In § 8 we looked at an experiment with a lamp and two spirals (resistors). We noted that by changing the current we mean a change in the flow of electrons passing through the conductor. This phrase referred to solid metal conductors. In liquid metals (for example, mercury), in molten or dissolved substances (for example, in salts, acids and alkalis), as well as gases, the current is created by electrons and ions (see § 8). All of them are carriers of electric charge.
Therefore, by current strength it is more convenient to understand not the number of various charged particles (electrons and/or ions) passing through a conductor over a period of time, but the total charge transferred through a conductor per unit time. In formula form it looks like this:

So, current strength - a physical quantity showing the charge passing through a conductor per unit time.

A device is used to measure current strength ammeter. It is connected in series with the section of the circuit in which the current is to be measured. Unit of current - 1 ampere(1 A). It is installed by measuring the force of interaction (attraction or repulsion) of conductors with current. For an explanation, see the picture with foil strips posted at the very beginning of this topic.
1 ampere is taken to be the strength of a current that, when passing through two parallel straight conductors of infinite length and small diameter, located at a distance of 1 m from each other in a vacuum, causes an interaction force equal to 0.0000002 N on a section of the conductor 1 m long.
Let's get to know laws of current distribution in circuits with different connections of conductors. In diagrams “a”, “b”, “c” the lamp and rheostat are connected sequentially. In diagrams “d”, “d”, “f” the lamps are connected parallel. Let's take an ammeter and measure the current in the places marked with red dots.
First, we turn on the ammeter between the rheostat and the lamp (circuit “a”), measure the current strength and designate it with the symbol Igenerally. Then we place the ammeter to the left of the rheostat (diagram “b”). Let's measure the current strength, denoting it with the symbol I1 . Then we place the ammeter to the left of the lamp, denote the current strength I2 (diagram “c”).


in all sections of the circuit with series connection of conductors, the current strength is the same:

Let us now measure the current in various sections of the circuit with a parallel connection of two lamps. In diagram "d", the ammeter measures the total current; in diagrams “d” and “f” - the strength of the currents passing through the upper and lower lamps.


Numerous measurements show that the current strength in the unbranched part of the circuit with parallel connection of conductors (total current strength) is equal to the sum of the current strengths in all branches of this circuit.

In this article, you will learn the definitions of electric current, amperage and voltage. Let's understand the main characteristics and formulas of current, and how to protect yourself from electric current.

Definition

In a physics textbook there is a definition:

ELECTRICITY- this is the ordered (directed) movement of charged particles under the influence of an electric field. Particles can be: electrons, protons, ions, holes.

In academic textbooks the definition is described as follows:

ELECTRICITY is the rate of change of electric charge over time.

• The electron charge is negative.
• protons- particles with a positive charge;
• neutrons- with a neutral charge.

CURRENT STRENGTH is the number of charged particles (electrons, protons, ions, holes) flowing through the cross section of the conductor.

All physical substances, including metals, consist of molecules consisting of atoms, which in turn consist of nuclei and electrons rotating around them. During chemical reactions, electrons pass from one atom to another, therefore, the atoms of one substance lack electrons, and the atoms of another substance have an excess of them. This means that substances have opposite charges. If they come into contact, electrons will tend to move from one substance to another. It is this movement of electrons that is ELECTRICITY. A current that will flow until the charges of the two substances are equal. The departed electron is replaced by another. Where? From the neighboring atom, to it - from its neighbor, so to the extreme, to the extreme - from the negative pole of the current source (for example, a battery). From the other end of the conductor, electrons go to the positive pole of the current source. When all the electrons on the negative pole are gone, the current will stop (the battery is dead).

VOLTAGE is a characteristic of the electric field and represents the potential difference between two points inside the electric field.

It seems like it’s not clear. Conductor- in the simplest case, this is a wire made of metal (copper and aluminum are more often used). The mass of the electron is 9.10938215(45)×10 -31 kg. If an electron has mass, then this means that it is material. But the conductor is made of metal, and metal is solid, so how do some electrons flow through it?

