What does electric current represent? What is electric current and what are the conditions for its existence. What is current
Abstract concept Every person has information about electric current. For an electrical appliance, the power source is something like the air source for any breathing organism. But these comparisons limit the understanding of the nature of the phenomenon, and only specialists understand the essence more deeply.
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IN school curriculum everyone takes a physics course that covers the basic concepts and laws of electricity. Dry, scientific approach is not of interest to children, so most adults have no idea what electric current is, why it occurs, how it has a unit of measurement, and how anything can move through stationary metal wires and even make it work electrical appliances.
In simple words about electric current
The standard definition from a school physics textbook succinctly describes the phenomenon of electric current. But to be honest, you can fully understand this if you study the subject much more deeply. After all, the information is presented in a different language - scientific. It is much easier to understand nature physical phenomenon, if you describe everything in familiar language, understandable to any person. For example, current in metal.
We should start with the fact that everything that we consider solid and motionless is so only in our imagination. A piece of metal lying on the ground is a monolithic motionless body in human understanding. For an analogy, let’s imagine our planet in space, looking at it from the surface of Mars. The earth seems like a complete, motionless body. If you get closer to its surface, it will become obvious that this is not a monolithic piece of matter, but a constant movement: water, gases, living beings, lithospheric plates- all this moves non-stop, although this is not visible from deep space.
Let's return to our piece of metal lying on the ground. It is motionless because we look at it from the outside as a monolithic object. At the atomic level, it consists of constantly moving tiny elements. They are different, but among all, we are interested in electrons, which create an electromagnetic field in metals, generating that same current. The word “current” must be taken literally, because when elements with an electric charge move, that is, “flow,” from one charged object to another, then “electric current” occurs.
Having understood the basic concepts, we can derive a general definition:
Electricity is a stream of charged particles moving from a body with a higher charge to a body with a lower charge.
To understand the essence even more precisely, you need to delve into the details and get answers to several basic questions.
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Answers to the main questions about electric current
After formulating the definition, several logical questions arise.
- What makes current “flow”, that is, move?
- If the smallest elements of metal are constantly moving, then why does it not deform?
- If something flows from one object to another, does the mass of these objects change?
The answer to the first question is simple. Just as water flows from a high point to a low point, so electrons will flow from a body with a high charge to a body with a low one, obeying the laws of physics. And “charge” (or potential) is the number of electrons in a body, and the more there are, the higher the charge. If a contact is made between two bodies with different charges, electrons will flow from the more charged body to the less charged one. This will create a current that will end when the charges of the two contacting bodies are equal.
To understand why a wire does not change structure, despite the fact that there is constant movement in it, you need to imagine it in the form of a large house in which people live. The size of the house will not change depending on how many people enter and leave it, or move around inside it. Man in in this case an analogue of an electron in metal - it moves freely and does not have much mass compared to the whole building.
If electrons move from one body to another, why does the mass of the bodies not change? The fact is that the weight of an electron is so small that even if all the electrons are removed from the body, its mass will not change.
What is the unit of current?
- Current strength.
- Voltage.
- Resistance.
If we try to describe the concept of current strength in simple words, it is best to imagine the flow of cars passing through the tunnel. Cars are electrons and the tunnel is a wire. The more cars pass through the cross-section of the tunnel at one point in time, the greater the current strength, which is measured by a device called an “ammeter” in Amperes (A), and in formulas is denoted by the letter (I).
Voltage is a relative quantity that expresses the difference in the charges of bodies between which current flows. If one object has a very high charge and another very low, then there will be a high voltage between them, which is measured using a device called a voltmeter and a unit called Volt (V). Identified in formulas by the letter (U).
Resistance characterizes the ability of a conductor, conventionally a copper wire, to pass a certain amount of current, that is, electrons, through itself. A resistive conductor generates heat by expending some of the energy of the current passing through it, thereby reducing its strength. Resistance is calculated in Ohms (Ohm), and the letter (R) is used in formulas.
Formulas for calculating current characteristics
Using three physical quantities, current characteristics can be calculated using Ohm's Law. It is expressed by the formula:
Where I is the current strength, U is the voltage on the circuit section, R is the resistance.
