells are called a battery. The electrodes of commercially available dry cell batteries are mostly composed of carbon and zinc (Figure 6). Carbon-zinc cells create a voltage of 1.5 V. When the electric current is removed, the less noble pole of the battery - the zinc vessel - is destroyed.

Rice. 4. Voltage due to induction

Rice. 5. Generator principle

Rice. 6. Carbon-zinc element

Discharged batteries should be removed from devices powered by these batteries, as they may be damaged by leaking electrolyte. The same applies to devices that have not been used for a long time. Used batteries must be collected and destroyed.

Creating tension through friction . Synthetic materials are generally good dielectrics. They can be charged with a higher electrical charge by friction with other materials. Due to the insulation, voltages cannot go to the ground (static charges). So, for example, a car on a dry road can charge up to a voltage of 1000 V. The action of electrostatic charges is, for example, attracting dust particles to the glass and attracting the film to the substrate. When a static charge is discharged, sparking and an explosion of solvent vapors or dust-air mixtures may occur.

Action of electric current

The action of electric current is manifested in the transformation of electrical energy into thermal, light, mechanical and chemical energy.

Thermal Effect In all conductors, the flow of electrons is limited by the resistance of the conductor. In this case, the conductor heats up. The thermal effect of electric current is used, for example, in electric boilers, kitchen stoves, electric soldering irons, fuses and electric arc welding (Fig. 7).

Rice. 7. Electric boiler

Light Effect In incandescent lamps, an electric current heats a tungsten wire to a white heat so that it emits light (Fig. 8). However, 95% of the electricity is converted into heat and only 5% is converted into light energy. Fluorescent lamps use the properties of certain gases, such as neon or mercury vapor, to glow when an electric current passes through them. The efficiency of such lamps ranges from 15 to 20%.

Rice. 8. Lamp

Mechanical action Each conductor through which electric current flows forms a magnetic force field around itself. These magnetic actions are converted into motion, for example, in electric motors, magnetic lifting devices, magnetic valves and relays (Fig. 9).

Rice. 9. Electric motor

Chemical action Electrically conductive liquids (electrolytes) contain ions as voltage carriers. If you pass an electric current through an electrolyte, negatively charged ions will be attracted to the positive pole, and positively charged ions will be attracted to the negative pole. This phenomenon is called electrolysis . It is used to decompose water into its constituent parts, when applying electroplating and in obtaining pure metals (Fig. 10).

Rice. 10. Nickel electroplating

Types of current

Among the types of electric current there are:

• Direct Current: Symbol (—) or DC (Direct Current).
• Alternating Current: Symbol (~) or AC (Alternating Current).

In the case of direct current (—), the current flows in one direction (Fig. 11). Direct current is supplied, for example, by dry batteries, solar panels and batteries for devices with low current consumption. For the electrolysis of aluminum, electric arc welding and the operation of electrified railways, high-power direct current is required. It is created using AC rectification or using DC generators. The technical direction of the current is that it flows from the contact with the “+” sign to the contact with the “–” sign. In the case of alternating current (~), a distinction is made between single-phase alternating current, three-phase alternating current and high-frequency current (see Fig. 11).

Rice. 11. Types of current

With alternating current, the current constantly changes its magnitude and direction. In the Western European power grid, the current changes its direction 50 times per second. The frequency of change of oscillations per second is called the frequency of the current. The unit of frequency is hertz (Hz). Single-phase alternating current requires a voltage conductor and a return conductor. Alternating current is used on the construction site and in industry to operate electrical machines such as hand sanders, electric drills and circular saws, as well as for job site lighting and construction site equipment.

Three-phase alternating current generators produce alternating voltage with a frequency of 50 Hz on each of their three windings. This voltage can supply three separate networks and use only six wires for forward and return conductors. If you combine the return conductors, you can limit yourself to only four wires (Fig. 12).