The number of electrons in a substance equal to the number of protons only ensures its neutrality, and the chemical element itself is determined by the number of protons and neutrons based on Mendeleev’s periodic law. If, purely theoretically, we subtract all its electrons from the mass of any chemical element, it will practically not approach the mass of the nearest chemical element. The difference between the masses of the electron and the nucleus is too large (the mass of only the 1st proton is approximately 1836 times greater than the mass of the electron). A decrease or increase in the number of electrons should only lead to a change in the total charge of the atom. The number of electrons in an individual atom is always variable. They either leave it due to thermal movement, or return back, having lost energy.

If electrons move in a direction, it means that they “leave” their atom, and the atomic mass will not be lost and, as a result, the chemical composition of the conductor will change? No. A chemical element is determined not by atomic mass, but by the number of PROTONS in the nucleus of an atom, and nothing else. In this case, the presence or absence of electrons or neutrons in an atom does not matter. Let's add - subtract electrons - we get an ion; add - subtract neutrons - we get an isotope. In this case, the chemical element will remain the same.

With protons it’s a different story: one proton is hydrogen, two protons are helium, three protons are lithium, etc. (see periodic table). Therefore, no matter how much current you pass through the conductor, its chemical composition will not change.

Electrolytes are another matter. This is where the CHEMICAL COMPOSITION CHANGES. Electrolyte elements are released from the solution under the influence of current. When everyone is released, the current will stop. This is because charge carriers in electrolytes are ions.

There are chemical elements without electrons:

1. Atomic cosmic hydrogen.

2. Gases in the upper layers of the atmosphere of the Earth and other planets with an atmosphere.

2. All substances are in a plasma state.

3. In accelerators, colliders.

When exposed to electric current, chemicals (conductors) can “scatter”. For example, a fuse. Moving electrons push atoms apart along their path; if the current is strong, the crystal lattice of the conductor is destroyed and the conductor melts.

Let's consider the operation of electric vacuum devices.

Let me remind you that during the action of an electric current in an ordinary conductor, an electron, leaving its place, leaves a “hole” there, which is then filled with an electron from another atom, where in turn a hole is also formed, which is subsequently filled by another electron. The entire process of electron movement occurs in one direction, and the movement of “holes” occurs in the opposite direction. That is, the hole is a temporary phenomenon; it fills up anyway. Filling is necessary to maintain charge equilibrium in the atom.

Now let's look at the operation of an electric vacuum device. For example, let's take the simplest diode - a kenotron. Electrons in the diode during the action of electric current are emitted by the cathode towards the anode. The cathode is coated with special metal oxides, which facilitate the escape of electrons from the cathode into vacuum (low work function). There is no reserve of electrons in this thin film. To ensure the release of electrons, the cathode is strongly heated with a filament. Over time, the hot film evaporates, settles on the walls of the flask, and the emissivity of the cathode decreases. And such an electronic vacuum device is simply thrown away. And if the device is expensive, it is restored. To restore it, the flask is unsoldered, the cathode is replaced with a new one, after which the flask is sealed back.

The electrons in the conductor move “carrying” the electric current, and the cathode is replenished with electrons from the conductor connected to the cathode. The electrons that leave the cathode are replaced by electrons from the current source.

The concept of “speed of movement of electric current” does not exist. At a speed close to the speed of light (300,000 km/s), an electric field propagates through the conductor, under the influence of which all electrons begin to move at a low speed, which is approximately equal to 0.007 mm/s, not forgetting to also rush chaotically in thermal motion.

Let's now understand the main characteristics of current

Let's imagine the picture: You have a standard cardboard box of 12 bottles of strong drink. And you're trying to put another bottle in there. Let's say you succeeded, but the box barely held up. You put another one in there, and suddenly the box breaks and the bottles fall out.

