Current strength. Current units

And the design of electrical appliances.

Electric current is the unidirectional movement of charged particles. Current strength is a concept that characterizes this process. Its physical meaning is the amount of charge flowing through the cross section of a conductor per unit time.

Units

In the System of International Units, current is usually measured in amperes (A). This is what the international conference of electricians decided in 1881.

Ampere Andre-Marie is a French scientist who worked in the field of physics and mathematics and put a lot of work into the study of electricity. His merits in this field are so high that many representatives of the scientific world consider Ampere worthy of the title of founder of electrodynamics.

A current of 1 A is quite strong, so units of milliampere (mA, 0.001 A) and microampere (μA, 10^-6 A) are often used.

In the system of units:

• SGSM (electromagnetic), Much less common, current is measured in amperes or bio. The ratio of units is as follows: 1 ampere = 0.1 ampere;
• SGSE (electrostatic) The unit statampere is used. Ratio: 1 amp = 2997924536.843 statamps.

The units abampere and statampere are widely used in theoretical physics.

Formula

When performing calculations, the current strength is denoted by the letter I.

The formula for current strength is represented as follows: I = q / t, where:

• q - charge, C (coulomb);
• T - time, s.

This implies the dimension of the ampere: (A) = (C/s). 1 C is equal to the charge of 6.241509343 x 10 18 electrons. In 2011, it was decided to change the definition of the ampere unit, like some others, by linking it to the charge of an electron.

With known - and , the current strength is determined by I = U / R, where:

• U - voltage, V;
• R is the electrical resistance of the circuit section, Ohm.

Definition

In the SI system, a current of 1 A is defined as that which, when flowing through two infinitely long wires of negligible cross-section, located in a vacuum and separated by a distance of 1 m, causes an attractive force between them of 2 x 10 -7 newton (N ).

The abamper in the SGSM system is determined in the same way, only in this case the force is measured in dynes, and the distance is measured in centimeters. The attraction between the wires is due to the presence of magnetic fields that always arise around moving charged particles (Bio-Savart's law).

At the end of the 19th century, a different definition was in force, based on the ability of an electric current to carry out electrolysis, that is, to separate various elements from a solution.

This ability is due to the fact that complex chemicals always contain two components: an oxidizing agent and a reducing agent.

The oxidizing agent attracts electrons from the reducing agent and acquires a negative charge, while the reducing agent acquires a positive charge.

When current is passed through a solution, negatively charged oxidizing atoms are attracted to the electrode with a positive potential, and reducing agent atoms are attracted to a negative one. The amount of substance released depends on the amount of electricity passed through the solution.

During the experiments, it was determined that a current of 1 A releases 4.025 g of this metal per hour (0.001118 g per second) from a solution of silver salt.

Current strength of different devices

The strength of current flowing in different devices and circuits varies quite greatly, here are some examples:

• hearing aid: 0.7 mA;
• 56-inch plasma TV: 250–290 mA;
• toaster, mini-oven: 5-6 A;
• : 500–830 mA;
• hair dryer: 4.5 A.

In an electrical circuit

The current in an electrical circuit obeys the laws discovered by G. Kirchhoff:

The operation of a differential current switch, commonly known as it, is based on this phenomenon. One contact of the device is connected to the phase, the other to the neutral wire, which are essentially the beginning and end of the circuit served by this RCD.

According to this law, the currents in both parts of the device during normal operation of the circuit will be equal, regardless of the type and power of the connected load. If a difference (differential current) suddenly appears, this will indicate a current leak.

In turn, a leak means one of three things:

1. a phase has broken on an electrical appliance;
2. contact has occurred between live parts and grounded metal structures, fraught with fire.

The RCD is designed to switch off in the presence of differential current. The signal is the magnetic field that appears in the device during a leak, while at equal currents the magnetic fields they create cancel each other out.

An ammeter, unlike a voltmeter, is connected in series with the load, that is, in an open circuit (the voltmeter is connected in parallel).

Wire size

Electric current flowing in a conductor acts in two ways:

• creates an electromagnetic field;
• causes heating of the conductor.

