Electric current

Dynamic electricity, or electric current, is the uniform motion of electrons through an electrical conductor. Static electricity is an unmoving, accumulated charge formed by either an excess or deficiency of electrons in an object. Although it is electrons which are the mobile charge carriers that are responsible for electric current in conductors, it has long been the convention to take the direction of electric current as if it were the positive charges which are moving.

Electric current is a physical quantity of fundamental importance in technology related to circuit theory, electrical engineering, and electronics, having a large number of applications such as the transport of electricity or information via signals (for example in communications).

With electric current we usually deal with negative charges, electrons, which “flow” into electrical conductors, usually metallic. But in other cases, a positive charge shift occurs, such as positive ions of electrolytic solutions. Since the direction of the charges depends on whether they are positive or negative, the direction of the current is conventionally defined as the direction of the flow of positive charge. This convention is due to Benjamin Franklin. In practical applications, however, the direction of the electric current is important for the correct functioning of the electronic circuits, while it has less importance in the electrical circuits. Besides the advantage of agreeing in direction with most texts, the conventional current direction is the direction from high voltage to low voltage, high energy to low energy, and thus has some appeal in its parallel to the flow of water from high pressure to low.

A schematic picture showing the direction of electric current as opposite to those of the negative charge carriers in a conductor.

The intensity of the electric current is generally measured with an ammeter, but to do this there are two different methods: one method requires the interruption of the circuit, which can sometimes be an inconvenience, while the other method is much less invasive and uses the detection of the magnetic field generated by the current flow, but in this case, a certain amount of field is required, which is not always present in some low power circuits. The instruments used for the latter method include Hall effect sensors or clamps and Rogowski coils.

The intensity of the electric current, indicated by the symbol \(I\), is assumed as a fundamental quantity in the international system (SI). Its unit of measure is the ampere (A), and from it, the unit of measure of the electric charge, the coulomb, is obtained, which corresponds to the amount of charge carried by a current with an intensity equal to 1 ampere in the unit of time of 1 second (1 C = 1 A·s).

The operational definition of electric current and the mathematical relationship associated with it is expressed by ohm’s law. Consider an electrical conductor of section \(S\) through which there is an ordered motion of charges. Electric current is defined as the amount of electric charge \(\Delta Q\) which in the time interval \(\Delta t\) crosses the surface \(S\):

\[I=\dfrac{\Delta Q}{\Delta t}\]

The motion of the charges that make up the electric current is achieved by generating an electric field in a conductor, the intensity of which is directly proportional to the force to which the charges are subjected. The existence of an electric field in the conductor implies the presence of electric potential: considered two points of the current-carrying conductor, the difference \(\Delta V\) between the respective electrical potentials is called electromotive force. If there are electric charges in the conductor, the electromotive force is directly proportional to the difference between the potential energy of the charges at the two points.

The ordered motion of charges is therefore due to the fact that electric charges minimize their potential energy by moving from the point of the greatest potential to the point of least potential. The electric field in the conductor, therefore, performs work on the charges, realizing a transfer of power from the field to the charges in motion. This work is given by:

\[dW=dq\Delta V=I\Delta Vdt\]

The power developed by the electric field is therefore:

\[P=\dfrac{dW}{dt}=I\Delta V\]

Conventional versus electron flow

When Benjamin Franklin made his conjecture regarding the direction of charge flow (from the smooth wax to the rough wool), he set a precedent for electrical notation that exists to this day, despite the fact that we know electrons are the constituent units of charge, and that they are displaced from the wool to the wax – not from the wax to the wool – when those two substances are rubbed together. This is why electrons are said to have a negative charge: because Franklin assumed electric charge moved in the opposite direction that it actually does, and so objects he called negative(representing a deficiency of charge) actually have a surplus of electrons.

By the time the true direction of electron flow was discovered, the nomenclature of positive and negative had already been so well established in the scientific community that no effort was made to change it, although calling electrons positive would make more sense in referring to “excess” charge. You see, the terms “positive” and “negative” are human inventions, and as such have no absolute meaning beyond our own conventions of language and scientific description. Franklin could have just as easily referred to a surplus of charge as “black” and a deficiency as “white,” in which case scientists would speak of electrons having a “white” charge (assuming the same incorrect conjecture of charge position between wax and wool).

However, because we tend to associate the word “positive” with “surplus” and “negative” with “deficiency,” the standard label for electron charge does seem backward. Because of this, many engineers decided to retain the old concept of electricity with “positive” referring to a surplus of charge, and label charge flow (current) accordingly. This became known as conventional flow notation:

Conventional flow notation.

Others chose to designate charge flow according to the actual motion of electrons in a circuit. This form of symbology became known as electron flow notation:

Electron flow notation.

