Electric current
Legend:
1. Electron
2. Sensor or probe Hall
3. Imanes
4. Magnetic field
5. Source of energy
Description
In image A, a negative load appears on the upper edge of the Hall sensor (symbolized with the blue color), and a positive on the lower edge (red color). In B and C, the electric or magnetic field is invested, causing the polarity to be reversed. Investing both the current and the magnetic field (image D) causes the probe to again assume a negative load in the upper corner.
An electrical current is the flow of electrical charge through a material. It can also be defined as a flow of charged particles, such as electrons or ions, moving through an electrical conductor or a space. It is measured as the net rate of electric charge flow through a surface or in a control volume. It is due to the movement of charges (usually electrons) within it. The current flow (amount of charge per unit of time) is called electrical current intensity (commonly represented by the letter I). In the International System of Units it is expressed in coulombs per second (C/s), a unit called ampere (A). An electric current, since it is a movement of charges, produces a magnetic field, a phenomenon that can be used in the electromagnet.
The instrument used to measure the intensity of the electrical current is the galvanometer which, calibrated in amperes, is called an ammeter, placed in series with the conductor through which the current to be measured circulates.
History
Historically, electric current was defined as a flow of positive charges (+) and the conventional direction of flow of current was established as a flow of charges from the positive to the negative pole. However, later it was observed thanks to the Hall effect, that in metals the charge carriers are negative, electrons, which flow in the opposite direction to the conventional one. In conclusion, the conventional sense and the real one are true as long as the electrons like protons flow from the negative pole until they reach the positive one (real sense), which does not contradict that said movement begins next to the positive pole where the first electron it is attracted to said pole, creating a hole to be covered by another electron from the next atom and so on until reaching the negative pole (conventional sense). That is to say, the electric current is the passage of electrons from the negative to the positive pole, beginning said progression at the positive pole.
In the 18th century when the first experiments with electricity were made, only electric charge generated by friction was available (static electricity) or by induction. It was achieved (for the first time, in 1800) to have a constant movement of charge when the Italian physicist Alessandro Volta invented the first electric battery.
Conventions
In a conductive material, the moving charged particles that make up the electric current are called charge carriers. In metals, which make up the wires and other conductors of most electrical circuits, the atomic nuclei of the positively charged atoms are held in a fixed position, and the negatively charged electrons are the charge carriers, free to move in the metal. In other materials, especially semiconductors, charge carriers can be positive or negative, depending on the dopant used. Positive and negative charge carriers may even be present at the same time, as in an electrolyte in an electrochemical cell.
A flow of positive charges gives the same electrical current, and has the same effect in a circuit, as an equal flow of negative charges in the opposite direction. Since current can be the flow of positive or negative charges, or both, a convention for the direction of current is needed that is independent of the type of charge carrier. The direction of the "conventional current" is arbitrarily defined as the direction in which positive charges flow. Negatively charged carriers, such as electrons (the charge carriers in metal wires and many other components of electronic circuits), therefore flow in the opposite direction to conventional current flow in an electrical circuit.
Current direction
A current in a cable or circuit element can flow in either of the two directions. By defining a variable I{displaystyle I} To represent the current, you must specify the direction that the positive current represents, usually by an arrow in the schematic diagram of the circuit. This is called the reference address of the current I{displaystyle I}. When electrical circuits are analyzed, the actual direction of the current through a specific element of the circuit is normally unknown until the analysis is completed. Consequently, the reference directions for currents are often arbitrarily assigned. When the circuit is resolved, a negative value for the current implies that the actual direction of the current through that element of the circuit is opposed to that of the chosen reference address.
Situations
Observable natural examples of electric current include lightning, static electric discharge, and the solar wind, the source of polar auroras.
Man-made electrical current includes the flow of conduction electrons in metallic wires, such as overhead power lines that deliver electrical power over long distances, and the smaller wires inside electrical and electronic equipment. Eddy currents are electrical currents that occur in conductors exposed to changing magnetic fields. In the same way, electric currents are produced, especially on the surface, of conductors exposed to electromagnetic waves. When oscillating electrical currents flow at the proper voltages inside radio antennas, radio waves are generated.