If the magnetic field is negligible (the wire is not wound in), almost all the current power is spent on heating.

Wire cross-section for current and power

The heating power is determined by the formula W = I 2 * R, where:

• W - heating power, W;
• I - current strength, A;
• R - conductor resistance, Ohm.

The resistance of the wires depends on their area: the larger it is, the lower the resistance. Therefore, when designing electrical wiring, it is important to select the cross-section of the wires (special tables are used) so that they do not overheat at rated load. Otherwise, the insulation may melt, followed by a short circuit or fire.

Short circuit current

Above was a formula linking current strength with voltage and resistance: I = U / R. Obviously, when the value of R is close to zero, which is found, for example, in aluminum and (used for the manufacture of cable cores), the current strength tends to infinity .

This phenomenon is called “short circuit current” (SC). It occurs when electrical contact occurs between the phase and neutral conductors, bypassing the load.

Short circuit current causes significant heating of the wires, which can lead to fire. Therefore, electrical networks are protected with special devices - circuit breakers or fuses.

When the current strength is higher than the rated value, the conductor inside the device melts (fuses) or the thermal relay (circuit breakers) is triggered, as a result of which the circuit is disconnected.

AC power

Carrying out calculations using the instantaneous value is quite inconvenient: you have to deal with extremely difficult to solve trigonometric equations. To simplify the task, alternating current is replaced by its effective value. This is equivalent to a given variable, that is, producing the same work.

The effective value of a sinusoidal alternating current is 1.41 times less than its amplitude value. That is, if it is said that a current of 5 A flows in an alternating current circuit, then in fact the current in it fluctuates between 7.05 A and -7.05 A.

The same applies to alternating voltage. That is, in a single-phase 220-volt network, the voltage actually fluctuates with an amplitude of 311 V.

Video on the topic

What is current strength? Explanation in video:

Current strength is the most important parameter characterizing the state of the electrical circuit. Therefore, a radio amateur often has to measure it using an ammeter or. It is important to remember that some devices do not have overload protection and, as a result, the measurement range when the order of the measured value is unknown should be selected starting from the largest value.

The concept of current strength is the basis of modern electrical engineering. Without this basic knowledge, it is impossible to make calculations for circuits, perform electrical operations, prevent, identify and eliminate damage in the circuit.

How does it arise

To understand what current strength is, you should know the condition for its occurrence - the existence of particles with a free charge. It moves through the conductor (its cross section) from one point to another. The physics of current consists of the ordered movement of electrons, which are acted upon by an electric field from a power source. The more charged particles are transferred, and the faster they move in one direction, the more charge will reach its destination.

In addition to the power source, the elements of a closed circuit are connecting wires through which electricity passes and energy consumers (installations, resistors).

Additional Information. In metal conductors, electrons act as charge transmitters; in gaseous conductors, ions act; in liquid conductors, the transfer of charged particles is carried out using both types of particles. Violation of the order of passage indicates a chaotic movement of charges, in which the circuit will become de-energized.

Definition

The current strength in a conductor is the amount of electricity moved through a cross section in a unit time interval. To increase this value, you need to remove the lamp from the circuit or increase the magnetic field created by the battery.

The unit of measurement of electric current according to the SI system (Systeme International) is the ampere (A), named after the outstanding French scientist of the 19th century, Andre-Marie Ampere.

Additional Information. Ampere is a fairly impressive electrical measure. A current value of up to 0.1A poses a mortal danger to human life. A burning 100 W household light bulb transmits approximately 0.5 A of electricity. In a room heater, this value reaches 10 A; a portable calculator will need one thousandth of an ampere.

In electrical engineering practice, measurements of small quantities can be expressed in micro- and milliamperes.

The current strength is determined by a measuring device (ampere or galvanometer), sequentially connecting it to the desired section of the circuit. Small quantities are measured with a micro- or milliammeter. The main methods for finding the amount of electricity using instruments are:

• Magnetoelectric – with a constant current value. This method is characterized by increased accuracy and low energy consumption;
• Electromagnetic – for stationary and changing quantities. Using this method, the current in the circuit is found as a result of the conversion of the magnetic field into the output signal of the modulation sensor;
• Indirect - based on measuring voltage at a known resistance. Next, calculate the desired value using Ohm’s law, shown below.