In conventional flow notation, we show the motion of charge according to the (technically incorrect) labels of + and -. This way the labels make sense, but the direction of charge flow is incorrect. In electron flow notation, we follow the actual motion of electrons in the circuit, but the + and – labels seem backward. Does it matter, really, how we designate charge flow in a circuit? Not really, so long as we’re consistent in the use of our symbols.

You may follow an imagined direction of current (conventional flow) or the actual (electron flow) with equal success insofar as circuit analysis is concerned. Concepts of voltage, current, resistance, continuity, and even mathematical treatments such as Ohm’s Law and Kirchhoff’s Laws remain just as valid with either style of notation.

Ampere (unit of electric current)

Electric units, called “international units,” for current and resistance, were introduced by the International Electrical Congress held in Chicago in 1893, and definitions of the “international ampere” and “international ohm” were confirmed by the International Conference in London in 1908.

Although it was already obvious on the occasion of the 8th CGPM (1933) that there was a unanimous desire to replace those “international units” by so-called “absolute units,” the official decision to abolish them was only taken by the 9th CGPM (1948), which adopted the ampere for the unit of electric current, following a definition proposed by the CIPM (1946):

The ampere (A) is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to \(2\cdot 10^{−7}\) newton per meter of length.

It follows that the magnetic constant \(\mu_0\), also known as the permeability of vacuum, is exactly \(4\pi \cdot 10^{−7}\) henries per meter, \(\mu_0 = 4\pi \cdot 10^{−7}\) H/m.

The expression “MKS unit of force” which occurs in the original text of 1946 has been replaced here by “newton,” a name adopted for this unit by the 9th CGPM (1948).

Direct current (DC)

Direct current (DC) is electricity flowing in a constant direction. So, electrons always flow constantly in the same direction within the electrical circuit, and/or possessing a voltage with constant polarity, therefore they will always circulate in the same direction.

Direct current was produced in 1800 by Italian physicist Alessandro Volta’s battery, by his “Voltaic pile.” Direct current is the type of electricity made by a battery (with definite positive and negative terminals), or the kind of charge generated by rubbing certain types of materials against each other. Direct current was adopted by Thomas Edison in the late 19th century at the beginning of industrial electrical distribution. Subsequently, however, the technology moved to alternating current, invented by Nikola Tesla, more convenient for the transmission of electricity remotely. The efficiency of the alternating current made it possible to drastically decrease energy losses over long distances, thanks to the increase in electrical voltage which allowed to transmit high electrical power at high voltage and low current, drastically reducing losses due to dissipation on the line and therefore the thickness of the electrical conductor used for transport, compared to Edison direct current.

In a direct current system, unlike in alternating current ones, it is very important to respect the direction of the current, that is the polarity. There are in fact in the batteries a “positive” and a “negative” pole, which must be correctly connected to the load. For example, a DC motor, if powered backward, rotates in the opposite direction, unlike a single-phase AC motor. Many electronic circuits, if powered incorrectly, can fail, particularly if they are not protected by an anti-reverse diode.

The direct current can be produced both with a dynamo and through an alternator starting from an alternating current (AC) with, following, a rectification process (by use of a rectifier), carried out with diodes or rectifiers bridges.

Alternating current (AC)

Alternating current describes the flow of charge that changes direction periodically. As a result, the voltage level also reverses along with the current. In other words, it is a type of electric current characterized by the fact of inverting the electric polarity continuously over time. Basically, unlike the direct current, in which the polarity is fixed and invariable, in alternating current the positive pole becomes negative and vice versa with an alternation (hence the name) which occurs with a fixed frequency (typically 50 Hz or 60 Hz).

The polarity inversion, however, does not occur abruptly but with a progressive variation in a pattern called sinusoidal, in which the value of current (and therefore voltage) starts at zero and increases gradually in a given direction, reaches its maximum value and then decrease to zero and return in the opposite direction with the same trend (the cycle repeats).

Overcurrent

In an electric power system, overcurrent or excess current is a situation where a larger than intended electric current exists through a conductor, leading to excessive generation of heat, and the risk of fire or damage to equipment. There are two types of overcurrents:

  • overload currents;
  • short-circuit currents.

Possible causes for overcurrent include short circuits, excessive load, incorrect design, an arc fault, or a ground fault. The electrical conductors present in electrical systems are protected from the presence of any overcurrents by using protective equipment such as fuses and magneto-thermic switches, circuit breakers, and current limiters are commonly used overcurrent protection (OCP) mechanisms.

The fuse is the simplest among the protection devices and is generally used to protect a line from short-circuit currents. It is placed in series with the line to be protected and intervenes by interrupting the line when the value of the current that passes through it is such as to cause the melting of a fuse element, hence the name of the fuse.