Rice. 12. Three-phase alternating current generator with four-wire network

The common return wire will be the neutral conductor (N). As a rule, it is grounded. The other three conductors (outer conductors) are abbreviated as L1, L2, L3. In the German grid, the voltage between the outer conductor and the neutral conductor, or earth, is 230 V. The voltage between two outer conductors, for example between L1 and L2, is 400 V. High-frequency current is said to be when the oscillation frequency is significantly higher than 50 Hz (from 15 kHz to 250 MHz). Using high-frequency current, you can heat conductive materials and even melt them, such as metals and some synthetic materials.

Electrical appliances in the electric current network

Electrical machines and devices are called consumers. They convert electrical energy into other forms of energy, such as heat in a heating device or, in an electric motor, mechanical energy. Each consumer has its own electrical resistance. The consumer resistance is greater the longer the conductor, the smaller its cross-section and the worse the conductor material conducts current. The resistance of a conductor with a length of 1 m and a cross-section of 1 mm2 is called specific resistance p (“rho”). Its value depends on the material and temperature and can be determined from the material tables. Conductor resistance is calculated using the following formula:

R=l*p0/A,

where R is resistance in Ohms; l is the length of the conductor in m; p0—resistivity in Ohm*mm2/m; A is the cross-section of the conductor in mm2.

Example A three-core copper wire extension cable is 50 m long. The cross-section of each core is 1.5 mm2. The resistivity of copper is 0.0178 (Ohm*mm2)/m. The working length of the wire is 100 m (forward and return conductors are 50 m each).

R = (100 m * 0.0178 (Ohm*mm2)/m)/1.5 mm2; R = 1.2 Ohm.

Ohm's Law The current flowing through the resistance is greater, the lower the resistance and the higher the voltage. Electric current calculation:

I=U/R,

where I is the current in amperes (A); U - voltage in volts (V); R is resistance in ohms (Ohm).

1 Ampere = 1 Volt / 1 Ohm; 1 A = 1 V / 1 Ohm.

Example What current passes through an electrical device with a resistance R = 10 Ohm, which is connected to a voltage Uv 6 V and, accordingly, 230 V?

• I=U/R; I=6V/10Ohm; I=0.6A.
• I=U/R; I=230V/10Ohm; I=23A.

If a device with a resistance of 10 Ohms is connected to a voltage of 6 V, then a current of 0.6 A flows in it. If the same device is connected to a voltage of 230 V, then the current will be 23 A. Each device can only be connected to that voltage for which it is designed. The permissible operating voltage is indicated on a special plate on the device body (Fig. 13). If the device is designed to be connected to a voltage of 230 V, then it cannot work normally at 6 V, the current is too small. On the contrary, a device designed to operate at a voltage of 6 V will be destroyed when connected to a voltage of 230 V, since the current is too high.

Electrical power (P) The electrical power of the device, both with direct and alternating current, is proportional to the voltage U and current I. The power is also indicated on the plate on the body of the device. In the case of electric motors, this is the mechanical power at the drive shaft (see Fig. 13).

Rice. 13. Electric motor data plate

Electrical power P is the product of voltage and current. The unit of power is watt (W). Electric power calculation:

Р=U*I,

where P is electrical power in W; U - electrical voltage in V; I—electric current in A.

1 watt = 1 volt * 1 ampere; 1 W = 1 V • 1 A

Example Determine the current strength in a 3 kW heating device, which is connected to a voltage of 230 V. I=P/U; I=3000W/230V; I=13.0A.

If electrical machines or devices are connected through an extension cord, for example through a cable drum, then due to the resistance of this conductor, a voltage loss occurs. The voltage loss from the meter to the consumer can be no more than 1.5% of the rated voltage in the network. With a nominal voltage of 230 V, this is 3.45 V. In the case of electric motors, the voltage loss in the network can be no more than 3%. Network voltage losses:

U=I*R

Example Determine the voltage loss of a 3 kW heater if it is connected to a 50-meter extension cord with a resistance R of 1.2 Ohm.