A box of bottles can be compared to a cross-section of a conductor:

The wider the box (thicker the wire), the greater the number of bottles (CURRENT POWER) it can accommodate (provide).

You can place from one to 12 bottles in a box (in a conductor) - it will not fall apart (the conductor will not burn), but it cannot accommodate a larger number of bottles (higher current strength) (represents resistance).
If we place another box on top of the box, then on one unit of area (conductor cross-section) we will place not 12, but 24 bottles, another one on top - 36 bottles. One of the boxes (one floor) can be taken as a unit similar to the VOLTAGE of electric current.

The wider the box (less resistance), the more bottles (CURRENT) it can supply.

By increasing the height of the boxes (voltage), we can increase the total number of bottles (POWER) without destroying the boxes (conductor).

Using our analogy we got:

The total number of bottles is POWER

The number of bottles in one box (layer) is the CURRENT POWER

The number of boxes in height (floors) is VOLTAGE

The width of the box (capacity) is the RESISTANCE of the electrical circuit section

Through the above analogies, we came to “ OMA'S LAW“, which is also called Ohm’s Law for a section of a circuit. Let's represent it as a formula:

Where I – current strength, U R - resistance.

In simple terms, it sounds like this: Current is directly proportional to voltage and inversely proportional to resistance.

In addition, we came to " WATT'S LAW". Let’s also depict it in the form of a formula:

Where I – current strength, U – voltage (potential difference), R – power.

In simple terms, it sounds like this: Power is equal to the product of current and voltage.

Electric current strength measured by an instrument called an Ammeter. As you guessed, the amount of electric current (the amount of charge transferred) is measured in amperes. To increase the range of unit of change designations, there are multiplicity prefixes such as micro - microampere (µA), miles - milliampere (mA). Other consoles are not used in everyday use. For example: They say and write “ten thousand amperes”, but they never say or write 10 kiloamperes. Such meanings are not real in everyday life. The same can be said about nanoamps. Usually they say and write 1×10 -9 Amperes.

Electrical voltage(electric potential) is measured by a device called a Voltmeter, as you guessed it, voltage, i.e. the potential difference that causes current to flow, is measured in Volts (V). Just as for current, to increase the range of designations, there are multiple prefixes: (micro - microvolt (μV), miles - millivolt (mV), kilo - kilovolt (kV), mega - megavolt (MV). Voltage is also called EMF - electromotive force.

Electrical resistance measured by a device called an Ohmmeter, as you guessed it, the unit of resistance is Ohm (Ohm). Just as for current and voltage, there are multiplicity prefixes: kilo - kiloohm (kOhm), mega - megaohm (MOhm). Other meanings are not real in everyday life.

Earlier, you learned that the resistance of a conductor directly depends on the diameter of the conductor. To this we can add that if a large electric current is applied to a thin conductor, it will not be able to pass it, which is why it will heat up very much and, in the end, may melt. The operation of fuses is based on this principle.

The atoms of any substance are located at some distance from each other. In metals, the distances between atoms are so small that the electron shells practically touch. This allows electrons to wander freely from nucleus to nucleus, creating an electric current, which is why metals, as well as some other substances, are CONDUCTORS of electricity. Other substances, on the contrary, have widely spaced atoms, electrons tightly bound to the nucleus, which cannot move freely. Such substances are not conductors and are usually called DIELECTRICS, the most famous of which is rubber. This is the answer to the question why electrical wires are made of metal.

The presence of electric current is indicated by the following actions or phenomena that accompany it:

;1. The conductor through which current flows may become hot;

2. Electric current can change the chemical composition of a conductor;

3. The current exerts a force on neighboring currents and magnetized bodies.

When electrons are separated from the nuclei, a certain amount of energy is released, which heats the conductor. The “heating” capacity of a current is usually called power dissipation and is measured in watts. The same unit is used to measure mechanical energy converted from electrical energy.

