In electrical engineering, electric current is the movement of charged particles along a conductor. This quantity is not characterized only by the amount of electrical energy passing through the conductor, since both different and different currents can be passed through the same conductor. equal strength for different periods of time. That is why not everything is as simple as it seems. It is recommended that you familiarize yourself with more detailed definitions of electric current, what it is equal to and how it is calculated. This article will explain how to find the current in a conductor and will give the formula for this equation.
Considering the amount of electricity that flows through a certain conductor over various time intervals, it will become clear that in a short period of time the current will flow more intensely, so one more definition needs to be introduced. It means the current flowing in a conductor per second of time.
Basic quantities characterizing the flow of electrons
If we formulate a definition based on all of the above, then the strength of the electric current is the amount of electricity passing through cross section conductor in a second. The value is marked with the Latin letter “I”.
Galvanometer for measuring small currents
Important! Experts define the strength of an electric current as one ampere when one coulomb of electricity passes through the cross-section of a conductor in one second.
Often in electrical engineering you can see other units for measuring electric current: milliamps, microamps, and so on. This is due to the fact that such quantities will be quite sufficient to power modern circuits. 1 amp is very great value, since a person can be killed by a current of 100 milliamps, and therefore an electrical outlet is no less dangerous for a person than, for example, a speeding car.
Scheme defining the concept under consideration
If you know the amount of electricity that passed through the conductor in a specific period of time, then the force (not power) can be calculated using the formula shown in the picture.
When the electrical network is closed and has no branches, the same amount of electricity flows through each cross-section per second. Theoretically, this is justified as follows: charge cannot accumulate in a certain place, and the strength of the electric current is the same everywhere.
Types of currents
Current sources
An electric current source is an electrical device that converts a certain type of energy into electrical energy. Such devices are divided into physical and chemical.
The operating principle of chemical sources is based on the conversion of chemical energy into electrical energy. This transformation occurs independently and does not require outside participation. Depending on the renewability of elements and the type of reactions, they are divided into:
- Primary (batteries) Primary sources cannot be used a second time if they are discharged, since the chemical reactions occurring in them are irreversible. They are divided into fuel and semi-fuel cells. Fuel ones are similar to batteries, but chemicals they are filled separately, like food chemical reaction they go outside. This helps them work for a long time. Semi-fuel include one of chemical elements, and the second gradually arrives throughout use. Their service life is determined by the supply of non-renewable substance. If regeneration through charging is possible for such an element, then it resumes its capabilities as a battery.
Batteries - as primary chemical sources of current
- Secondary (batteries) undergo a charging cycle before use. The charge they receive during the process can be transported along with the devices. After the charge is consumed, it can be regenerated due to charging and the reversibility of the chemical reaction. Also classified as secondary are renewable elements that are mechanically or chemically charged and restore their ability to power devices. They are designed in such a way that after a certain period of time they require replacement of certain parts in order for the reaction to continue.
Types of electric power sources
Important! It should be understood that the division into batteries and accumulators is conditional. The properties of the battery can be manifested, for example, in alkaline batteries, which can be revived at a certain state of charge.
Also, according to the type of reagents, chemical sources are divided into:
- Acidic.
- Saline.
- Alkaline.
Physical sources of electric current are based on the conversion of mechanical, as well as nuclear, thermal or light energy into electrical energy.
Industrial three-phase current generator
Current strength - what it is equal to, in what units it is measured, how to find current strength using the formula
As has already become clear, the strength of an electric current is a physical quantity that shows the charge that passes through a conductor per unit time. The basic formula for calculating it looks like this: I = q/t, where q is the charge that flows along the conductor in coulombs, and t is the time interval in seconds.
You can also calculate the strength of electric current using Ohm's law. It states that this value is equal to the network voltage in volts divided by its resistance in ohms. In this regard, there is a formula of this kind - I = U/R. This law is applicable for calculating values DC.
To calculate variable parameters electricity, you need to divide the found values by square root out of two.
For your information! This is a more practical measurement method and must be used frequently since all appliances in the home or office are powered by outlets that supply alternating current. This is done because it is easier to work with and more convenient to transform.
Ohm's law in the table
Important! A clear example of the operation of alternating electric current can be observed when fluorescent lamps are turned on. Until they light up completely, they will blink because the current moves back and forth in them.
The unit of current is ampere. It is defined as the strength of a constant current that passes through infinite parallel conductors with the smallest circular cross-section (minimum area circular section), spaced 1 meter apart and located in an airless vacuum space. This interaction over one meter of length of these conductors is equal to 2 × 10 to the minus 7th power of Newton. If one coulomb of charge passes through a conductor in one second of time, then current strength it is equal to one ampere.
Rechargeable batteries are secondary sources, but are inextricably linked to batteries
Why do you need to measure current?