U=13.0A*1.2Ohm; U=15.6V.

This tension is unacceptable!

Extension cable heating corresponds to power

P=15.6V*30A; P = 202.8 W.

In addition, the extension cord is heated by the current. An extension cord wrapped around a reel can be damaged by the heat of the electrical current. When connecting devices with high power, the extension cord must be unwound from the drum to its entire length.

Electrical power with inductive or capacitive reactances

Inductive reactances are, for example, the windings of electric motors or coils, capacitive reactances are capacitors. When these resistances operate, the actual power decreases. This is taken into account by the power coefficient cos φ. Electrical power at alternating current:

P=U*I*cos φ.

Electrical power at three-phase alternating current:

P =√3*U*I*cos φ.

With three-phase alternating current, due to the formation of a circuit of three external conductors, an increase in power is obtained compared to single-phase alternating current with a coefficient of √3 = 1.172.

Electrical work and its cost

The greater the power and the longer the operating time of the connected device, the greater the electrical work. Electrical work is obtained as the product of electrical power and duration of operation. The units of electrical work are watt * second and joule, and the larger unit is kilowatt * hour.

1 kW*h = 3600000W*s = 3600000J.

Electrical work taken from the network is measured by a meter in kilowatt-hours (kWh). The cost of electricity is obtained from the product of consumed electrical work and the electricity tariff. Along with the cost of electrical work, most enterprises supplying consumers with electricity calculate fixed prices. These prices depend on the type of building and the volume of installed electrical capacity. Electrical work cost calculation:

W=P*t,

where W is electrical work in kilowatt * hours; P - connected power in kilowatts; t — duration of work (time) in hours:

1 kilowatt * hour = 1 kilowatt - 1 hour; 1 kWh = 1 kW * 1 h.

Example What is the cost of electrical work if a heating device with a power of 2 kW at an electricity tariff of 0.15 Euro/kWh operates for 6 hours?

W=P*t; W = 2kW*hx6h; W= 12 kWh.

Cost of work = 12 kWh x 0.15 Euro / kWh. Cost of work = 1.80 Euro.

Electrical energy distribution

Wires, fuses and switches are used to distribute electrical energy. The conductors necessary to form a closed circuit from the point of connection to the electrical appliance and back are connected to the general network using insulated wires, also called cores. To prevent mechanical damage, the wires are protected by special boxes that contain a third core, which serves as a protective conductor and is not under current. Local networks are supplied with electricity using high voltage lines, switches and transformers connected to power plants. The consumer is connected to the local network through a cable or overhead wires to the home connection cabinet. This sealed box contains a safety device for connecting to the house.

Copper is most often used for electrical wires due to its good electrical conductivity. But copper wire, due to its resistance, heats up when current flows through it. Excessive current can cause the conductors to become very hot, damage the insulation, and cause a fire. The permissible current for the conductor can be exceeded due to overload or short circuit. Overload occurs when connected devices together create too much current in the circuit. A short circuit is a direct connection between electrical wires. In this case, the resistance of the conductors becomes very small. The consequence is a very high current in the network.

In order to avoid overloading wires and devices, they are protected by fuses. Fuses are devices that, when the maximum current in a circuit is exceeded, interrupt the circuit. There are fuses (Fig. 14) and automatic fuses (safety circuit breakers) (Fig. 16). Fuses contain a thin wire or strip of fusible conductor inside, which, if the current in the network is too high, melts and breaks the circuit (Fig. 15). Depending on the disconnection method, fast, medium-slow and slow fuses are distinguished. The adjusting nut in the fuse holder must prevent the fuse body from being screwed in with unacceptably high force (see Fig. 15).

The adjusting nuts and corresponding fuse holders are strictly standardized. The adjusting nut is sized to match the conductor cross-section and can only be replaced by qualified personnel.