In electronics, other forms of electrical current include the flow of electrons through resistors or through the vacuum in a vacuum tube, the flow of ions within a battery or a neuron, and the flow of holes within metals and semiconductors.
Electric driving
A conductive material has a large number of free electrons, so it is possible for electricity to pass through it. The free electrons, although they exist in the material, cannot be said to belong to any particular atom.
A current of electricity exists at a location when a net charge is transported from that location to another location in that region. Suppose charge moves through a wire. If the charge q is transported through a given cross section of the wire, in a time t, then the current intensity I, a through the wire is:
- I=qt{displaystyle I={frac {q}{t}},!}
Here q is given in coulombs, t in seconds, and I in amperes. Therefore, the equivalence is:
- 1A=1Cs{displaystyle 1 mathrm {A} =1 {frac {mathrm {C}{mathrm {s}}}{,!}
A characteristic of free electrons is that, even without applying an electric field to them from outside, they move through the object randomly due to heat energy. In the event that they have not applied any electric field, they comply with the rule that the average of these random movements within the object is equal to zero. That is: given an unreal plane drawn through the object, if we add the charges (electrons) that cross said plane in one direction, and subtract the charges that cross it in the opposite direction, these quantities cancel out.
When an external voltage source (such as a battery) is applied to the ends of a conducting material, an electric field is applied to the free electrons. This field causes them to move in the direction of the positive terminal of the material (the electrons are attracted [taken] by the positive terminal and rejected [injected] by the negative). That is, free electrons are the carriers of electric current in conductive materials.
If the intensity is constant over time, the current is said to be continuous; otherwise, it is called a variable. If there is no storage or decrease of charge at any point on the conductor, the current is steady.
To obtain a current of 1 ampere, it is necessary that 1 coulomb of electric charge per second is passing through an imaginary plane traced in the conductive material.
The value I of the instantaneous intensity will be:
I=dqdt{displaystyle I={frac {dq}{dt}}}}
If the intensity remains constant, in which case it is denoted Im, using finite time increments it can be defined as:
Im=Δ Δ qΔ Δ t{displaystyle I_{m}={frac {Delta q}{Delta t}}}}
If the intensity is variable, the previous formula gives the average value of the intensity in the considered time interval.
According to Ohm's law, the intensity of the current is equal to the voltage of the source divided by the resistance that the bodies oppose:
I=VR{displaystyle I={frac {V}{R}}}}
Referring to power, current is equal to the square root of power divided by resistance. In a circuit containing several generators and receivers, the current is equal to:
I=・ ・ E− − ・ ・ E♫・ ・ R+・ ・ r+・ ・ r♫{displaystyle I={frac {Sigma {mathcal {E}}}-Sigma {mathcal {E}}{Sigma R+Sigma r+Sigma r'}}}}}
where ・ ・ ε ε {displaystyle sigma epsilon } is the sum of the electromotric forces of the circuit, ・ ・ ε ε ♫{displaystyle sigma epsilon} is the sum of all counter-electromotric forces, ・ ・ R{displaystyle Sigma R} is the equivalent resistance of the circuit, ・ ・ r{displaystyle sigma r} is the sum of the internal resistance of the generators and ・ ・ r♫{displaystyle Sigma r'} is the sum of the internal resistance of the receptors.
Current intensity in a volume element: dI=n⋅ ⋅ q⋅ ⋅ dS⋅ ⋅ v,{displaystyle dI=ncdot qcdot dScdot v,} where we find n as the number of loads carriers per unit of volume dV; q referring to the load of the carrier; v the speed of the carrier and finally dS as the area of the section of the driver volume element.[chuckles]required]
Definition by means of magnetism
Electric current is the flow of electric charge carriers, usually through a metallic cable or any other electrical conductor, due to the potential difference created by a current generator. The equation that describes it in electromagnetism is:
- I=∫ ∫ SJ→ → ⋅ ⋅ dS→ → =∫ ∫ SJ→ → ⋅ ⋅ n→ → dS{displaystyle I=int _{S}{vec {J}}}{cdot d{vec {S}}}=int _{S}{vec {J}{cdot {vec {n}}}dS}
Where J→ → {displaystyle {vec {J}}} is driving current density, dS→ → {displaystyle d{vec {s}} is the vector perpendicular to the surface differential, n→ → {displaystyle {vec {n}}} is the normal unit vector to the surface, and dS{displaystyle dS} It's the surface differential.