Electrical hazards and other hazardous properties of electricity and safety precautions

Electric current heats the conductor through which it flows. That's why:

1. If a household electrical network is overloaded, the insulation gradually chars and crumbles. There is a possibility of a short circuit, which is very dangerous.

2. Electric current flowing through wires and household appliances encounters resistance, so it “chooses” the path with the least resistance.

3. If a short circuit occurs, the current increases sharply. This generates a large amount of heat that can melt the metal.

4. A short circuit can also occur due to moisture. If a fire occurs in the case of a short circuit, then in the case of exposure to moisture on electrical appliances, it is the person who suffers first.

5. Electric shock is very dangerous and can be fatal. When electric current flows through the human body, tissue resistance decreases sharply. Processes of tissue heating, cell destruction, and death of nerve endings occur in the body.

How to protect yourself from electric shock

To protect yourself from exposure to electric current, use means of protection against electric shock: work in rubber gloves, use a rubber mat, discharge rods, grounding devices for equipment, workplaces. Automatic switches with thermal protection and current protection are also a good means of protection against electric shock that can save human life. When I am not sure that there is no danger of electric shock, when performing simple operations in electrical panels or equipment units, I usually work with one hand and put the other hand in my pocket. This eliminates the possibility of electric shock along the hand-to-hand path in case of accidental contact with the shield body or other massive grounded objects.

To extinguish a fire that occurs on electrical equipment, only powder or carbon dioxide fire extinguishers are used. Powder extinguishers are better, but after covering the equipment with dust from a fire extinguisher, it is not always possible to restore this equipment.

In electrical engineering, it is generally accepted that a simple circuit is a circuit that reduces to a circuit with one source and one equivalent resistance. You can collapse a circuit using equivalent transformations of serial, parallel, and mixed connections. The exception is circuits containing more complex star and delta connections. Calculation of DC circuits produced using Ohm's and Kirchhoff's laws.

Example 1

Two resistors are connected to a 50 V DC voltage source, with internal resistance r = 0.5 Ohm. Resistor values R 1 = 20 and R2= 32 Ohm. Determine the current in the circuit and the voltage across the resistors.

Since the resistors are connected in series, the equivalent resistance will be equal to their sum. Knowing it, we will use Ohm's law for a complete circuit to find the current in the circuit.

Now knowing the current in the circuit, you can determine the voltage drop across each resistor.

There are several ways to check the correctness of the solution. For example, using Kirchhoff's law, which states that the sum of the emf in the circuit is equal to the sum of the voltages in it.

But using Kirchhoff's law it is convenient to check simple circuits that have one circuit. A more convenient way to check is power balance.

The circuit must maintain a power balance, that is, the energy given by the sources must be equal to the energy received by the receivers.

The source power is defined as the product of the emf and the current, and the power received by the receiver as the product of the voltage drop and the current.

The advantage of checking the power balance is that you do not need to create complex cumbersome equations based on Kirchhoff’s laws; it is enough to know the EMF, voltages and currents in the circuit.

Example 2

Total current of a circuit containing two resistors connected in parallel R 1 =70 Ohm and R 2 =90 Ohm, equals 500 mA. Determine the currents in each of the resistors.

Two resistors connected in series are nothing more than a current divider. We can determine the currents flowing through each resistor using the divider formula, while we do not need to know the voltage in the circuit; we only need the total current and the resistance of the resistors.

Currents in resistors

In this case, it is convenient to check the problem using Kirchhoff’s first law, according to which the sum of currents converging at a node is equal to zero.

If you do not remember the current divider formula, then you can solve the problem in another way. To do this, you need to find the voltage in the circuit, which will be common to both resistors, since the connection is parallel. In order to find it, you must first calculate the circuit resistance

And then the tension

Knowing the voltages, we will find the currents flowing through the resistors

As you can see, the currents turned out to be the same.

