A blinding flash of lightning, rolling thunder. For a long time, mankind has observed these terrible natural phenomena and, not understanding them, felt fear in front of them. And just over a hundred years ago, people taught the electric forces of nature to serve themselves.

Express Physics

In nature, there are tiny charged particles. There are particles that are charged and have a charge with a plus sign, and there are particles that have a negative charge with a minus sign. Particles that have a negative charge are called electrons. They can run on metal conductors. And this flow of charged particles scientists called electric current.

What are the characteristics of current? Firstly, this is the strength of the current and its density, and secondly, this is the power of the current. We will consider the density and power of the current in another article, now we will pay attention to the strength of the current. Let's consider what it is, what definition and meaning in physics this quantity performs. What is the symbol for current? How to find current strength? We learn interesting and informative facts about the strength of the current.

Formula language

The current strength is a physical quantity that determines not the variety of particles that have passed through the cross section of the conductor, but the total charge that is transferred through the conductor per unit time. It looks like this:

• I=q/t

Where I is our current strength measured in Amperes (A), q is the charge that passes through the conductor, its units are Coulomb (C), and t is the observation time measured in seconds (s).

And according to Ohm's law, you can determine the current strength as follows, and for this we will need to know the voltage of the circuit section U is measured in volts (V), and its resistance R is measured in Ohms (Ohms):

• I=U/R

And how to determine the strength of the current, if we do not know the charge passing through the conductor? How to find the current strength if this is not a school task? To do this, there is a special device - an ammeter. To determine the current strength, we must connect our device in series with the section of the circuit in which we measure the current strength. To be able to determine the strength of the current is very important and simply necessary in everyday life. The current strength of 0.01 Amperes is not felt or felt, but very weakly. But the current strength of 0.1 Amperes leads to great disturbances in the human body. And a current of more than 0.2 Amperes is fatal, resulting in severe burns and respiratory arrest. Be extremely careful and careful with the current strength!

Physics grade 8. CURRENT

The directed movement of charged particles is called electric current.

Conditions of existence electric current in the conductor:
1. availability free charged particles (in a metal conductor - free electrons),
2. availability electric field in explorer
(The electric field in the conductor is created by current sources.).

Electric current has a direction.
The direction of movement of positively charged particles is taken as the direction of the current.

The current strength (I) is a scalar quantity equal to the ratio of the charge q passed through the cross section of the conductor to the time interval t during which the current flowed.

The current strength shows how much charge passes through the cross section of the conductor per unit of time.

unit of measurement current strength in the SI system:
[I] = 1 A (amps)

In 1948, it was proposed to base the definition of the unit of current strength on the phenomenon interactions of two guides with current:

........................

When current flows through two parallel conductors in the same direction, the conductors attract, and when current passes through the same conductors in opposite directions, they repel.

per unit of current 1 A take the current strength at which two parallel conductors 1 m long, located at a distance of 1 m from each other, interact with a force of 0.0000002 N.

ANDRE-MARI AMPERE
(1775 - 1836)
- French physicist and mathematician

Introduced such terms as electrostatics, electrodynamics, solenoid, EMF, voltage, galvanometer, electric current, etc.;
- suggested that a new science about the general laws of control processes would probably arise and suggested calling it "cybernetics";
- discovered the phenomenon of mechanical interaction of conductors with current and the rule for determining the direction of the current;
- has works in many fields of science: botany, zoology, chemistry, mathematics, cybernetics;

The unit of measurement of current strength - 1 Ampere - is named after him.

ELECTRIC CURRENTS IN NATURE.

We live in an ocean of electric discharges created by machines, machines and people. These discharges - short-term electric currents are not so powerful, and we often do not notice them. But they still exist and can bring a lot of harm!

What is lightning?

