In the material, Motion of a charged particle in an electric field, electrically charged particles whose source was in an electric field moved due to the electric force acting on them. We assumed in our minds that the space in which these particles move is empty space - a vacuum. The electric force affected the particles by changing their speed or direction of motion. The distance between the particles that accelerated in the electric field was large enough so that they did not interfere with each other's motion. The particles could therefore reach very high speeds, up to slightly less than the speed of light.
Moving electrically charged particles can also be present in a substance itself. Such a substance is called an electrical conductor. It can be a gas, liquid, or solid. Under the influence of the electric field in the substance, these particles move.
Any moving electrically charged particles, be it in vacuum or in matter, is called electric current.
What is the difference between the movement of electrically charged particles in vacuum and in matter?
Let's take a copper wire through which an electric current flows. The current in the wire is the movement of free electrons present in the copper under the influence of the force caused by the electric field. The number of free electrons in copper is large. However, due to the large number of electrons, their speed is very small compared to comparably large currents in vacuum. Despite the fact that the electrons in the substance are slower, the electric field inside the conductor is established in an instant. Therefore, all the electrons in the conductor start to move at the same time.
In this material we will learn:
what electric current is and its unit;
we will learn the elements and properties of a simple electrical circuit;
how the electric current flows through the conductors.
Electric current is the movement of electrically charged particles in solids (metals, semiconductors), liquids, or gases.
An electric current flows in a substance if the following conditions are met:
there must be moving electrically charged particles: electrons or ions.
There must be an electric field caused by electric voltage. This acts with force on the electrically charged particles and causes them to move.
In the material, Electric charge we saw that the quantity 
of every electric charge is 
-times the base charge 
:
where the base charge is given as:
The base charge can be negative (electron) or positive (proton).
The unit of the electric charge quantity is 
(Coulomb) and is also the same as 
(ampere second).
Electric current 
is determined by the quantity 
of charge that flows through a conductive substance in a certain time 
:
When we insert the units of quantities in the formula above, we get the unit of electric current as:
The unit for current is therefore 
(ampere). A current of 
flows in a wire if a charge of 
flows through it every second.
If the electric current does not change with time, the equation can be simply expressed as:
Such a current is called direct current.
By convention, the direction of the current is the same as the direction of motion of a positively charged particle in an electric field or opposite to the direction of motion of a negatively charged particle.
Electric current is equal to the charge that flows through a conductor in 
.
The unit of current is (ampere).
An ampere is a base unit similar to a metre, kilogram, or second.
A simple electrical circuit is a closed circuit through which an electric current flows. It consists of:
a voltage source (e.g. battery),
an electric consumer or load (e.g. light bulb),
electric conductors through which current flows from the voltage source to the load and back to the source (e.g. copper wire):
Current in an electric circuit is the circular movement of electrically charged particles from the voltage source towards the consumer and back to the source. Let's think of a battery as a source of voltage. There is an excess of electrons on the negative pole of the battery, but there is a lack of them on the positive pole. When we make an electrical circuit, a lot of electrons flow along the conductor and through the consumer from the negative pole of the battery towards the positive pole. The direction of electron movement is therefore such that it tends to balance the charge on both poles of the source. When the charges balance out, the battery is said to be dead.
The same current flows in the same direction through all the elements of a simple electric circuit. By convention, current flows through the circuit from the positive terminal of the voltage source to the negative terminal. Inside, it originates in the reverse direction, i.e. from the negative terminal to the positive. The direction of movement of the electrons in the circuit is the reverse of the direction of the current since the electrons move from minus to plus.
The source voltage is the cause of the electric current, which is why it is also called the driving voltage. The driving voltage creates an electric field inside the circuit and thus forces on the electrically moving particles. This causes them to move and thus an electric current is produced.
The driving voltage is distributed within the circuit between the circuit elements. We are talking about voltage drops on circuit elements.
A simple electrical circuit consists of a voltage source, an electrical conductor, and an electric consumer or load. The voltage source provides the driving voltage for the electric current, there are voltage drops on the circuit elements. The current through all elements is the same.
By definition, the direction of current is the same as the direction of movement of positively charged particles. Inside the circuit, the current flows from the positive pole of the voltage source towards the negative pole, and through the voltage source the direction is reversed.
A voltage source creates an electric voltage. This drives the electric current, which is why it is also called the driving voltage.
How do we get electric voltage?
A substance that is externally neutral contains an equal number of positive and negative base charges. The total charge within a molecule or atom is zero. Since the charge is zero, there is also no external electric field.
