Electric current is the ordered movement of charged particles.
The words “electric current” and “electricity” are familiar to everyone and are used in homes for lighting (light bulbs, spotlights), transportation, factories, etc. To understand what electric current is, it is necessary to familiarize oneself with a significant range of phenomena.
- Electrification of bodies
- Conductors, semiconductors, insulators
- The action of current
- Current strength
- Ohm’s law for a section of a circuit
- Work of electric current
- Power of current
- Expression of work of current through power
- Heating of conductors by electric current (Joule-Lenz law)
Electrification of a body
The ancient Greeks noticed that when amber was rubbed with wool, it attracted other objects to itself. The ancient Greeks called it “electron” because of its property of attracting objects. Hence the name “electricity” came about.
A body that attracts objects to itself after rubbing is said to be electrified. Knowledge about the structure of atoms helps to understand the electrification of bodies.
The nucleus, which consists of protons and neutrons, is located at the center of an atom, and electrons move around it. The atom as a whole has no charge, it is neutral because the positive charge of its nucleus is equal to the negative charge of all its electrons.
An atom that has lost one or several electrons is no longer neutral, but has a positive charge. Then it is called a positive ion.
An extra electron that joins a neutral atom makes the atom negative ion.
Under normal conditions, the number of electrons in a body equals the number of protons. All electrons are the same, and each has the smallest negative charge. All protons are also the same, and each has a positive charge that is equal to the electron charge. Therefore, the sum of all negative charges in the body equals the sum of all positive charges in it, and the body as a whole has no charge: it is electrically neutral.
When a neutral body receives electrons from another body, it acquires a negative charge. A negatively charged body has an excess of electrons.
When a neutral body loses electrons, it acquires a positive charge. A body is positively charged if it has a shortage of electrons.
A body acquires an electric charge or is electrified when it receives or loses electrons.
If an ebonite rod is rubbed with wool, the ebonite becomes negatively charged, while the wool becomes positively charged. During rubbing, electrons from the wool transfer to the ebonite, from the material where the gravitational force towards the atom’s nucleus is weaker to the material where this force is stronger. The ebonite rod will have an excess of electrons, while the wool will have a deficiency. The charges of the wool and ebonite rod are equal in absolute value, as many electrons as the wool lost, the ebonite rod gained. Therefore, during electrification, charges are not created, but only separated: some negative charges are transferred from one object to another.
Electrons that move within an atom and are located farther from the nucleus are less attracted to it than electrons closer to it. These distant electrons are especially weakly held by atoms of metals. The electrons that are farthest from the nucleus leave their place and move freely between atoms; these electrons are called free electrons. It is these free electrons that create an electric current.
Conductors, semiconductors, dielectrics
Materials that have free electrons are called conductors, while materials whose electrons are strongly bound to their atoms and cannot move freely are called non-conductors or dielectrics. In addition to conductors and dielectrics, there is a group of materials whose conductivity occupies an intermediate position between conductors and dielectrics. These materials are called semiconductors. They either conduct or do not conduct electricity depending on their environment. For example, silicon has poor electrical conductivity at low temperatures, but its conductivity increases under the influence of light or heat.
Electric current is the orderly movement of charged particles.
To generate a current in a conductor, an electric field must be created within it. Under the influence of this field, free electrons will move in the direction of the electric force acting upon them. When the body loses its charge, the electric field in the conductor disappears, along with the current.
An electric field is a space that surrounds an electric charge. The force with which the electric field acts on a charged object placed within it is called the electric force.
To maintain an electric current in a conductor for a long time, an electric field must be maintained within it. This role is performed by current sources, which come in various forms, each distributing negative and positive charged particles. The separated particles accumulate at the poles of the power source. In the process of working on particle distribution, mechanical, chemical, or other types of energy are transformed into electrical energy.
Action of Electric Current
It is impossible to see electrons moving in a conductor or ions in an electrolyte. The presence of current can be judged by the effects that are usually caused by electric current, which are called actions of current.
Thermal Action of Current.
It is known that the temperature of a conductor increases when current flows through it. Various metals or their alloys, semimetals or semiconductors, as well as electrolytes and plasmas act as such conductors. For example, when an electric current is passed through a wire made of nichrome, strong heating occurs.
A piece of wire heats up when an electric current flows through it. The more current in the conductor, the more it heats up. The length of the heated conductor increases.
