Electric current: what is it and how does it arise

It is impossible to imagine the life of a modern person without electricity. Volts, Amperes, Watts - these words sound in a conversation about devices that run on electricity. But what is this electric current and what are the conditions for its existence? We will talk about this further, providing a short explanation for beginner electricians.

Definition

Electric current is the directional movement of charge carriers - this is a standard formulation from a physics textbook. In turn, certain particles of a substance are called charge carriers. They can be:

• Electrons are negative charge carriers.
• Ions are positive charge carriers.

But where do charge carriers come from? To answer this question, you need to remember the basic knowledge about the structure of matter. Everything that surrounds us is matter, it consists of molecules, its smallest particles. Molecules are made up of atoms. An atom consists of a nucleus around which electrons move in given orbits. Molecules also move chaotically. The movement and structure of each of these particles depend on the substance itself and the influence of the environment on it, for example, temperature, stress, and so on.

An ion is called an atom in which the ratio of electrons and protons has changed. If the atom is initially neutral, then the ions, in turn, are divided into:

• Anions are a positive ion of an atom that has lost electrons.
• Cations are an atom with "extra" electrons attached to the atom.

The unit of current measurement is Ampere, according to Ohm's law it is calculated by the formula:

I = U / R,

where U is voltage, [V], and R is resistance, [Ohm].

Or it is directly proportional to the amount of charge transferred per unit of time:

I = Q / t,

where Q - charge, [Cl], t - time, [s].

Conditions for the existence of electric current

We figured out what electric current is, now let's talk about how to ensure its flow. For the flow of electric current, two conditions must be met:

1. The presence of free charge carriers.
2. Electric field.

The first condition for the existence and flow of electricity depends on the substance in which the current flows (or does not flow), as well as its state. The second condition is also feasible: for the existence of an electric field, the presence of different potentials is necessary, between which there is a medium in which charge carriers will flow.

Let's remind: Voltage, EMF is the potential difference. It follows from this that to fulfill the conditions for the existence of a current - the presence of an electric field and an electric current, voltage is needed. These can be plates of a charged capacitor, a galvanic cell, an EMF generated by a magnetic field (generator).

How it arises, we figured it out, let's talk about where it is directed. The current, mainly in our usual use, moves in the conductors (electrical wiring in the apartment, light bulbs incandescent) or in semiconductors (LEDs, your smartphone's processor and other electronics), less often in gases (fluorescent lamps).

So the main charge carriers in most cases are electrons, they move from the minus (points with a negative potential) to plus (a point with a positive potential, you will learn more about this below).

But an interesting fact is that the direction of current movement was taken as the movement of positive charges - from plus to minus. Although, in fact, everything happens the other way around. The fact is that the decision on the direction of the current was made before studying its nature, and also before it was determined due to what the current flows and exists.

Electric current in different environments

We have already mentioned that in different environments the electric current can differ in the type of charge carriers. The media can be divided according to the nature of conductivity (in decreasing conductivity):

1. Conductor (metals).
2. Semiconductor (silicon, germanium, gallium arsenide, etc.).
3. Dielectric (vacuum, air, distilled water).

In metals

There are free charge carriers in metals, they are sometimes called "electric gas". Where do free charge carriers come from? The fact is that a metal, like any substance, consists of atoms. Atoms move or oscillate in one way or another. The higher the temperature of the metal, the stronger this movement. At the same time, the atoms themselves generally remain in their places, in fact, forming the structure of the metal.

In the electron shells of an atom, there are usually several electrons, which have a rather weak bond with the nucleus. Under the influence of temperatures, chemical reactions and the interaction of impurities, which are in any case in the metal, electrons are detached from their atoms, positively charged ions are formed. Detached electrons are called free and move chaotically.

If they are affected by an electric field, for example, if you connect a battery to a piece of metal, the chaotic movement of electrons will become orderly. Electrons from the point to which the negative potential is connected (the cathode of a galvanic cell, for example) will begin to move to the point with a positive potential.