Positive and negative charges within matter attract each other. If we want to get a voltage source, we have to space them, e.g. by distance 
. We must act on them with a force 
along the distance and therefore perform work 
:
By separating the positive and negative charges with the help of work, we got a source of voltage. The work we do can be:
electric work (e.g. charging electrical batteries),
mechanical work (e.g. water turbines of electric generators or windmills that use the kinetic energy of the wind),
solar energy (e.g. there are solar power plants that convert the energy of solar radiation directly into electrical work).
An electrical consumer or load is a device that consumes electrical work. These can be electrical household appliances, electrical mechanical machines, or electrical elements, e.g. electrical resistors.
In the case of a simple electrical circuit, the driving voltage 
of the source is approximately equal to the voltage drop across the load. The voltage across the consumer is equal to the electrical work per unit of charge transferred:
An electric current flows through the consumer. Electrons in the consumer flow from a higher energy level to a lower one and do work in the process.
At the consumer, the electrical work is converted into:
internal energy - e.g. heating of the electrical resistors,
kinetic energy - e.g. rotation of electric motors,
potential energy - e.g. lifting of the elevator.
There are larger or smaller distances between the voltage source and the consumer, which are bridged by electrical conductors. These can be high-voltage transmission lines, underground or overhead cables, house wiring, or just cables that connect the outlet in the apartment to the electrical consumer. We want the voltage drop on electrical conductors to be as small as possible. The voltage drop on the electrical conductors is equal to the electrical work per unit of charge flowing through the conductor. This work is always transformed into internal energy. The conductor heats up, which represents energy loss.
What is the driving voltage of the source and what are the voltage drops on the circuit elements?
Let's illustrate this using the following example.
This chapter goes beyond the high school level. It is intended for those who would like to know a little more about electric current in metals.
The carriers of electric current in metals are electrons on the outer, valence shell of metal atoms. These electrons are not bound to the atomic nucleus so can move freely between the atoms in the structure of the metal. They move within the same energy level of the electron shells - the valence or conduction band, i.e. away from the atomic nuclei. We say that the valence and conduction bands of most metals overlap. All metals have the common property of conducting electricity.
Under the influence of the electric field, the electrons move towards the direction of the electric field. This is the opposite direction from the agreed direction of the electric current.
base charges pass through a certain cross-sectional area 
of a wire in time . The electric current is therefore:
Let's define the current density 
. This tells us how much current flows through a cross-sectional area of a wire:
The unit is 
The smaller the cross-sectional area of the wire, the higher the electric current density . Due to the smaller distance between the electrons, there will be greater repulsive forces between them. The movement of electrons through a conductor with a small cross-sectional area is difficult. We say that its resistance increases.
But, how does the electric current density depend on the electric field strength 
?
They are related by the formula:
where 
is the specific conductivity. It tells us how well a conductor conducts electric current. It depends on the metal from which the conductor is made. We will learn more about this in the material, Electrical resistance.
Let's take an electrical conductor, e.g. copper. Copper has 29 protons in its nucleus and 29 electrons around its nucleus. The last electron is not bound to the atomic nucleus and can move freely between the copper atoms. An atom that loses an electron becomes a positive ion.
In the absence of an electric field, free electrons move in disordered zigzag tracks, similar to the thermal motion of gas or liquid molecules. The movement of electrons is stopped by collisions with positive ions in the crystal lattice of the metal - we call them gaps. The electrons themselves also partially collide with each other, but the effect of these collisions can be neglected. After each collision with a metal ion, the electron stops and then moves again in the other direction. Despite the thermal motion, the mean position of the electrons in the conductor does not change, so the current is equal to zero.
The solid line in the figure above shows the possible path of the thermal motion of the electron without the presence of an electric field. During movement, it reaches a high speed - around 
. When an electron collides with a positive ion, it changes direction. Between two collisions, it covers an average distance of 
(it travels a distance of about 200 atoms). The average time between two collisions is 
.
At the moment when an electric field is established, the electrons begin to move additionally in the direction of the force acting on them, i.e. against the direction of the force of the electric field. A possible path of movement is the dashed red line in the image above. The component of the speed of the movement of electrons along the conductor, which represents the electric current, is very small. We will calculate it in the next subsection.
We will look at the process of calculating the speed of electrons along a conductor using an example.
We can see that the movements of current carriers (electrons) are uniform and the speed is low. Given that there is a constant force 
acting on them, we would expect them to move with uniform acceleration according to Newton's second law, similar to what is described in the material, Motion of a charged particle in an electric field. Obviously, there is another opposite, retarding force, which is the result of stopping electrons upon collisions with positive ions.