As a result of movement and friction against each other, the air layers in the atmosphere are electrified. Large charges build up in clouds over time. They are the cause of the lightning.
At the moment when the charge of the cloud becomes large, between its parts having charges opposite in sign, a powerful electric spark - lightning. Lightning can form between two neighboring clouds and between a cloud and the Earth's surface. In this case, under the action of the electric field of the negative charge of the lower part of the cloud, the surface of the Earth under the cloud is electrified positively. As a result, lightning strikes the ground.
The nature of lightning began to be clarified after studies carried out in the 18th century by Russian scientists M.V. Lomonosov and G. Richman and the American scientist B. Franklin.

Usually lightning is drawn striking from top to bottom. Meanwhile in reality the glow
starts from below and only then spreads along the vertical channel.
Lightning - more precisely, its visible phase, it turns out, strikes from the bottom up!

LOOK AT THE BOOKSHELF!

DO YOU HAVE A LIGHTNING WIRE IN YOUR COTTAGE?

One of the first in the world lightning rods (lightning rods) hoisted over the cross of his temple a village priest from Moravia named Prokop Divish, a peasant son, scientist and inventor.
This was in June 1754.
___

The first lightning rod in Russia appeared in 1756 above the Peter and Paul Cathedral in St. Petersburg.
It was built after lightning struck the cathedral's spire twice and set it on fire.

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 actions, prevent, identify and repair damage in the circuit.

How does

To understand what current strength is, one 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 strength lies in the orderly movement of electrons, which are affected by an electric field from a power source. The more charged particles are transported, and the faster they move in one direction, the more charge will reach their destination.

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

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

Definition

The current strength in a conductor is the amount of electricity moving through the 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 measure for the strength of electric current according to the international SI system (Systeme International) is the ampere (A), named after the outstanding French scientist of the 19th century, Andre-Marie Ampère.

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

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

The current strength is found with a measuring device (ampere, - or galvanometer), sequentially including it in 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 - at a constant current value. This method is distinguished by increased accuracy and low energy consumption;
• Electromagnetic - for stationary and changing quantities. With this method, the current in the circuit is the result of converting the magnetic field into the output signal of the modulation sensor;
• Indirect - based on measuring voltage with a known resistance. Next, calculate the desired value according to Ohm's law, shown below.

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

I = q/t, where:

• q is the charge going across the conductor (C);
• t is the duration of time spent on the movement of particles (s).

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

Note! The current strength 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.

Ohm's law formula will help you find the current strength, which looks like a 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 multi-phase circuits.

Note! If the conductors are connected in series, then the electricity of each of them is equal. Parallel connection provides for the number of amperes, which is the sum of the current values ​​\u200b\u200bof each conductor.

How to find power (rate of transfer or energy 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 variable electricity, its strength is of a different nature. For a chain with particles moving in a constant direction, all parameters remain unchanged. A variable view is able to change its value with the same or changing direction. The amount of electricity in this case is:

• instantaneous, depending on the amplitude value and frequency of oscillations associated with the angular frequency;
• amplitude - the maximum value of the instantaneous current strength 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 strength is a concept closely related to electrical energy, which is of great importance for the sphere of everyday life, the national economy, and strategic facilities. 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

Gentlemen, hello everyone!

Today we will talk about such a fundamental concept of physics in general and electronics in particular as current strength. Each of you, for sure, has heard this term more than once. Today we will try to understand it a little better.

Today we are going to talk about DC. That is, about such a thing, the magnitude of which is always constant in strength and direction. Dear gentlemen, bores can start to dig - what does "all the time" mean? There is no such term. This can be answered that the magnitude of the current should not change throughout the entire time observations.

So current. Current strength. What is it? Everything is quite simple. Current is the directed movement of charged particles. Note, gentlemen, directed. Random - thermal - movement, from which electrons in a metal or ions in a liquid / gas rush back and forth, is of little interest to us. But if this chaotic movement is superimposed with the movement of all particles in one direction, then this is a completely different calico.

What are charged particles? And in general, do not care what, no matter. Positive ions, negative ions, electrons, it doesn't matter. If we have a directed movement of these respected comrades, then there is an electric current.

Obviously, the current has some direction. Per current direction It is customary to take the motion of positive particles. That is, although the electrons run from minus to plus, it is believed that the direction of the current in this case is the opposite - from plus to minus. This is how it's all wrapped up. What can you do - a tribute to tradition.