From the formula we see that the current is calculated by dividing the voltage by the resistance. Hence we have the wording of the law:
The current strength is directly proportional to the voltage and inversely proportional to the resistance of the conductor.
From this formula, you can mathematically calculate its other components.
Resistance:
Voltage:
It is important to note that the formula is only valid for a specific section of the chain. For a complete, closed circuit, as well as other special cases, there are other Ohm's laws.
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The effect of current on various materials and living beings
Different chemical elements Under the influence of current they behave differently. Some superconductors offer no resistance to electrons moving through them, causing no chemical reaction. Metals, when exposed to excessive stress, can break down and melt. Dielectrics that do not allow current to pass through do not interact with it in any way and thereby protect against it. environment. This phenomenon is successfully used by humans when insulating wires with rubber.
For living organisms, current is an ambiguous phenomenon. It can have both beneficial and destructive effects. People have long used controlled shocks for therapeutic purposes: from mild brain-stimulating shocks to powerful electric shocks that can restart a stopped heart and bring a person back to life. A strong discharge can lead to serious health problems, burns, tissue death and even instant death. When working with electrical devices, you must follow safety regulations.
In nature, you can find many phenomena in which electricity plays a key role: from deep-sea creatures (electric rays) that can deliver electric shocks, to lightning during a thunderstorm. Man has long been mastering this natural force and skillfully using it, which is why all modern electronics work.
It should be remembered that natural phenomena can be both beneficial and harmful to humans. Studying from school and further education helps people to competently use the phenomena of the world for the benefit of society.
Electricity
What is electric current called?
The ordered (directed) movement of charged particles is called electric current. Moreover, an electric current whose strength does not change over time is called constant. If the direction of current movement changes, so does the change. are repeated in the same sequence in magnitude and direction, then such a current is called alternating.
What causes and maintains the orderly movement of charged particles?
An electric field causes and maintains the ordered movement of charged particles. Does electric current have a specific direction?
It has. The direction of electric current is taken to be the movement of positively charged particles.
Is it possible to directly observe the movement of charged particles in a conductor?
No. But the presence of electric current can be judged by the actions and phenomena that accompany it. For example, a conductor along which charged particles move heats up, and in the space surrounding the conductor, a magnetic field is formed and the magnetic needle near the conductor with electric current turns. In addition, the current passing through gases causes them to glow, and when passing through solutions of salts, alkalis and acids, it decomposes them into their component parts.
How is the strength of electric current determined?
The strength of the electric current is determined by the amount of electricity passing through the cross-section of the conductor per unit time.
To determine the current strength in a circuit, the amount of electricity flowing must be divided by the time during which it has flowed.
What is the unit of current?
The unit of current strength is taken to be the strength of a constant current, which, passing through two parallel straight conductors of infinite length of extremely small cross-section, located at a distance of 1 m from each other in a vacuum, would cause between these conductors a force equal to 2 Newtons per meter. This unit was named Ampere in honor of the French scientist Ampere.
What is the unit of electricity?
The unit of electricity is the Coulomb (Ku), which passes in one second at a current of 1 Ampere (A).
What devices measure the strength of electric current?
The strength of electric current is measured by instruments called ammeters. The ammeter scale is calibrated in amperes and fractions of an ampere according to the readings of precise standard instruments. The current strength is counted according to the readings of the arrow, which moves along the scale from the zero division. The ammeter is connected in series to the electrical circuit using two terminals or clamps located on the device. What is electric voltage?
Electric voltage is the potential difference between two points electric field. It is equal to the work done by the electric field forces when moving positive charge, equal to one, from one point of the field to another.
The basic unit of voltage is the Volt (V).
What device measures the voltage of an electric current?
The voltage of the electric current is measured by the device; rum, which is called a voltmeter. A voltmeter is connected in parallel to the electric current circuit. Formulate Ohm's law on a section of the circuit.
What is conductor resistance?
There is conductor resistance physical quantity, characterizing the properties of the conductor. The unit of resistance is Ohm. Moreover, a resistance of 1 ohm has a wire in which a current of 1 A is established with a voltage at its ends of 1 V.