Rice. 14. Fuse

Rice. 15. Fuses and their designations

Rice. 16. Motor safety switch with bimetallic breaker

Electrical fuses (thin fuses) are used to protect measuring instruments and electronics, such as controls and instruments in cars. Defective fuses should not be repaired. Fuses cannot be bugged. Motor protection switches have the advantage that they can be used to switch the motor on and off and at the same time protect the connected motor from overload. The bimetallic strip heats up when the current is too high and, using mechanics, turns off the motor (see Fig. 16).

Network safety switches (safety circuit breakers) can be turned on again after an operation. They have a magnetic breaker, which, for example, in the event of a short circuit, interrupts the electric current circuit, and a bimetallic breaker, which operates with a delay during a long-term overload. If the safety circuit breaker is switched off using a bimetallic breaker, it can only be switched on again after the bimetallic strip has cooled down (Fig. 17).

Rice. 17. Mains fuse-switch

Industrial and occupational safety

Accidents when handling electric current in most cases occur due to technical deficiencies, ignorance, carelessness or inattention. Therefore, knowledge of the causes of accident hazards and measures to prevent accidents is mandatory for everyone involved in what is happening on a construction site.

The effect of electric current on the human body If an electric current flows through the human body, for example when touching a live wire, then if a certain current value is exceeded, the muscles of the respiratory organs can be paralyzed. If it is impossible to disconnect from a live wire, muscle cramps, imbalance, and respiratory and cardiac arrest may occur.

Currents above 50 mA and voltages above 50 V are life-threatening! Therefore, working with live parts is strictly prohibited.

First aid measures in case of accidents:

• break the chain;
• clear the airways;
• cardiac massage, as well as artificial respiration;
• immediately call an ambulance.

Malfunctions of electrical installations If the insulation in electrical installations is damaged, a short circuit, a ground fault, a wire short, and a short circuit to the frame may occur (Fig. 18).

Rice. 18. Short circuit, short circuit to frame, short circuit to ground, short circuit of wires

A short circuit occurs between two live electrical wires when they come into contact without insulation. A fuse included in the circuit cuts off the resulting large short circuit current. A ground fault occurs when one of the live wires is directly connected to the ground or to grounded parts. And in this case, the fuse cuts off the ground fault current.

A short circuit in the wires occurs, for example, when a “bug” is criminally installed on a fuse, when the installation cannot be turned off. A short-circuit to the frame occurs when, due to an insulation failure, voltage enters parts of the installation that should not be energized, for example the frame of an electrical machine. In this case, initially there is no current and the fuse does not respond. Thus, a short circuit to the housing with a well-insulated base of the installation remains unrecognized for a long time. Upon contact with the installation, current flows through a person into the ground (Fig. 19).

Rice. 19. Dangerous contact voltage

The magnitude of this current depends on the resistance of the human body and on the conductivity of the connection between the person and the earth. If a person comes into contact with grounding, for example with water, gas or heating pipes, dangerously high current can flow through it (Fig. 20).

Rice. 20. Emergency current circuit

Protective measures

Protective low voltage . Where there is a risk that a person may come into contact with live wires, for safety reasons only low voltages of not more than 50 V may be used, for example in welding machines or in lamps when working in tanks or confined spaces. In children's toys, the voltage may not exceed 25 V. In all installations with operating voltages greater than 25 V AC or 60 V DC, other protective measures against electric shock are prescribed.

Protective insulation With protective insulation, all metal parts that could become live in the event of an accident must be insulated using special measures. Protective insulation is often used in small machines and household electrical appliances. In insulated hand drills, for example, a plastic gear prevents conductive communication in the drive between the motor and the drill spindle. The wire and plug in devices equipped with protective insulation are made two-core or two-pole.

Protective measures in the TN system In the TN system, the neutral wire N of the transformer is directly grounded (T from the French terre - earth). The housing and casing of the connected devices are connected by a PE protective wire (green-yellow color) to a neutral wire (Fig. 21). The connection for wires with a cross-section of more than 6 mm2 can be made with one common PEN wire (PEN = PE and N conductors connected together).