According to the definition, the current strength (I) can be found using the formula:

I = q/t, where:

• q – charge passing across the conductor (C);
• t is the duration of time spent moving particles (s).

The formula for current strength reads as follows: the required value I is the ratio of the charge passed through the conductor to the period of time used.

Note! The strength of the current is determined not only through the charge, but also by calculation formulas based on Ohm's law, which states: the strength of electricity is directly proportional to the voltage of the conductor and inversely proportional to its resistance.

The formula of Ohm's law will help you find the current strength, which looks like the ratio:

I = U/R, here:

• U – voltage (V);
• R – resistance (Ohm).

This established relationship of physical quantities is used for various calculations:

• taking into account the characteristics of the power source;
• for calculations in current circuits of any direction;
• for multiphase circuits.

Note! If the conductors are connected in series, then the electricity of each of them is equal. A parallel connection provides a number of amperes, which is the sum of the current values ​​of each conductor.

How to find power (rate of energy transfer or conversion) using current value? To do this you need to use the formula:

P = U*I, where the multiplied values ​​were mentioned above.

Kinds

With constant and alternating electricity, its strength varies. For a chain with particle motion in a constant direction, all parameters remain unchanged. A variable species is capable of changing its magnitude with the same or changing direction. The amount of electricity in this case is:

• instantaneous, depending on the amplitude and frequency of oscillations associated with the angular frequency;
• amplitude - the maximum value of instantaneous current for a certain period;
• efficient - when converting energy, the amount of heat from both types of current is the same.

Household electrical networks pass alternating current, which is converted to direct current when passing through the power supply of an electrical appliance (computer, TV).

The magnitude of the current is a concept closely related to electrical energy, which is of great importance for everyday life, the national economy, and strategic objects. Moreover, the electric power industry is the economic basis of the state and the determining vector of development within the country and at the international level.

Video

« Physics - 10th grade"

Electricity- directed movement of charged particles. Thanks to electric current, apartments are illuminated, machine tools are set in motion, burners on electric stoves are heated, the radio operates, etc.

Let's consider the simplest case of directed motion of charged particles - direct current.

What electric charge is called elementary?
What is the elementary electric charge?
What is the difference between charges in a conductor and a dielectric?

When charged particles move in a conductor, electric charge is transferred from one point to another. However, if charged particles undergo random thermal motion, such as free electrons in a metal, then charge transfer does not occur (Fig. 15.1, a). The cross-section of a conductor, on average, crosses the same number of electrons in two opposite directions. Electric charge is transferred through the cross section of a conductor only if, along with random movement, electrons participate in directed movement (Fig. 15.1, b). In this case, they say that the conductor goes electricity.

Electric current is the ordered (directed) movement of charged particles.

Electric current has a certain direction.

The direction of current is taken to be the direction of movement of positively charged particles.

If you move a generally neutral body, then, despite the ordered movement of a huge number of electrons and atomic nuclei, no electric current will arise. The total charge transferred through any cross section will be equal to zero, since charges of different signs move with the same average speed.

The direction of the current coincides with the direction of the electric field strength vector. If the current is formed by the movement of negatively charged particles, then the direction of the current is considered opposite to the direction of movement of the particles.

The choice of current direction is not very successful, since in most cases the current represents the ordered movement of electrons - negatively charged particles. The choice of current direction was made at a time when nothing was known about free electrons in metals.

Action of current.

We do not directly see the movement of particles in a conductor. The presence of electric current must be judged by the actions or phenomena that accompany it.

Firstly, the conductor through which the current flows heats up.

Secondly, electric current can change the chemical composition of the conductor: for example, release its chemical components (copper from a solution of copper sulfate, etc.).

Thirdly, the current exerts a force on neighboring currents and magnetized bodies. This action of current is called magnetic.

Thus, a magnetic needle near a current-carrying conductor rotates. The magnetic effect of current, in contrast to chemical and thermal, is the main one, since it manifests itself in all conductors without exception. The chemical effect of current is observed only in solutions and melts of electrolytes, and heating is absent in superconductors.