Chemical Action of Current.
Chemical action manifests itself in the fact that various chemical reactions can be initiated when an electric current flows. Substances settle on electrodes – plates immersed in a solution and connected to a power source. During electrolysis, anions are connected to the anode and cations to the cathode. Such an action of current is used in electroplating to coat metal surfaces. Nickel plating, brass plating, chrome plating, as well as silver plating and gold plating of surfaces are applied.
Magnetic effect of current.
When electric current flows through any conductor (solid, liquid, or gas), a magnetic field is observed around the conductor, meaning that the conductor with current acquires magnetic properties. The nature of the magnetic field always exists in the presence of electric current.
Mechanical effect of current.
A magnetic field arises around a conductor with electric current. All magnetic effects are transformed into motion. Examples include electric motors, installations, relays, and others.
The effect of current can manifest in different intensities – stronger or weaker. The intensity depends on the charge that comes through the circuit per unit of time – a second. When a free charged particle moves through an electric circuit, there is a movement of charge. The more particles moved from one pole to the other, the greater the total charge transferred by the particles. This total charge is called the quantity of electricity that passes through the conductor. The stronger the current passing through the cross-sectional area of the conductor in 1 second, the stronger the current in the conductor.
The quantity of electricity (charged particles) passing through a conductor’s cross-sectional area in one second is called electric current. To determine the electric current in a circuit, the amount of charge that passes through it must be divided by the time it takes to pass.
I – electric current;
q – quantity of electricity or electric charge (C);
t – time.
Electric current is a physical quantity, and to measure it, a unit of measurement must be established. This unit is called the ampere (A), named after the French scientist Andre Marie Ampere. At the International Conference on Weights and Measures in 1948, it was decided to base the definition of the ampere on the phenomenon of the interaction of two conductors with a current. However, a new definition of the ampere was adopted on November 16, 2018, at the XXVI General Conference on Weights and Measures.
Two flexible straight conductors are placed parallel to each other. Both conductors are connected to a current source. When the circuit is closed, a current flows through the conductors, causing them to interact with each other – attracting or repelling each other – depending on the direction of the currents in them. The force of interaction can be measured, and this force depends on the length of the conductors, the distance between them, the external environment, and most importantly, the strength of the current in the conductors. If all conditions are the same except for the strength of the currents, the greater the strength of the current in each conductor, the greater the force with which they interact with each other.
Let’s imagine that the parallel conductors are very thin and infinitely long. The distance between them is 1 meter, and they are in a vacuum. The electric current is the same.
The unit of electric current was defined as the current that causes parallel conductors with a length of 1 meter to interact with a force of 2 × 10−7 N (0.0000002 N).
The idea behind the new definition was that it should be based not on human-made artifacts through a thought experiment, but on fundamental physical constants or properties of atoms.
Therefore, the new definition is expressed only through one constant – the charge of an electron.
The new definition: based on the numerical value of the elementary electric charge. The formulation, which came into effect on May 20, 2019, states: The ampere, symbol A, is the unit of electric current in SI. It is defined by fixing the numerical value of the elementary charge equal to 1.602 176 634 × 10−19 C, when it is expressed in coulombs, which is equal to:
1 coulomb = 1 ampere × 1 second
1 ampere = 1 coulomb / 1 second
The current is measured using ammeters or combined instruments called multimeters. It is represented on the scale by the letter A and in circuit diagrams as a circle with the letter A inside.
When measuring current, the ammeter is connected in series with the instrument whose current needs to be measured. All current flows through the ammeter to avoid additional resistance in the circuit. The resistance of the ammeter should be low.
Electric potential difference
In every closed circuit, an electric current performs work. It can be said that the work depends on the strength of the current, but it also depends on another quantity called electric potential difference.
The electric potential difference between the ends of a segment of a circuit is numerically equal to the work done by one positive charge passing through that segment. The greater the work, the greater the potential difference at the ends of the circuit segment. The potential difference is denoted by the letter U.
The unit of electric potential is called the volt, symbolized as (V) in honor of the Italian scientist Alessandro Volta.
One volt is the potential difference across a conductor when a current of one ampere dissipates one watt of power. It is also equal to one joule per coulomb of electric charge.
To measure electric potential, voltmeters are used. Voltmeters are always connected in parallel with the circuit. The voltmeter terminals are connected to the two points between which the potential difference is to be measured.