Rice. 21. Protection in the TN system

Portable devices are connected to sockets using a protective contact - a “sound plug” (Fig. 22). In this case, the connection wire must be three-wire.

Rice. 22. Protective contact

Protective gap When there is a protective gap between the network and the electrical appliance, the isolation transformer is switched on. In this case, an ungrounded voltage is obtained (Fig. 23). Only one device with an operating current of no more than 16 A can be connected to the isolation transformer. The protective gap is used in construction machines, such as concrete mixers, concrete vibrators or wet grinders.

Rice. 23. Security gap

Safety switch Safety switches provide the greatest safety for electrical machines. Therefore, many enterprises supplying electricity require the use of protective switches against emergency currents. In this way, it is possible to monitor both live networks and individual devices and, if a malfunction occurs, turn them off (Fig. 24).

Rice.
24. Emergency Current Safety Switch The current in the incoming wire is usually the same value as the current in the outlet wire. In the event of a malfunction in the machine, for example, a short circuit to the frame, some part of the reverse current goes into the ground. The safety switch is switched off within 0.2 s. Using the test button T, you can simulate an emergency current. If you press the test button, the switch should operate. To ensure good protection of people, F1 safety switches with a current limit of 30 or 10 mA should be used.

Types of protection, protection classes

Depending on the application and installation location, electrical devices and installations must be protected from inadvertent influence, as well as from the penetration of foreign bodies and water. In the case of lamps, heating devices, devices with electric motors, power tools and electromedical treatment devices, the types of protection can be shown on a plate where the type of device is indicated in the form of a semantic picture. Types of protection are described by a short designation, which consists of the letters IP (IP - International Protection) and two numbers indicating the degree of protection (Table 2).

Table 2. Image signs to indicate types of IP protection

Example for type of protection IP: IP 44 = Protection against penetration of solid bodies with a diameter greater than 1.0 mm. Protection against splashing water from all directions.

Electrical devices are divided into protection classes (Table 3). Protection classes show what protective measures are applied during installation against direct and indirect exposure to them. There are protection classes I, II and III.

Table 3. Protective classes

Protection class I, for example, contains all devices with a metal housing, which must have a connection terminal for the PE conductor (yellow-green protective conductor) with an appropriate designation.

Electrical installations on construction sites

All electrically powered machinery and appliances on a construction site must be connected to a central electrical distribution board. The central electrical distribution board must comply with current requirements (VDE 0612). The housing of the central electrical distribution board must be made of metal or plastic; a wooden cabinet is unacceptable. In the cabinet of the central electrical distribution board (cabinet AV) there is a connection to the current network (Fig. 25). In addition, it contains a meter, safety switches F1, fuses, as well as sockets and terminals.

Rice. 25. Central electrical distribution cabinet

The cabinet must be locked. Particularly important is the perfect grounding of the electrical distribution board on the construction site. Fire-galvanized strip or rod grounding elements must be connected to the grounding terminals with well-insulated braided copper wires with a cross-section of at least 16 mm2. After equipping the construction site, it is necessary to test all electrical installations by a responsible specialist to ensure correct connection and operation of protective measures. For legal reasons, the test results must be documented in a test report.

On large construction sites, it is advisable to install several electrical distribution panels so that when one of the safety switches F1 is turned off, the entire construction site is not disconnected from the city network. For this purpose, electrical distribution boards with several connection circuits are also used, each of which is equipped with its own safety switch F1. In addition, distribution cabinets (cabinets V) without electricity meters are used. Electrical devices, connecting sockets and cables must comply with VDE requirements (VDE - Association of German Electrical Engineers) and must bear the mark that they have passed VDE tests (Fig. 26).