In semiconductors

Semiconductors are materials in which there are no free charge carriers in the normal state. They are in the so-called forbidden zone. But if you apply external forces, such as an electric field, heat, various radiation (light, radiation, etc.), they overcome the forbidden zone and go into a free zone or a zone conductivity. Electrons are detached from their atoms and become free, forming ions - positive charge carriers.

Positive carriers in semiconductors are called holes.

If you simply transfer energy to a semiconductor, for example, heat it, the chaotic movement of charge carriers will begin. But if we are talking about semiconductor elements, such as a diode or transistor, then at opposite ends of the crystal (a metallized layer is applied to them and the leads are soldered) EMF will occur, but this does not apply to the topic of today's articles.

If you apply an EMF source to a semiconductor, then the charge carriers will also go into the conduction band, and will also begin their directional movement - the holes will go to the side with a lower electric potential, and the electrons - to the side with great.

In vacuum and gas

A vacuum is called a medium with a complete (ideal case) absence of gases or its minimized (in reality) amount. Since there is no substance in a vacuum, there is nowhere for charge carriers to come from. However, the flow of current in a vacuum marked the beginning of electronics and a whole era of electronic elements - vacuum tubes. They were used in the first half of the last century, and in the 50s they began to gradually give way to transistors (depending on the specific field of electronics).

Let us assume that we have a vessel from which all the gas has been pumped out, i.e. there is a complete vacuum in it. Two electrodes are placed in the vessel, let's call them anode and cathode. If we connect the negative potential of the EMF source to the cathode, and positive to the anode, nothing will happen and the current will not flow. But if we start heating the cathode, the current will begin to flow. This process is called thermionic emission - the emission of electrons from the heated surface of an electron.

The figure shows the process of current flow in a vacuum lamp. In vacuum tubes, the cathode is heated by a nearby filament in Fig (H), such as in a lighting lamp.

In this case, if you change the polarity of the power supply - apply a minus to the anode, and apply a plus to the cathode - the current will not flow. This will prove that the current in vacuum flows due to the movement of electrons from the CATHODE to the ANODE.

Gas, like any substance, consists of molecules and atoms, which means that if the gas is under the influence of an electric field, then at With a certain strength (ionization voltage), the electrons will detach from the atom, then both conditions for the flow of electric current - the field and free media.

As already mentioned, this process is called ionization. It can occur not only from the applied voltage, but also when the gas is heated, X-rays, under the influence of ultraviolet radiation and other things.

The current will flow through the air even if a burner is installed between the electrodes.

The flow of current in inert gases is accompanied by gas luminescence; this phenomenon is actively used in fluorescent lamps. The flow of an electric current in a gas medium is called a gas discharge.

In liquid

Let's say that we have a vessel with water in which two electrodes are placed, to which a power source is connected. If the water is distilled, that is, pure and does not contain impurities, then it is a dielectric. But if we add a little salt, sulfuric acid or any other substance to the water, an electrolyte is formed and a current begins to flow through it.

An electrolyte is a substance that conducts an electric current due to dissociation into ions.

If copper sulfate is added to the water, then a layer of copper will settle on one of the electrodes (cathode) - this is called electrolysis, which proves that the electric current in a liquid is carried out due to the movement of ions - positive and negative carriers charge.

Electrolysis is a physicochemical process, which consists in the release of the components that make up the electrolyte on the electrodes.

Thus, copper plating, gilding and coating with other metals occurs.

Conclusion

To summarize, free charge carriers are needed for the flow of electric current:

• electrons in conductors (metals) and vacuum;
• electrons and holes in semiconductors;
• ions (anions and cations) in liquids and gases.

In order for the movement of these carriers to become ordered, an electric field is needed. In simple words, apply a voltage at the ends of the body or install two electrodes in an environment where an electric current is supposed to flow.

It is also worth noting that the current affects the substance in a certain way, there are three types of exposure:

• thermal;
• chemical;
• physical.

Finally, we recommend watching a useful video, which discusses in more detail the conditions for the existence and flow of electric current:

Useful on the topic:

• The dependence of the resistance of the conductor on temperature
• Joule-Lenz law in simple words
• Which electric current is more dangerous for a person: direct or alternating