Circuit breakers are mechanical power cut-off devices. They are equipped with a special release device which causes the automatic opening of the contacts when the current flowing through it exceeds a certain value for a certain time. The tripping time depends on the value of the fault current.

Physiological effects of electricity

As electric current is conducted through a material, any opposition to that flow of electrons (resistance) results in a dissipation of energy, usually in the form of heat. This is the most basic and easy-to-understand effect of electricity on living tissue: current makes it heat up. If the amount of heat generated is sufficient, the tissue may be burnt. The effect is physiologically the same as damage caused by an open flame or another high-temperature source of heat, except that electricity has the ability to burn tissue well beneath the skin of a victim, even burning internal organs.

Another effect of electric current on the body, perhaps the most significant in terms of hazard, regards the nervous system. By ”nervous system” I mean the network of special cells in the body called ”nerve cells” or ”neurons” which process and conduct the multitude of signals responsible for regulation of many body functions. The brain, spinal cord, and sensory/motor organs in the body function together to allow it to sense, move, respond, think, and remember.

Nerve cells communicate to each other by acting as ”transducers:” creating electrical signals (very small voltages and currents) in response to the input of certain chemical compounds called neurotransmitters, and releasing neurotransmitters when stimulated by electrical signals. If an electric current of sufficient magnitude is conducted through a living creature (human or otherwise), its effect will be to override the tiny electrical impulses normally generated by the neurons, overloading the nervous system and preventing both reflex and volitional signals from being able to actuate muscles. Muscles triggered by an external (shock) current will involuntarily contract, and there’s nothing the victim can do about it.

This problem is especially dangerous if the victim contacts an energized conductor with his or her hands. The forearm muscles responsible for bending fingers tend to be better developed than those muscles responsible for extending fingers, and so if both sets of muscles try to contract because of an electric current conducted through the person’s arm, the ”bending” muscles will win, clenching the fingers into a fist. If the conductor delivering current to the victim faces the palm of his or her hand, this clenching action will force the hand to grasp the wire firmly, thus worsening the situation by securing excellent contact with the wire. The victim will be completely unable to let go of the wire.

Medically, this condition of involuntary muscle contraction is called tetanus. Electricians familiar with this effect of electric shock often refer to an immobilized victim of electric shock as being ”froze on the circuit.” Shock-induced tetanus can only be interrupted by stopping the current through the victim.

Even when the current is stopped, the victim may not regain voluntary control over their muscles for a while, as the neurotransmitter chemistry has been thrown into disarray. This principle has been applied in ”stun gun” devices such as Tasers, which on the principle of momentarily shocking a victim with a high-voltage pulse delivered between two electrodes. A well-placed shock has the effect of temporarily (a few minutes) immobilizing the victim.

Electric current is able to affect more than just skeletal muscles in a shock victim, however. The diaphragm muscle controlling the lungs, and the heart – which is a muscle in itself – can also be ”frozen” in a state of tetanus by electric current. Even currents too low to induce tetanus are often able to scramble nerve cell signals enough that the heart cannot beat properly, sending the heart into a condition known as fibrillation. A fibrillating heart flutters rather than beats and is ineffective at pumping blood to vital organs in the body. In any case, death from asphyxiation and/or cardiac arrest will surely result from a strong enough electric current through the body. Ironically, medical personnel uses a strong jolt of electric current applied across the chest of a victim to ”jump start” a fibrillating heart into a normal beating pattern.

That last detail leads us into another hazard of electric shock, this one peculiar to public power systems. Though our initial study of electric circuits will focus almost exclusively on DC (Direct Current, or electricity that moves in a continuous direction in a circuit), modern power systems utilize alternating current or AC.

Direct current (DC), because it moves with continuous motion through a conductor, has the tendency to induce muscular tetanus quite readily. Alternating current (AC), because it alternately reverses the direction of motion, provides brief moments of opportunity for an afflicted muscle to relax between alternations. Thus, from the concern of becoming ”froze on the circuit,” DC is more dangerous than AC.

However, AC’s alternating nature has a greater tendency to throw the heart’s pacemaker neurons into a condition of fibrillation, whereas DC tends to just make the heart standstill. Once the shock current is halted, a ”frozen” heart has a better chance of regaining a normal beat pattern than a fibrillating heart. This is why ”defibrillating” equipment used by emergency medics works: the jolt of current supplied by the defibrillator unit is DC, which halts fibrillation and gives the heart a chance to recover. In either case, electric currents high enough to cause involuntary muscle action are dangerous and are to be avoided at all costs.

References

  1. Electric current flow direction. Wikimedia. https://commons.wikimedia.org/wiki/File:Direzione_convenzionale_della_corrente_elettrica.svg
  2. Lessons in Electric Circuits, Volume I – DC. By Tony R. Kuphaldt
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