Electric charge can move when it is in an object and the object is moved, such as an electrophore. An object is electrically charged or discharged when there is movement of charge inside it.
Direct current
The flow of electrical charges that does not change direction over time is called direct current or direct current (CC in Spanish, in English DC, from direct current). The electric current through a material is established between two points of different potential. When there is direct current, the higher and lower potential terminals are not exchanged with each other. The identification of direct current with constant current is wrong (none are, not even that supplied by a battery). Any current whose direction of circulation is always the same is continuous, regardless of its absolute value.
Its discovery dates back to the invention of the first voltaic battery by the Italian count and scientist Alessandro Volta. It was not until Edison's work on the generation of electricity, in the late 19th century, that direct current began to used for the transmission of electrical energy. Already in the XX century this use declined in favor of alternating current, which presents lower losses in transmission over long distances, although it is preserved in the connection of electrical networks of different frequencies and in the transmission through submarine cables.
Since 2008, the use of direct current generators from photoelectric cells that make it possible to take advantage of solar energy has been spreading.
When it is necessary to have direct current for the operation of electronic devices, the alternating current of the electricity supply network can be transformed through a process called rectification, which is carried out with devices called rectifiers, based on the use of semiconductor diodes or thyristors (formerly, also from vacuum tubes).
Alternating current
Alternating current (symbolized by CA in Spanish and AC in English, from alternating current) is the electric current in which the magnitude and direction vary cyclically. The most commonly used alternating current waveform is a sine wave. In colloquial usage, "alternating current" refers to the way in which electricity reaches homes and businesses.
The system used today was primarily devised by Nikola Tesla, and alternating current distribution was commercialized by George Westinghouse. Others who contributed to the development and improvement of this system were Lucien Gaulard, John Gibbs and Oliver B. Shallenberger between the years 1881 and 1889. Alternating current overcame the limitations that appeared when using direct current (CC), which constitutes a system inefficient for large-scale power distribution due to problems in power transmission.
The reason for the wide use of alternating current, which minimizes power transmission problems, is determined by its ease of transformation, a quality that direct current lacks. The electrical energy transmitted is given by the product of voltage, intensity and time. Given that the section of the conductors of the electric power lines depends on the intensity, it is possible, by means of a transformer, to modify the voltage up to high values (high voltage), decreasing the current intensity in equal proportion. This allows the conductors to have a smaller section and, therefore, lower cost; Furthermore, it minimizes the losses due to the Joule effect, which depend on the square of the intensity. Once at the point of consumption or in its vicinity, the voltage can again be reduced to allow its industrial or domestic use in a comfortable and safe way.
The frequencies used in the distribution networks are 50 and 60 Hz. The value depends on the country.
Three-phase current
The set of three alternating currents of equal frequency, amplitude and rms value that present a phase difference of 120° between them, and are given in a certain order, are called triphasic currents. Each of the currents that form the system is designated by the name of phase.
Three-phase power generation is more common than single-phase and provides more efficient use of conductors. The use of electricity in triphasic form is the majority to transport and distribute electrical energy and for its industrial use, including the drive of motors. Three-phase currents are generated by alternators equipped with three coils or groups of coils, wound in a system arranged at 120 electrical degrees between each phase.
The conductors of the three electromagnets can be connected in star or delta. In the star arrangement, each coil is connected to a phase at one end and to a common conductor at the other, called neutral. If the system is balanced, the sum of the line currents is null, with which the transport can be carried out using only three cables. In the delta or delta arrangement, each coil is connected between two phase wires, so that one end of each coil is connected to the other end of another coil.