Rice. 26. Test marks

Sockets . Three-phase current sockets must comply with the international standard for round sockets according to CEE standards (CEE is the international commission for the rules and examination of electrical products) (Fig. 27). They allow the use of high currents and are available in splash-proof and waterproof versions. In addition, they satisfy the safety requirement that only plug-socket systems designed for the same voltage can be matched to each other.

Rice. 27. Three-phase current plug

A person responsible for the condition of electrical installations must be appointed at the construction site, as well as his deputy, who must be known to everyone on the construction site. The person in charge has the duty to check daily by pressing all buttons the operation of all safety switches F1, to switch off the electrical installation after finishing work and to lock the AV cabinet. The following rules must be repeated regularly to those working at the enterprise.

• Faulty devices must be switched off immediately. The creation, modification and repair of electrical appliances and installations may only be carried out by a qualified electrician.
• In the event of malfunctions of electrical installations or unusual manifestations during their operation, such as the smell of fire, sparking or unusual sounds, the installation must be turned off. The person in charge should be notified of this.
• Cables must not be tampered with, pulled over sharp edges, buried in the ground, or subjected to stretching.
• When transporting electrical machines, the plug must be removed from the socket. Portable devices must be disconnected from the network again after finishing work.
• Appliances marked “protect from moisture” should not be operated in the rain or stored outdoors.
• Do not hang clothes or place other objects on electrical machines and electric heating devices.

Medical and biological properties of humans

Analysis of accidents involving electric shock shows that the outcome of the injury is related to the medical and biological characteristics of a person and his state of health. Physically healthy and strong people tolerate electrical injuries more easily than unhealthy and weak people. People suffering from skin diseases, cardiovascular, and nervous diseases are more susceptible to electric current.

Therefore, safety regulations for the operation of electrical installations provide for the medical selection of personnel to service electrical installations. Selection is carried out upon admission to work, periodic examinations are carried out within the time frame established by the Ministry of Health in accordance with the list of diseases and disorders that prevent access to work. Selection also has another goal: to prevent people with diseases from servicing electrical installations that may interfere with their production work or cause erroneous actions that are dangerous for other persons (failure to distinguish the color of a signal due to a visual impairment, inability to give a clear command due to a sore throat or stuttering, etc.).

In addition, safety regulations do not allow persons under 18 years of age and without specific knowledge in the field of electrical safety appropriate to the scope and conditions of the work they perform to service electrical installations.

Galvanometer device

The device was named galvanometer in honor of the Italian physicist and physician Luigi Galvani. This device is capable of measuring small electrical currents (direct).

On diagrams, the device is designated by a circle, inside of which there is a large Latin letter G. On some diagrams, inside the circle there is an arrow pointing vertically upward.

Galvanometer contains:

• horseshoe magnet and
• a frame located inside it containing turns of thin copper wire (Fig. 8).

Rice. 8. How the galvanometer is tripled

The movable frame is located on an axis and can rotate around it.

An arrow is attached to the frame. It indicates at what angle the frame rotated while electric current flowed through it.

The angle of rotation is marked by scale divisions.

Rice. 9. What does a device for measuring small currents look like?

Who is Luigi Galvani

Galvani was one of the founders of the doctrine of electricity.

I discovered that electrical voltage occurs at the points of contact between different types of metals.

Conducted experiments using an iron key and a silver coin.

He studied muscle contractions under the influence of electricity and came to the conclusion that muscles are controlled by electrical impulses traveling along nerve fibers from the brain.

In the Italian city of Bologna, not far from the building of the University of Bologna, there is a monument to Galvani. It is located in Piazza Luigi Galvani, named after the scientist.

One of the craters on the far side of the Moon was also named in his honor.

And the Bologna Lyceum has been named after Galvani since 1860.

About magnetoelectric system devices

Such devices, containing a conducting frame and a small magnet, are called magnetoelectric system devices. They are widely used due to their relatively simple design.

Instrument scales can be graduated in various units of measurement, depending on the physical quantities being measured. Based on such devices, voltmeters, ammeters, ohmmeters, etc. are made.