Example 3

In the electrical circuit shown in the diagram R 1 =50 Ohm, R 2 =180 Ohm, R 3 =220 Ohm. Find the power released by the resistor R 1, current through resistor R 2, voltage across resistor R 3 if it is known that the voltage at the circuit terminals is 100 V.

To calculate the DC power dissipated by resistor R 1, it is necessary to determine the current I 1, which is common to the entire circuit. Knowing the voltage at the terminals and the equivalent resistance of the circuit, you can find it.

Equivalent resistance and current in the circuit

Hence the power allocated to R 1

Many of us, even from school, cannot understand what aspects distinguish current from voltage. Of course, teachers constantly argued that the difference between these two concepts is simply huge. However, only some adults have the opportunity to boast of having the relevant knowledge, and if you are not one of them, then it’s time for you to pay attention to our review today.

What is current and voltage?

In order to talk about what current strength is and what nuances may be associated with it, we consider it necessary to draw your attention to what it is in itself. Current is a process during which, under the direct influence of an electric field, the movement of certain charged particles begins to occur. The latter may be a whole list of various elements; in this regard, everything depends on the specific situation. So, for example, if we are talking about conductors, then in this case, electrons will act as the above-mentioned particles.


Perhaps some of you didn’t know this, but current is actively used in modern medicine and, in particular, to save a person from a whole list of all kinds of diseases, such as epilepsy, for example. Current is also indispensable in everyday life, because with its help, the lights are on in your home and some electrical appliances work. Current strength, in turn, implies a certain physical quantity. It is designated by the symbol I.


In the case of voltage, everything is much more complicated, even if you compare it with such a concept as “current strength”. There are single positive charges that must move from different points. In addition, voltage is the energy through which the above-mentioned movement occurs. In schools, to understand this concept, they often give the example of the flow of water that occurs between two banks. In this situation, the current will be the flow of water itself, while the voltage will be able to show the difference in levels in these two banks. Therefore, the flow will be observed until both levels in the banks are equal.

What is the difference between current and voltage?

We dare to suggest that the main difference between these two concepts is their immediate definition:

1. The words “current” and “current”, in particular, represent a certain amount of electricity, while voltage is usually considered a measure of potential energy. In simple words, these two concepts are quite dependent on each other, while maintaining some distinctive features, at the same time. Their resistance is influenced by a huge number of different factors. The most important of them is the material from which a particular conductor is made, external conditions, and temperature.
2. There is also some difference in receiving them. So, if the effect on electric charges creates a voltage, then the current is obtained by applying voltage between the points of the circuit. By the way, such devices can be ordinary batteries or more advanced and convenient generators. For this reason, we can say that the main differences between these two concepts come down to their definition, as well as the fact that they are obtained as a result of completely different processes.

Current should not be confused with energy consumption. These concepts are completely different and their main difference should be perceived precisely power. So, in the event that the voltage is intended for that. to characterize potential energy, then in the case of current, this energy will already be kinetic. In our modern realities, the vast majority of pipes correspond to analogies from the world of electricity. We are talking about the load that is created when a light bulb or the same TV is connected to the network. During this, a consumption of electricity is created, which ultimately leads to the appearance of current.

Of course, if you do not connect any electrical appliances to the outlet, the voltage will remain unchanged, while the current will be zero. Well, if there is no provision for flow, then how can we even talk about current and any of its strength? Therefore, current is just a certain amount of electricity, while voltage is considered a measure of the potential energy of a certain source of electricity.

To measure current, a measuring device called is used. Current strength has to be measured much less often than voltage or resistance, but, nevertheless, if you need to determine the power consumption of an electrical appliance, then without knowing the amount of current it consumes, the power cannot be determined.

Current, like voltage, can be constant or variable, and different measuring instruments are required to measure their values. Current is designated by the letter I, and to the number, to make it clear that this is the current value, a letter is added A. For example, I=5 A means that the current in the measured circuit is 5 Amps.