The three-phase system presents a series of advantages such as the economy of its energy transmission lines (thinner wires than in an equivalent single-phase line) and of the transformers used, as well as its high performance of the receivers, especially motors, to which the three-phase line feeds with constant power and not pulsed, as in the case of the single-phase line.
Tesla was the inventor who discovered the principle of the rotating magnetic field in 1882, which is the basis of alternating current machinery. He invented the system of polyphase alternating current motors and generators that powers the planet.
Single-phase current
The one obtained by taking a phase of the three-phase current and a neutral wire is called single-phase current. In Spain and other countries that use similar values for the generation and transmission of electrical energy, this type of current provides a voltage of 230 volts, which makes it appropriate for most household appliances and lighting fixtures in homes to function properly..
Four wires are arranged from the nearest transformation center to the houses: a neutral (N) and three phases (R, S and T). If the voltage between any two phases (line voltage) is 400 volts, between one phase and the neutral it is 230 volts. The neutral and one of the phases enter each house, connecting several houses to each of the phases and to the neutral; this is called single-phase current. If high electrical power devices are installed in a home (air conditioning, motors, etc., or if it is a workshop or an industrial company) they are usually supplied directly with three-phase current that offers a voltage of 400 volts.
Standing electric current
Stationary electric current is called the electric current that is produced in a conductor in such a way that the charge density ρ of each point of the conductor is constant, that is to say that it is fulfilled that:
- dρ ρ dt=0{displaystyle {d{rho } over dt}=0}
Current measurement
Current can be measured with an ammeter.
Electrical current can be measured directly with a galvanometer, but this method involves breaking the electrical circuit, which is sometimes inconvenient.
Current can also be measured without breaking the circuit by sensing the magnetic field associated with the current. Devices, at the circuit level, use several techniques to measure current:
- Derivative resistance
- Effect Hall current sensor transducers
- Transformers (they can't measure the continuous current)
- Magnetoresistiva field sensors
- Rogowski Coil
- Amperimetrical clamp
Drift Speed
Mobile charged particles inside a conductor are constantly moving in random directions, like particles in a gas. (More accurately, a Fermi gas.) To create a net flow of charge, the particles must also move together with a mean drift velocity. Electrons are the charge carriers in most metals, and they follow an erratic path, bouncing from atom to atom, but generally drifting in the opposite direction of the electric field. The speed at which they drift can be calculated from the equation:
- I=nAvQ,{displaystyle I=nAvQ,,}
where
- I{displaystyle I} is the intensity of the electric current
- n{displaystyle n} is the number of charged particles per volume unit (or load carrier density)
- A{displaystyle A} is the area of the transverse section of the driver
- v{displaystyle v} is the speed of drift and
- Q{displaystyle Q} is the load of each particle.
Normally, electric charges in solids flow slowly. For example, in a copper cable with a cross section of 0.5 mm², which carries a current of 5 A, the drift speed of the electrons is of the order of one millimeter per second. To take a different example, in the near-vacuum inside a cathode ray tube, electrons travel in nearly straight lines at one-tenth the speed of light.
Any accelerating electric charge, and therefore any changing electric current, gives rise to an electromagnetic wave that propagates at very high speed outside the surface of the conductor. This speed is usually a significant fraction of the speed of light, as can be deduced from Maxwell's equations, and is therefore many times faster than the drift speed of electrons. For example, in alternating current lines, waves of electromagnetic energy propagate through the space between the wires, moving from a source to a distant load, even though the electrons in the wires only move back and forth in a tiny distance.
The ratio between the speed of the electromagnetic wave and the speed of light in free space is called the speed factor, and it depends on the electromagnetic properties of the conductor and surrounding insulating materials, as well as their shape and size.
The magnitudes (not the natures) of these three velocities can be illustrated by an analogy with the three similar velocities associated with gases. (See also hydraulic analogy.)
- The low rate of drift from the load carriers is analogous to the air movement; in other words, the winds.
- The high speed of electromagnetic waves is approximately analogous to the speed of sound in a gas (the sound waves move through the air much faster than large-scale movements such as convection)
- The random movement of the loads is analogous to heat – the thermal speed of the gas particles that vibrate at random.
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