Does resistance in conductors depend on the amount of electric current flowing through them?
The resistance of a homogeneous metal conductor of a certain length and cross-section does not depend on the magnitude of the current flowing through it.
What determines the resistance in electrical conductors?
Resistance in electrical conductors depends on the length of the conductor, its cross-sectional area and the type of material of the conductor (material resistivity).
Moreover, the resistance is directly proportional to the length of the conductor, inversely proportional to the cross-sectional area and depends, as mentioned above, on the material of the conductor.
Does resistance in conductors depend on temperature?
Yes, it depends. An increase in the temperature of a metal conductor causes an increase in the speed of thermal movement of particles. This leads to an increase in the number of collisions of free electrons and, consequently, to a decrease in the free travel time, as a result of which the conductivity decreases and the resistivity of the material increases.
The temperature coefficient of resistance of pure metals is approximately 0.004 °C, which means their resistance increases by 4% for every 10 °C increase in temperature.
As the temperature in the carbon electrolyte increases, the free path time also decreases, while the concentration of charge carriers increases, as a result of which their resistivity decreases as the temperature increases.
Formulate Ohm's law for a closed circuit.
The current strength in a closed circuit is equal to the ratio of the electromotive force of the circuit to its total resistance.
This formula shows that the current strength depends on three quantities: electromotive force E, external resistance R and internal resistance r. Internal resistance does not have a noticeable effect on the current strength if it is small compared to external resistance. In this case, the voltage at the terminals of the current source is approximately equal to the electromotive force (EMF).
What is electromotive force (EMF)?
Electromotive force is the ratio of the work done by external forces to move a charge along a circuit to the charge. Like potential difference, electromotive force is measured in volts.
What forces are called external forces?
Any force acting on electrically charged particles, except potential forces electrostatic origin (i.e. Coulomb) are called external forces. It is due to the work of these forces that charged particles acquire energy and then release it when moving in conductors electrical circuit.
Third-party forces set in motion charged particles inside a current source, generator, battery, etc.
As a result, charges of opposite signs appear at the terminals of the current source, and a certain potential difference appears between the terminals. Further, when the circuit is closed, the formation of surface charges begins to act, creating an electric field throughout the entire circuit, which appears as a result of the fact that when the circuit is closed, a surface charge appears almost immediately on the entire surface of the conductor. Inside the source, charges move under the influence of external forces against forces electrostatic field(positive from minus to plus), and throughout the rest of the circuit they are driven by an electric field.
Rice. 1. Electrical circuit: 1- source, electricity (battery); 2 - ammeter; 3 - energy successor (lai pa incandescent); 4 - electrical wires; 5 - single-pole RuSidnik; 6 - fuses
TO Category: - Crane operators and slingers
The first discoveries related to the work of electricity began in the 7th century BC. Philosopher Ancient Greece Thales of Miletus discovered that when amber is rubbed on wool, it is subsequently able to attract lightweight objects. “Electricity” is translated from Greek as “amber.” In 1820, André-Marie Ampère established the law direct current. Subsequently, the magnitude of the current or what the electric current is measured in began to be denoted in amperes.
Meaning of the term
The concept of electric current can be found in any physics textbook. Electric current- this is the ordered movement of electrically charged particles in a direction. To understand to the common man what electric current is, you should use an electrician’s dictionary. In it, the term stands for the movement of electrons through a conductor or ions through an electrolyte.
Depending on the movement of electrons or ions inside a conductor, the following are distinguished: 
types of currents:
- constant;
- variable;
- periodic or pulsating.
Basic measurement quantities
Electric current strength- the main indicator that electricians use in their work. The strength of the electric current depends on the amount of charge that flows through the electrical circuit over a set period of time. The greater the number of electrons flowing from one beginning of the source to the end, the greater the charge transferred by the electrons will be.
A quantity that is measured by the ratio of the electric charge flowing through the cross-section of particles in a conductor to the time of its passage. Charge is measured in coulombs, time is measured in seconds, and one unit of electrical flow is determined by the ratio of charge to time (coulomb to second) or amperes. Determination of the electric current (its strength) occurs by sequentially connecting two terminals in the electrical circuit.