A schematic representation of a conductor with current is shown in Figure 1.

Figure 1 - Schematic representation of a conductor with current

Imagine a cloud with mosquitoes. Yes, I know, vile creatures, and the cloud is generally some kind of horror. But still, suppressing disgust, let's try to imagine them. So, in this cloud, every vile mosquito flies by itself. This is a random movement. Now imagine a saving breeze. It simultaneously carries away all this mosquito horde in one direction, hopefully away from us. This is a directed movement. Replacing mosquitoes with electrons, and the breeze with some mysterious driving force, we get, in general, some kind of analogy with an electric current.

Most often there is a current caused by the movement of electrons. Yes, friends, throughout our lives we are surrounded by poor electronics, forced to move in a direction, one might say in formation, under the influence of a coercive force. They run along the wires of power lines, in all our sockets, in all our smart devices - computers, laptops, smartphones and work just like Carlo's dad to make our hard life easier and fill it with amenities.

Mosquitoes - mosquitoes, it's all cool, but it's time for formal definitions.

So, gentlemen, the current strength is the ratio of the charge Δq, which is transferred through a certain section of the conductor S during the time ∆t. The current strength is measured, as many already know, in Amperes. So - the current in the conductor is 1 Ampere, if 1 Coulomb passes through this conductor in 1 second.

"Excellent!" - the dear reader will exclaim. And what should I do with this formula?! Well, the time is fine, I have a stopwatch in my iPhone, I will detect it. And what about the charge? Should I count the number of electrons in the wire and then multiply by the charge of one electron, since this is a known value in order to determine the current ?!

Calm, gentlemen! All will be. Do not hurry. For now, just remember that there was some kind of formula. Then it turns out that with its help you can count some cool things like the charge of capacitors and much more.

In the meantime ... For now, you can take an ammeter, measure the current in the circuit with a light bulb and find out how much charge flows every second through the conductor section q = I t = I 1c= I.

Yes, every second a charge flows through the cross section of the conductor, equal to the current strength in it. You can now multiply this value by the electron charge (for those who forgot, I remind you that it is equal) and find out how many electrons run in the circuit. There may be a voros - what for? The answer of the author - just like that, for the sake of interest. You are unlikely to get any practical benefit from this. If you please your teacher. This question is purely academic.

The question may arise - how does an ammeter measure current? Is he counting electrons? Of course not, gentlemen. Here we have indirect measurements. They are based on the magnetic effect of the current in old-fashioned analog pointer ammeters or on Ohm's law - by converting the flowing current through a known resistance into voltage and then processing it - in all modern multimeters. But more on that later.

Now I will give this calculation. It is quite simple and should be digested even by the humanities. If you have an individual intolerance to matan, well, you can just look at the result.

Let's remember our charge ∆q that passes in time ∆t through the cross section of the conductor ∆S which we talked about above. Like true mathematicians, we will complicate it to the point of disgrace, so that only after straining the brain it will be clear that we have written an identity.

Lord, chesslovo, no deception. e is the electron charge, n − concentration of electrons, that is, the number of pieces in one cubic meter, v is the speed of electrons. It's obvious that v∙∆t∙∆S is, in fact, the volume that the electrons will pass through. We multiply the concentration by the volume - we get pieces, how many pieces of electrons have passed. We multiply the pieces by the charge of one electron - we get the total charge that has passed through the section. I told you everything is fair!

Let us introduce the concept of current density. The bores who have already read something about it will now exclaim - aha, this is a vector quantity! I do not argue, gentlemen, vector. But we, to simplify the already difficult life, we will assume that the direction of the current density vector coincides with the axis of the conductor, which happens in most cases. Therefore, vectors immediately become scalars. Roughly speaking, current density is how many amperes per square meter of conductor cross section. Obviously, for this it is necessary to divide the current strength by the area. We have

Now, I hope, it is clear why we have so transformed the formula? To cut a bunch of stuff!