Resistance (electric resistance) is the property of any conductor to resist the flow of electric current through it. The thinner and longer the conductor, the greater its resistance to electric current. The material it is made of also plays an important role.
If electrons in the conductor had no obstacles in their movement, they would move indefinitely without the influence of an electric field. In reality, electrons interact with the ions of the crystal lattice, slowing down the orderly movement, reducing the current strength, and increasing the temperature.
To find the resistance, it is necessary to divide the voltage across the conductor by the current.
The unit of resistance is defined as the resistance of such a conductor in which, at a voltage of 1 volt across its ends, the current strength equals 1 ampere.
Ohm’s Law for a Circuit Element
Three quantities were considered: electric current, voltage, and resistance. These quantities are related to each other.
The dependence of electric current on the voltage across a circuit element and the resistance of this element is called Ohm’s Law, named after the German scientist Georg Ohm who discovered this law in 1827.
Ohm’s Law reads as follows: the electric current in a conductor is directly proportional to the applied voltage and inversely proportional to the conductor’s resistance.
I – electric current;
U – voltage;
R – resistance.
The voltage across a circuit element is numerically equal to the work done when one coulomb of charge passes through it. When several coulombs pass through it, the work done is several times greater.
Electrical work is a physical quantity that characterizes the change in electrical energy of a current as it is transformed into other forms of energy.
When charges move in an electric circuit, work is done. The numerical value of the work done when transferring an electric charge q between two points with a potential difference of U can be determined by the formula:
A = Uq
A – work;
U – potential difference;
q – amount of electricity (amount of electric charge)
The amount of electricity that has passed through a section of the circuit is calculated by:
q = It
Using this relationship, we obtain the formula for the work of the current, which is convenient to use in calculations:
A = UIt
The work done by an electric current in a circuit is directly proportional to the current strength in the circuit, the voltage across the circuit, and the time the current is active.
Work is measured in joules (1 J = 1 V · A · s). To calculate the work, an ammeter, a voltmeter, and a timer are required.
Special devices called meters are used to measure and calculate the work of a current. These meters include the three instruments mentioned above.
Electric current power
Power is numerically equal to the work done in 1 second and is denoted by the letter P.
To find the power, divide the work by time:
Knowing that A = UIt, we can obtain the formula:
P = UI
The unit of power is 1 watt, which is equal to 1 joule per second.
1 watt = 1 J/s.
From the formula for current power, it follows that:
1 watt = 1 volt · 1 ampere, or 1 W = 1 V · A
In practice, power units are used:
1 hectowatt (hW) = 100 watts
1 kilowatt (kW) = 1000 watts
1 megawatt (MW) = 1000000 watts.
The power of the current is measured by a wattmeter.
Expression of electrical work through power
Using the formula
Expressing power in watts and time in seconds, we obtain the work in joules:
1 W = 1 J/s, where 1 J = 1 W · s
In practice, for convenience of calculations, the work of electrical current is often expressed not in joules, but in other units:
1 watt-hour (Wh) = 3600 J
1 gigawatt-hour (GWh) = 1000 MW · h = 3.6 × 109 J
1 kilowatt-hour (kWh) = 1000 W · 1 h = 3.6 × 106 J
Heating of conductors by electric current (Joule-Lenz law)
Electric current heats the conductor. This is due to the fact that free electrons in metals or ions in electrolytes, moving under the influence of an electric field, interact with molecules or atoms of the conductor substance and transfer their energy to them.
In stationary metal conductors, all the work of the current goes to heating the conductor, that is, to increasing its internal energy. The measure of the internal energy of a body is the amount of heat released.
So, the amount of heat released in the conductor is equal to the work of the current.
The work of the current is calculated by the formula A=UIt. Let’s denote the amount of heat by the letter Q. So, Q=A or Q=UIt. The amount of heat is expressed in joules.
Using Ohm’s law, we can express the amount of heat released in a section of a circuit when current is flowing through it, in terms of the current strength, the resistance of the section, and the time. To do this, we replace the voltage in the formula Q=UIt with the product of current and resistance: U=IR.
Thus, we obtain Q=IRIt
which is equivalent to
The amount of heat released when a conductor is heated by electric current is proportional to the square of the current strength, the resistance of the conductor, and the time.