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.

electric current
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 ΔQ which in the time interval Δt 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 ΔV 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
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
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.

  • Direct current (DC)
  • Alternating current (AC)
  • Physiological effects of electricity

Related keywords

  • Ampere (unit of electric current)
  • Overcurrent

References

  • Image credits: Electric current flow direction. Wikimedia.
  • Lessons in Electric Circuits, Volume I – DC. By Tony R. Kuphaldt
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