Remember the main thing - we are looking for speed. We express it:

Everything would be fine, but we do not know the concentration yet. We remember chemistry. There was a formula

Where ρ=8900 kg/m 3 is the density of copper, N A \u003d 6 10 23 avogadro number, M=0.0635 kg/mol- molar mass.

Gentlemen, I hope there will be no need to explain where this formula came from. I'm not very good with chemistry, to be honest. Although I spent all 11 years at a school with in-depth study of chemistry, however, in the 8th grade I entered the physics and mathematics class, became interested in physics, especially the part that talks about electricity, but I can say that I forgot about chemistry. Actually, they didn’t ask us deeply about it, we were physimatics. However, if all of a sudden the need arises, I'm still ready to delve into this chemical jungle and tell you what's what here.

Thus, the speed of electrons in a current-carrying conductor is

Substitute specific numbers. For definiteness, let's set the current density to 5 A/mm 2 .

We already have all the other numbers. The question may arise - why exactly 5 A / mm 2.

It's simple, gentlemen. People are not in the first year involved in electronics. Some experience has been accumulated in this area, or, in the language of science, empirical data. So, these empirical data say that the allowable current density in copper wires is usually 5-10 A / mm 2. With a higher current density, unacceptable overheating of the conductor is possible. However, for tracks on a printed circuit board, this value is much larger and amounts to 20 A / mm 2 and even more. However, this is a topic for a completely different discussion. Let's return to our problem, namely, to the calculation of the speed of electrons in a conductor. Substituting the numbers, we get that

Gentlemen, the calculation irrefutably shows that the electrons in a current-carrying conductor move only at a speed of 0.37 millimeters per second! So slow. True, it should be remembered that this is not a thermal movement, but a directed one. Thermal motion is much, much larger, on the order of 100 km/s. A reasonable question - why does the light flash instantly when I turn the switch? And remember, I spoke about some coercive force? It's about her! But more on that in the next article. Good luck to you all, and see you soon!

Join our

Probably, everyone at least once in their life felt the effect of current. An ordinary battery barely perceptibly pinches when applied to the tongue. The current in the apartment socket beats quite strongly if you touch the bare wires. But the electric chair and power lines can take lives.

In all cases, we are talking about the action of electric current. How is one current so different from another that the difference in its effect is so significant? Obviously, there is some quantitative characteristic that can explain such a difference. Current, as you know, is electrons moving along a conductor. It can be assumed that the more electrons run through the cross section of the conductor, the greater the effect of the current.

Current formula

In order to characterize the charge passing through the conductor, a physical quantity called the strength of the electric current was introduced. The current strength in a conductor is the amount of electricity passing through the cross section of the conductor per unit of time. The current strength is equal to the ratio of the electric charge to the time of its passage. To calculate the current strength, the formula is used:

where I is the current strength,
q - electric charge,
t - time.

The unit of current in a circuit is 1 ampere (1 A) in honor of the French scientist André Ampère. In practice, multiple units are often used: milliamps, microamps and kiloamps.

Current measurement with an ammeter

Ammeters are used to measure current. Ammeters are different depending on what measurements they are designed for. Accordingly, the scale of the instrument is calibrated in the required values. The ammeter is connected anywhere in the network in series. Where the ammeter is connected does not matter, since the amount of electricity passing through the circuit will be the same in any place. Electrons cannot accumulate in any places in the circuit, they flow evenly through all the wires and elements. When connecting an ammeter before and after the load, it will show the same values.

The first scientists who studied electricity did not have instruments for measuring current strength and charge magnitude. They checked the presence of current with their own sensations, passing it through their body. Pretty ugly way. At that time, the strength of the currents with which they worked were not very high, so most researchers got off with only unpleasant sensations. However, in our time, even in everyday life, not to mention industry, very large currents are used.

You should know that for the human body, a current value of up to 1 mA is recognized as safe. A current greater than 100 mA can cause serious damage to the body. A current of a few amperes can kill a person. At the same time, it is also necessary to take into account the individual susceptibility of the body, which is different for each person. Therefore, one should remember the main requirement when operating electrical appliances - safety.