When an electric current operates, the movement of charged particles is accomplished using an electric field and depends on the force of electron movement. The value on which the work of an electric current depends is called voltage and is determined by the ratio of the work of the current in a specific part of the circuit and the charge passing through the same part. The unit of measurement volts is measured by a voltmeter when two terminals of the device are connected to a circuit in parallel.
The amount of electrical resistance is directly dependent on the type of conductor used, its length and cross-section. It is measured in ohms.
Power is determined by the ratio of the work done by the movement of currents to the time when this work occurred. Power is measured in watts.
A physical quantity such as capacitance is determined by the ratio of the charge of one conductor to the potential difference between the same conductor and the neighboring one. The lower the voltage when conductors receive an electrical charge, the greater their capacity. It is measured in farads.
The amount of work done by electricity at a certain interval in the chain is found using the product of current, voltage and the time period during which the work was carried out. The latter is measured in joules. The operation of electric current is determined using a meter that connects the readings of all quantities, namely voltage, force and time.
Electrical Safety Techniques
Knowledge of electrical safety rules will help prevent an emergency and protect human health and life. Since electricity tends to heat the conductor, there is always the possibility of a situation dangerous to health and life. To ensure safety at home must be adhered to the following simple but important rules:
- Network insulation must always be in good condition to avoid overloads or the possibility of short circuits.
- Moisture should not get on electrical appliances, wires, panels, etc. Also, a humid environment provokes short circuits.
- Be sure to ground all electrical devices.
- Avoid overloading electrical wiring as there is a risk of the wires catching fire.
Safety precautions when working with electricity involve the use of rubberized gloves, mittens, mats, discharge devices, grounding devices for work areas, circuit breakers or fuses with thermal and current protection.
Experienced electricians, when there is a possibility of electric shock, work with one hand, and the other is in their pocket. In this way, the hand-to-hand circuit is interrupted in the event of an involuntary touch to the shield or other grounded equipment. If equipment connected to the network catches fire, extinguish the fire exclusively with powder or carbon dioxide extinguishers.
Application of electric current
Electric current has many properties that allow it to be used in almost all areas human activity. Ways to use electric current:
Electricity today is the most environmentally friendly form of energy. In the modern economy, the development of the electric power industry is of planetary importance. In the future, if there is a shortage of raw materials, electricity will take a leading position as an inexhaustible source of energy.
What is electric current
Directed movement of electrically charged particles under the influence . Such particles can be: in conductors – electrons , in electrolytes – ions (cations and anions), in semiconductors – electrons and the so-called"holes" (“electron-hole conductivity”). There is also"displacement current ", the occurrence of which is due to the process of charging the capacitor, i.e., a change in the potential difference between the plates. No movement of particles occurs between the plates, but current flows through the capacitor.
In the theory of electrical circuits, current is considered to be the directional movement of charge carriers in a conducting medium under the influence of an electric field.
Conduction current (simply current) in the theory of electrical circuits is the amount of electricity flowing per unit time through the cross section of a conductor: i=q/t, where i is current. A; q = 1.6·10 9 - electron charge, C; t - time, s.
This expression is valid for DC circuits. For alternating current circuits, the so-called instantaneous current value is used, equal to speed charge changes over time: i(t)= dq/dt.
The first condition for the long-term existence of an electric current of the type under consideration is the presence of a source or generator that maintains a potential difference between charge carriers. The second condition is the closedness of the path. In particular, for direct current to exist, there must be a closed path along which charges can move within the circuit without changing their value.
As is known, in accordance with the law of conservation of electric charges, they cannot be created or disappeared. Therefore, if any volume of space where electric currents flow is surrounded by a closed surface, then the current flowing into this volume must be equal to the current flowing out of it.
A closed path along which electric current flows is called an electric current circuit, or electrical circuit. An electrical circuit is divided into two parts: an internal part, in which electrically charged particles move against the direction of electrostatic forces, and an external part, in which these particles move in the direction of electrostatic forces. The ends of the electrodes to which the external circuit is connected are called clamps.
So, electric current occurs when an electric field, or a potential difference between two points of a conductor, appears in a section of an electrical circuit. The potential difference between two points is called voltage or voltage drop in this section of the circuit.