The current strength in a conductor or section of an electrical circuit is measured in order to have an idea of the characteristics of a given conductor or circuit. Since current strength is one of the main parameters of electricity, it is inextricably linked with other values such as voltage and resistance. Moreover, as has already become clear, these three quantities can proportionally determine each other.
The solar panel is also a source that converts light energy
Calculations of electric current strength are made in different cases:
- When laying electrical networks.
- When creating devices.
- For educational purposes.
- When choosing the appropriate parts to perform certain actions.
Current generator device diagram
Electrical device for measuring current
To measure the strength of electric current, a special device called an ammeter is used. If it is necessary to measure currents of various strengths, then they resort to the use of milliammeters and macroammeters. To measure the required value with it, it is connected to the circuit in series. The current that passes through the device will be changed by it, and the data will be displayed on a digital display or analog scales.
Important! It is worth remembering that you can turn on the ammeter at any part of the network, since the current strength in a simple closed circuit without branches is the same at all points.
Modern testers and multimeters contain the function of measuring electric current, so there is no need to resort to large devices intended for industrial use
Current strength at home can be measured using a multimeter
Thus, the strength of electric current is a fundamental characteristic of moving particles. It not only makes it clear what voltage and resistance are in the network, but also determines other important quantities such as EMF, etc.
I think you have heard the phrase “current strength” more than once. What is strength needed for? Well, what for, in order to do useful or useless work. The main thing is to do something. Our body also has power. Some people have such strength that they can smash a brick to smithereens with one blow, while others cannot even lift a spoon. So, my dear readers, electric current also has power.
Imagine a hose with which you water your garden.
Let the hose be a wire, and the water in it be an electric current. We opened the faucet slightly and water ran through the hose. Slowly, but still she ran. The jet force is very weak. Now let's open the tap all the way. As a result, the stream will gush with such force that you can even water your neighbor’s garden.
Now imagine that you are filling a bucket. Will water pressure from a faucet or hose fill it faster? The diameter of the hose and faucet are the same
Of course, with pressure from the yellow hose! But why does this happen? The thing is that the volume of water coming out of the faucet and the yellow hose over an equal period of time is also different. Or in other words, a much larger number of water molecules will run out of a hose than out of a faucet in the same time.
What is current strength
It's exactly the same story with wires). That is, over an equal period of time, the number of electrons running along the wire can be completely different. From this we can derive the definition of current strength.
So, current is the number of electrons passing through the cross-sectional area of a conductor per unit of time, let's say, per second. Below in the figure, the same cross-sectional area of the wire through which the electric current flows is shaded with green lines.
And the greater the number of electrons “running” along the wire through the cross-section of the conductor over some time, the greater the current strength in the conductor will be.
Or in other words, the formula for a teapot:
Where
I – actual current strength
N – number of electrons
t is the period of time during which these electrons travel through the cross section of the conductor.
Current strength is measured in so-called Ampere, in honor of the French scientist André-Marie Ampère.
Also keep in mind that each individual hose can only withstand a certain maximum flow of water, otherwise it will either get a hole somewhere from such pressure, or it will simply be blown to pieces. It's the same with wires. We need to know what maximum current we can drive through this wire. For example, for a copper wire with a cross-section of 1 mm 2 the normal value is 10 Amperes. If we supply more, the wire will either start to heat up or melt. They are based on this principle. Therefore, power cables, through which hundreds and thousands of amperes “run,” are taken with a large diameter and try to be made of copper, since its specific power is very small.
Definition
Electric shock called the ordered movement of charge carriers. In metals, these are electrons, negatively charged particles with a charge equal to the elementary charge. The direction of current is considered to be the direction of movement of positively charged particles.
The current strength (current) through a certain surface S is a scalar physical quantity, which is denoted by I, equal to:
where q is the charge passing through the surface S, t is the charge travel time. Expression (1) determines the magnitude of the current at time t (instantaneous value of the current).
Some types of current
The current is called constant if its strength and direction do not change over time, then:
Formula (2) shows that the direct current is equal to the charge that passes through the surface S per unit time.
If the current is alternating, then the instantaneous current strength (1), amplitude current strength and effective force current The effective value of alternating current (I eff) is the strength of direct current that will perform work equal to the work of alternating current during one period (T):
If alternating current can be represented as sinusoidal:
then I m is the amplitude of the current ( is the frequency of the alternating current).
Current Density
The distribution of electric current over the cross-section of a conductor is characterized using the current density vector (). In this case:
where is the angle between the vectors and ( is the normal to the surface element dS), j n is the projection of the current density vector onto the direction of the normal ().