In an incandescent light bulb, due to the passage of electric current, visible light is emitted, and the electric motor performs mechanical work.

Current strength.

If an electric current flows in a circuit, this means that an electric charge is constantly being transferred through the cross-section of the conductor.

The charge transferred per unit time serves as the main quantitative characteristic of the current, called current strength.

If a charge Δq is transferred through the cross section of a conductor during a time Δt, then the average value of the current is equal to

The average current strength is equal to the ratio of the charge Δq passing through the cross section of the conductor during the time interval Δt to this period of time.

If the current strength does not change over time, then the current is called permanent.

The alternating current strength at a given time is also determined by formula (15.1), but the time period Δt in this case should be very small.

Current strength, like charge, is a scalar quantity. She might be like positive, so negative. The sign of the current depends on which of the directions around the circuit is taken as positive. Current strength I > 0 if the direction of the current coincides with the conditionally chosen positive direction along the conductor. Otherwise I< 0.

Relationship between current strength and the speed of directional movement of particles.

Let a cylindrical conductor (Fig. 15.2) have a cross section with area S.

For the positive direction of current in a conductor we take the direction from left to right. The charge of each particle will be considered equal to q 0. The volume of the conductor, limited by cross sections 1 and 2 with a distance Δl between them, contains nSΔl particles, where n is the concentration of particles (current carriers). Their total charge in the selected volume is q = q 0 nSΔl. If particles move from left to right with an average speed υ, then during the time all particles contained in the volume under consideration will pass through cross section 2. Therefore, the current strength is equal to:

The SI unit of current is ampere (A).

This unit is established on the basis of the magnetic interaction of currents.

Measure current strength ammeters. The design principle of these devices is based on the magnetic action of current.

The speed of ordered movement of electrons in a conductor.

Let's find the speed of ordered movement of electrons in a metal conductor. According to formula (15.2) where e is the modulus of the electron charge.

Let, for example, the current strength I = 1 A, and the cross-sectional area of ​​the conductor S = 10 -6 m 2. Electron charge modulus e = 1.6 10 -19 C. The number of electrons in 1 m 3 of copper is equal to the number of atoms in this volume, since one of the valence electrons of each copper atom is free. This number is n ≈ 8.5 10 28 m -3 (this number can be determined by solving problem 6 from § 54). Hence,

As you can see, the speed of ordered movement of electrons is very low. It is many times less than the speed of thermal motion of electrons in the metal.

Conditions necessary for the existence of electric current.

For the emergence and existence of a constant electric current in a substance, it is necessary to have free charged particles.

However, this is still not enough for a current to occur.

To create and maintain the ordered movement of charged particles, a force is required that acts on them in a certain direction.

If this force ceases to act, then the ordered movement of charged particles will cease due to collisions with ions of the crystal lattice of metals or neutral molecules of electrolytes, and electrons will move randomly.

Charged particles, as we know, are acted upon by an electric field with the force:

Typically, it is the electric field inside the conductor that causes and maintains the ordered movement of charged particles.
Only in the static case, when the charges are at rest, the electric field inside the conductor is zero.

If there is an electric field inside the conductor, then there is a potential difference between the ends of the conductor in accordance with formula (14.21). As the experiment showed, when the potential difference does not change over time, a direct electric current. Along the conductor, the potential decreases from the maximum value at one end of the conductor to the minimum at the other, since the positive charge, under the influence of field forces, moves in the direction of decreasing potential.

What is voltage and current?

Today we will talk about the most basic concepts of current and voltage, without a general understanding of which it is impossible to build any electrical device.

So what is tension?

Simply put voltage- potential difference between two points in an electrical circuit, measured in Volts. It is worth noting that voltage is always measured between two points! That is, when they say that the voltage on the controller leg is 3 Volts, it is meant that the potential difference between the controller leg and the ground is the same 3 Volts.