Instead of the term “current” (“current magnitude”), the term “current strength” is often used. However, the latter cannot be called successful, since the current strength is not any force in the literal sense of the word, but only the intensity of the movement of electrical charges in the conductor, the amount of electricity passing per unit time through the cross-sectional area of the conductor.
Current is characterized by , which in the SI system is measured in amperes (A), and by current density, which in the SI system is measured in amperes per square meter.
1A = 1C/s.
In the general case, denoting the current by the letter i and the charge by q, we obtain:
i = dq / dt.
The unit of current is called ampere (A). The current in a conductor is 1 A if it passes through the cross-section of the conductor in 1 second. electric charge, equal to 1 coulomb.
If a voltage is applied along a conductor, an electric field arises inside the conductor. At field strength E, electrons with charge e are acted upon by a force f = Ee. The quantities f and E are vector. During the free path time, electrons acquire directional motion along with chaotic motion. Each electron has a negative charge and receives a velocity component directed opposite to vector E (Fig. 1). Ordered motion, characterized by a certain average electron speed vcp, determines the flow of electric current.
Electrons can have directed motion in rarefied gases. In electrolytes and ionized gases, the flow of current is mainly due to the movement of ions. In accordance with the fact that in electrolytes positively charged ions move from the positive pole to the negative, historically the direction of current was taken to be opposite to the direction of electron movement.
The direction of the current is taken to be the direction in which positively charged particles move, i.e. the direction opposite to the movement of electrons.
In the theory of electrical circuits, the direction of current in a passive circuit (outside energy sources) is taken to be the direction of movement of positively charged particles from a higher potential to a lower one. This direction was adopted at the very beginning of the development of electrical engineering and contradicts the true direction of movement of charge carriers - electrons moving in conducting media from minus to plus.
The value equal to the ratio of current to cross-sectional area S is called current density: I/S
It is assumed that the current is evenly distributed over the cross-section of the conductor. Current density in wires is usually measured in A/mm2.
According to the type of electric charge carriers and the medium of their movement, they are distinguished conduction currents and displacement currents. Conductivity is divided into electronic and ionic. For steady-state conditions, two types of currents are distinguished: direct and alternating.
Electric current transfer call the phenomenon of transfer of electric charges by charged particles or bodies moving in free space. The main type of electric current transfer is movement in emptiness elementary particles, having a charge (movement of free electrons in vacuum tubes), movement of free ions in gas-discharge devices.
Electric displacement current (polarization current) called ordered movement related media electric charges. This type of current can be observed in dielectrics.
Total electric current- scalar quantity, equal to the sum electric conduction current, electric transfer current and electric displacement current through the surface in question.
Constant is a current that can vary in magnitude, but does not change its sign for an arbitrarily long time. Read more about this here:
Magnetization current - constant microscopic (ampere) current, which is the reason for the existence of its own magnetic field magnetized substances.
An alternating current is a current that periodically changes both in magnitude and sign.The quantity characterizing alternating current is frequency (measured in hertz in the SI system), in the case when its strength changes periodically.
High frequency alternating current is forced onto the surface of the conductor. High frequency currents are used in mechanical engineering for heat treatment of surfaces of parts and welding, and in metallurgy for melting metals.Alternating currents are divided into sinusoidal and non-sinusoidal. A current that varies according to a harmonic law is called sinusoidal:
i = Im sin wt,
where Im, - , A,
The rate of change of alternating current is characterized by it, defined as the number of complete repeating oscillations per unit time. Frequency is designated by the letter f and is measured in hertz (Hz). Thus, a current frequency in a network of 50 Hz corresponds to 50 complete oscillations per second. Angular frequency w is the rate of change of current in radians per second and is related to frequency by a simple relation:
w = 2pif
Steady (fixed) values of direct and alternating currents denote capital letter I unsteady (instantaneous) values - letter i. Conventionally, the positive direction of current is considered to be the direction of movement of positive charges.
This is a current that changes according to the sine law over time.
Alternating current also refers to current in conventional single- and three-phase networks. In this case, the alternating current parameters change according to a harmonic law.