The current strength in the conductor is determined using the formula:
where integration in expression (6) is carried out over the entire cross section of the conductor S
For direct current we have:
If we consider two conductors with cross sections S 1 and S 2 and direct currents, then the following relation holds:
Current strength in conductor connections
When conductors are connected in series, the current in each of them is the same:
When connecting conductors in parallel, the current strength (I) is calculated as the sum of the currents in each conductor (I i):
Ohm's law
Current strength is included in one of the basic laws of direct current - Ohm's law (for a section of a circuit):
where - is the potential difference at the ends of the section under consideration, is the emf of the source that is included in the circuit section, R is the resistance of the circuit section.
Current units
The basic unit of current in the SI system is: [I]=A(ampere)=C/s
Examples of problem solving
Example
Exercise. What charge (q) passes through the cross section of the conductor during the period of time from t 1 = 2c to t 2 = 6c, if the current strength changes in accordance with the equation: I = 2 + t, where the current strength is in amperes, time in seconds?
Solution. As a basis for solving the problem, we will take the definition of instantaneous current strength:
In this case, the charge that passes through the cross section of the conductor is equal to:
Let us substitute into expression (1.2) the equation for the current strength from the conditions of the problem, taking into account the boundaries of the change in the time segment:
Answer. q=24 Cl
Example
Exercise. A flat capacitor is made up of two square plates with side A, located at a distance d from each other. This capacitor is connected to a source of constant voltage U. The capacitor is immersed in a vessel with kerosene (the capacitor plates are vertical) at a speed v=const. What is the amount of current that will flow through the lead wires in the process described above. Assume that the dielectric constant of kerosene is .
Solution. The basis for solving the problem will be formulas for calculating the current strength of the form.
We remember from physics lessons high school basic postulate. It looks like this:
Current strength is a quantity that quantitatively characterizes the ordered movement of charged particles
To understand this definition, you first need to find out what “ordered movement of charged particles” is. This is exactly what electric current is. Thus, current strength allows a numerical measurement of electric current.
For example, a given amount of electrical charge can flow through a conductor in 1 hour or 1 second. It is clear that in the second case the intensity of the passage of charges will be much greater. Accordingly, the current will be greater. Since in international system The SI unit of time is considered to be 1 second, then we come to the definition of current strength.
Current strength is the amount of electricity passing through the cross-section of a conductor in one second.
Unit of current
The unit of current is Ampere. Ampere is the strength of electric current at which an amount of electricity equal to one coulomb passes through the cross-section of the conductor every second: 1 ampere = 1 coulomb/1 second.
Additional units of measurement most commonly found in the energy sector:
- 1 mA (milliamp) = 0.001 A;
- 1 µA (microamp) = 0.000001 A;
- 1 kA (kiloampere) = 1000 A.
Now we know how current is measured.
Current measurement
A device is used to measure current strength Ammeter. Milliammeters and microammeters are used to measure very low currents.
Symbols of ammeter and milliammeter
In order to measure the current, you need to connect the ammeter to the open circuit, that is, in series. The measured current passes from the source through the ammeter and receiver. The ammeter needle shows the current in the circuit. Where exactly to turn on the ammeter in the circuit does not matter, since the current strength in a simple closed circuit (without branches) will be the same at all points of the circuit.
Ammeter device
In technology there are very high currents (thousands of amperes) and very small ones (millionths of an ampere).
For example, the current strength of an electric stove is approximately 4 - 5 amperes, incandescent lamps - from 0.3 to 4 amperes (and more). The current passing through the photocells is only a few microamps. In the main wires of the substations that supply electricity to the tram network, the current reaches thousands of amperes.
Current is the movement of charged particles in one direction. You can find the current strength in practice using special measuring instruments, or you can calculate it using already derived ready-made formulas if you have the initial data.
The physical quantity that shows the charge that passes through a conductor in a certain unit of time is called current strength. The basic formula according to which this force can be calculated is: I = q/t. That is, the ratio of the charge passing through the cross section to the time interval during which electricity flowed is equal to the desired value I.Explanation of symbols:
- I – designation of the strength of electricity, measured in Amperes (A) or 1 Coulomb/second;
- q – charge moving along the conductor, unit of measurement Coulombs (C);
- t – charge passage interval, measured in seconds (s).
- E – electromotive force, EMF, Volt;
- R – external resistance, Ohm;
- r – internal resistance, Ohm.
Ohm's laws are applicable to calculate direct current, but if you want to know the magnitude of the power of alternating electricity, then the obtained values should be divided by the root of two.
The main ways to determine current strength using instrument systems in practice:- Magnetoelectric measurement method, the advantage of which is sensitivity and accuracy of readings, as well as low energy consumption. This method can only be used to determine the magnitude of direct current.
- Electromagnetic is the determination of the strength of alternating and direct currents by the method of transformation from electromagnetic field into the signal of the magnetic modular sensor.
- Indirect, using a voltmeter, the voltage is found at a certain resistance.
Determining the current consumed is not as often required as measuring resistance or voltage, but without finding physical quantity current, power consumption cannot be calculated.