Ground (Ground, Zero) is a point in an electrical circuit with a potential of 0 Volts. However, it is worth noting that voltage is not always measured relative to ground. For example, by measuring the voltage between two terminals of the controller, we will obtain the difference in the electrical potentials of these points in the circuit. That is, if on one leg there are 3 Volts (That is, this point has a potential of 3 Volts relative to the ground), and on the second 5 Volts (Again, potential relative to the ground), we get a voltage value equal to 2 volts, which is equal to the potential difference between points 5 and 3 Volta.

From the concept of voltage follows the next concept - electric current. From the course of general physics we remember that electric current is the directed movement of charged particles along a conductor, measured in Amperes. Charged particles move due to the potential difference between points. It is generally accepted that current flows from a point with a large charge to a point with a smaller charge. That is, it is the voltage (potential difference) that creates the conditions for the flow of current. In the absence of voltage, current is impossible, that is, there is no current between points with equal potential.

On its way, the current encounters an obstacle in the form of resistance, which prevents its flow. Resistance is measured in Ohms. We'll talk more about it in the next lesson. However, the following relationship has long been established between current, voltage and resistance:

Where I - Current in Amperes, U - Voltage in Volts, R - Resistance in Ohms.

This relationship is called Ohm's law. The following conclusions from Ohm’s law are also true:

If you still have questions, ask them in the comments. Only thanks to your questions we will be able to improve the material presented on this site!

That's all, in the next lesson we'll talk about resistance.

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Impossible. The concept of current is the basis on which, like a house on a reliable foundation, further calculations of electrical circuits are built and new and new definitions are given. Current strength is one of the international values, therefore the universal unit of measurement is Ampere (A).

The physical meaning of this unit is explained as follows: a current of one ampere arises when charged particles move along two conductors of infinite length, between which there is a gap of one meter. In this case, the energy generated on each meter section of conductors is numerically equal to 2*10 to the power of -7 Newton. It is usually added that the conductors are located in a vacuum (which makes it possible to neutralize the influence of the intermediate medium), and their cross section tends to zero (at the same time, the conductivity is maximum).

However, as is usually the case, the classical definitions are understandable only to specialists who, in fact, are no longer interested in the basics. But a person unfamiliar with electricity will become even more confused. Therefore, let us explain what current strength is, literally “on the fingers”. Let's imagine an ordinary battery, from the poles of which two insulated wires go to the light bulb. A switch is connected to the gap in one wire. As you know from the initial course of physics, electric current is the movement of particles that have their own. They are usually considered to be electrons (indeed, it is the electron that has a single negative charge), although in reality everything is a little more complicated. These particles are characteristic of conductive materials (metals), but in gaseous media ions additionally carry charge (remember the terms “ionization” and “air gap breakdown”); in semiconductors, conductivity is not only electronic, but also hole (positive charge); in electrolytic solutions the conductivity is purely ionic (for example, car batteries). But let's return to our example. In it, the current forms the movement of free electrons. Until the switch is turned on, the circuit is open, there is nowhere for the particles to move, therefore, the current strength is zero. But once you “assemble the circuit,” electrons rush from the negative pole of the battery to the positive, passing through the light bulb and causing it to glow. The force that makes them move comes from the electric field created by the battery (EMF - field - current).

Current is the ratio of charge to time. That is, in fact, we are talking about the amount of electricity passing through a conductor per conventional unit of time. An analogy can be made with water: the more the tap is opened, the greater the volume of water will pass through the pipeline. But if water is measured in liters (cubic meters), then current is measured in the number of charge carriers or, which is also true, in amperes. It's that simple. It is easy to understand that you can increase the current in two ways: by removing the light bulb from the circuit (resistance, an obstacle to movement), and also by increasing the electric field created by the battery.

Actually, we have come to how the current strength is calculated in the general case. There are many formulas: for example, for a complete circuit that takes into account the influence of the characteristics of the power source; for alternating and for multiphase systems, etc. However, they are all united by a single rule - the famous Ohm's law. Therefore, we present its general (universal) form:

where I is current, in Amperes; U is the voltage at the terminals of the power source, in Volts; R is the resistance of the circuit or section, in Ohms. This dependence only confirms all of the above: increasing the current can be achieved in two ways, through resistance (our light bulb) and voltage (source parameter).