Since alternating current changes over time, simple ways solutions to problems suitable for DC circuits are not directly applicable here. At very high frequencies, charges can make oscillatory motion- flow from one place in the chain to another and back. In this case, unlike direct current circuits, the currents in series-connected conductors may not be the same.
Capacitances present in AC circuits enhance this effect. In addition, when the current changes, self-induction effects occur, which become significant even at low frequencies if coils with high inductance are used.
At relatively low frequencies, AC circuits can still be calculated using , which, however, must be modified accordingly.
A circuit that includes various resistors, inductors, and capacitors can be treated as if it consists of a generalized resistor, capacitor, and inductor connected in series.
Let's consider the properties of such a circuit connected to a sinusoidal alternating current generator. To formulate rules for calculating AC circuits, you need to find the relationship between voltage drop and current for each of the components of such a circuit.
Plays completely different roles in AC and DC circuits. If, for example, an electrochemical element is connected to the circuit, then until the voltage on it becomes equal to the EMF of the element. Then charging will stop and the current will drop to zero.
If the circuit is connected to an alternating current generator, then in one half-cycle electrons will flow out of the left plate of the capacitor and accumulate on the right, and in the other - vice versa.
These moving electrons represent alternating current, the strength of which is the same on both sides of the capacitor. As long as the frequency of the alternating current is not very high, the current through the resistor and inductor is also the same.
In AC consuming devices, AC current is often rectified to produce DC current.
Electric current in all its manifestations is a kinetic phenomenon similar to the flow of fluid in closed hydraulic systems. By analogy, the process of current movement is called “flow” (current flows).
The material in which current flows is called. Some materials become superconducting at low temperatures. In this state, they offer almost no resistance to current; their resistance tends to zero.
In all other cases, the conductor resists the flow of current and, as a result, part of the energy of the electrical particles is converted into heat. The current strength can be calculated using the circuit section and Ohm's law for the complete circuit.
The speed of movement of particles in conductors depends on the material of the conductor, the mass and charge of the particle, the surrounding temperature, the applied potential difference and is much less than the speed of light. Despite this, the speed of propagation of the electric current itself is equal to the speed of light in a given medium, that is, the speed of propagation of the electromagnetic wave front.
How does current affect the human body?
Current passed through the body of a person or animal can cause electrical burns, fibrillation or death. On the other hand, electric current is used in intensive care, for treatment mental illness, especially depression, electrical stimulation of certain areas of the brain is used to treat diseases such as Parkinson's disease and epilepsy, a pacemaker that stimulates the heart muscle with a pulsed current is used for bradycardia. In humans and animals, current is used to transmit nerve impulses.
According to safety regulations, the minimum human-perceivable current is 1 mA. The current becomes dangerous to human life starting from a force of approximately 0.01 A. The current becomes fatal to a person starting from a force of approximately 0.1 A. A voltage of less than 42 V is considered safe.
Electric current is one of the main processes occurring in absolutely any electronic circuit (in an electrical circuit). Studying this process will make it much easier in the future to understand other processes inherent in electrical circuits.
For a deeper understanding of the essence of electric current, I recommend that you first familiarize yourself with the nature of its occurrence. Previously, we learned that when a plastic rod is rubbed against the wool, due to frictional forces, a certain number of electrons leave the surface layer of the rod, which becomes positively charged. When a glass rod is rubbed on silk, it becomes negatively charged as electrons leave the atoms from upper layers silk and settle on the glass.
Thus, we have one rod with an excess of electrons, so it is said to be negatively charged, and the second rod has a shortage of electrons, so it is said to have a predominant positive charge.
Since everything isIf electrons in nature tend to balance, then by connecting both oppositely charged rods with a conductor, free electrons will instantly move from the glass rod to the plastic one, from the zone of their excess to the zone of shortage. As a result, both rods will become neutrally charged and devoid of free electrons that could easily move around. The process of moving electrons along a conductor between rods is electricity .
Electric current can be carried out useful work, for example, light up the LED,placed in his path.