On measuring instruments for measuring alternating current, the letter A is preceded by the sign " ~ ", and those intended for measuring direct current are placed " – ". For example, -A means that the device is designed to measure direct current.

You can read about what current is and the laws of its flow in a popular form in the website article “The Law of Current Strength”. Before taking measurements, I strongly recommend that you read this short article. The photo shows an ammeter designed to measure direct current up to 3 Amperes.

Circuit for measuring current with an ammeter

According to the law, current flows through wires at any point in a closed circuit of the same magnitude. Therefore, in order to measure the current value, you need to connect the device by breaking the circuit in any convenient place. It should be noted that when measuring the current value, it does not matter what voltage is applied to the electrical circuit. The current source can be a 1.5 V battery, a 12 V car battery, or a 220 V or 380 V household power supply.

The measurement diagram also shows how an ammeter is indicated on electrical circuits. This is a capital letter A surrounded by a circle.

When starting to measure the current in a circuit, it is necessary, as with any other measurements, to prepare the device, that is, set the switches to the current measurement position, taking into account its type, constant or alternating. If the expected current value is not known, the switch is set to the maximum current measurement position.

How to measure current consumption of an electrical appliance

For the convenience and safety of measuring current consumption by electrical appliances, it is necessary to make a special extension cord with two sockets. In appearance, a homemade extension cord is no different from an ordinary extension cord.

But if you remove the covers from the sockets, it is not difficult to notice that their terminals are connected not in parallel, as in all extension cords, but in series.

As you can see in the photo, the mains voltage is supplied to the lower terminals of the sockets, and the upper terminals are connected to each other by a jumper made of wire with yellow insulation.

Everything is ready for measurement. Insert the plug of the electrical appliance into any of the sockets, and the ammeter probes into the other socket. Before measurements, it is necessary to set the device switches in accordance with the type of current (AC or DC) and to the maximum measurement limit.

As can be seen from the ammeter readings, the current consumption of the device was 0.25 A. If the device scale does not allow direct reading, as in my case, then it is necessary to calculate the results, which is very inconvenient. Since the ammeter measurement limit is 0.5 A, to find out the division value, you need to divide 0.5 A by the number of divisions on the scale. For this ammeter it turns out 0.5/100=0.005 A. The needle has deviated by 50 divisions. So now you need 0.005×50=0.25 A.

As you can see, taking current readings from dial gauges is inconvenient and you can easily make a mistake. It is much more convenient to use digital instruments, such as the M890G multimeter.

The photo shows a universal multimeter turned on in the AC current measurement mode to a limit of 10 A. The measured current consumed by the electrical device was 5.1 A at a supply voltage of 220 V. Therefore, the device consumes 1122 W of power.

The multimeter has two sectors for measuring current, indicated by letters A- for DC and Ah~ to measure a variable. Therefore, before starting measurements, you need to determine the type of current, estimate its magnitude and set the switch pointer to the appropriate position.

Multimeter socket with inscription COM is common to all types of measurements. Sockets marked mA And 10A are intended only for connecting a probe when measuring current. For a measured current of less than 200 mA, the probe plug is inserted into a mA socket, and for a current of up to 10 A, into a 10 A socket.

Attention, if you measure a current that is many times greater than 200 mA when the probe plug is in the mA socket, the multimeter can be damaged.

If the value of the measured current is not known, then measurements should be started by setting the measurement limit to 10 A. If the current is less than 200 mA, then switch the device to the appropriate position. Switching multimeter measurement modes can only be done by de-energizing the circuit being measured..

Calculation of the power of an electrical appliance based on current consumption

Knowing the current value, you can determine the power consumption of any electrical energy consumer, be it a light bulb in a car or an air conditioner in an apartment. It is enough to use a simple law of physics, which was established simultaneously by two physicists, independently of each other. In 1841 James Joule, and in 1842 Emil Lenz. This law was named after them - Joule–Lenz law.