The useful work of charges can be illustrated using the example of a bus. If a bus without passengers traveled from city A to city B, then the bus did not perform any useful work and wasted fuel. The bus that carried the passengers did useful work. Electric current works in a similar way, so a load is placed on its path, on which useful work is performed.
Connected by wires with rubbed sticks, the LED glows for a very short period of time, since the free negative charges will instantly move from the area of their excess to the area of shortage and equilibrium will come.
Generator
In order for the LED to glow for a long time, it is necessary to maintain the electric current by replenishing the charges on the sticks, that is, constantly rubbing them on wool and silk, respectively. But this method is difficult to implement in practice and ineffective. Therefore, a much more practical method is used to maintain the required amount of energy carriers.
A device that constantly creates or generates charges of different signs is called a generator or, more generally, a power source. The simplest generator is a battery, which is more correctly called a galvanic cell. Unlike rods, in which charges are formed due to frictional forces, in a galvanic cell, unlike charges are formed as a result of chemical reactions.
Electric current and conditions for its flow
Now we can draw the first important preliminary conclusions and identify the conditions for the flow of electric current.
- First. To generate an electric current, the path of movement of charges must be closed.
- Second. To maintain an electric current, it is necessary that at the beginning of the path the supply of charges is replenished, and at the end of the path they are taken away, making room for newly arrived charges.
- Third. In order for the charges to perform useful work, it is necessary to place in their path, for example, an incandescent lamp filament, an LED, or a motor winding, which in general is called a load or consumer.
In general, the simplest electrical circuit consists of a generator, a load, and wires connecting the generator to the load.
Electromotive force EMF
The main task of any power source is to form and maintain a constant value of opposite charges at the terminals, called electrodes. How larger number charges, the more they tend to attract each other and therefore move more intensely along the electrical circuit. And the force that causes electrons to move along a circuit is called electromotive force or for short EMF . Electromotive force is measured in volts [IN]. The EMF of a new (not discharged) battery is a little more than 1.5 V, and a crown is a little more than 9 V.
The value of electric current can be clearly quantified using the example of a water pipe. Let's mentally imagine water as a set of small droplets of the same size. Now let’s take and cut the pipe in some place and install a water droplet counter. Next, open the tap and record the time, for example one minute. After counting down the time, we will take the meter readings. Let's say the counter recorded 1 million drops in one minute. From this we conclude that the water flow is a million drops per minute. If we increase the water pressure - we make the pump pump it faster - then the water pressure will increase, while the droplets will begin to move more intensely and, accordingly, the water consumption will increase.
Electric current strength
The strength of the electric current is determined in a similar way. If we mentally cut the wire connecting the generator to the load and install a meter, then we will get the consumption of electrons per unit of time - this is the current strength.
As the electromotive force of the generator increases, electrons pass through the circuit more intensely, and the current increases.
Since the charge of the electron and their total number passing through the cross section of the conductor per unit time are known, the current strength can be quantitatively determined.
The charge of one electron has a very small value, and a huge number of them participate in the electric current. Therefore, 628∙10 16 was taken as a unit of electric charge, that is, 6280000000000000000 electron charges. This amount of electric charge is called pendant , abbreviated [Cl].
The unit for measuring current is called ampere [A]. The current strength is equal to one ampere when a total electric charge of one coulomb passes through the cross-section of the conductor in one second.
1 A = 1 C/1 sec
I = Q/t
If twice as many electrons pass through a conductor in one second, then I equals 2 amperes.
In a conductor made of metal, such as copper or aluminum, a lot of free electrons are formed. They easily leave atoms crystal lattice metal and move freely in interatomic space. However, they do not walk for long, since they are instantly attracted by another positively charged atom, which has lost a similar element. Therefore, by default, no current flows through the conductor. In addition, free electrons do not have an ordered movement, but move chaotically in the interatomic space. Such movement, which does not have a clear direction, is called Brownian motion. As the temperature rises, the intensity of traffic increases.
To leak I you need to create a shortage of electrical components at one end of the conductor, and an excess of them at the other, that is, connect opposite poles of the power source. Then the electric field of the power source will create such an electromotive force that will force the electrons in the conductor to move in strictly one direction. That's why electric current is the ordered movement of charges under the influence of an external electric field