How does electric current flow – Mechanisms of the current flow

Electric Current

Electric current is the flow of electric charge through a material. It is the rate at which electric charge flows past a point in a circuit. The flow of electric charge is typically carried by electrons, which are negatively charged particles.

The SI unit for current is the coulomb per second, or the ampere (A), which is an SI base unit: 

1 ampere = 1A = 1 coulomb per second = 1 C/s.

How does electric current flow – Mechanisms of the current flow

In electrostatic situations, the electric field is zero everywhere within the conductor, and there is no current. However, this does not mean that all charges within the conductor are at rest. In an ordinary metal such as copper or alumium, some of the electrons are free to move within the conducting material. These free electrons move randomly in all directions, somewhat like the molecules of a gas but with much greater speeds, of the order of 10⁶ m/s. The electrons nonetheless do not escape from the conducting material, because they are attracted to the positive ions of the material. The motion of the electrons is random, so there is no net flow of charge in any direction and hence no current.

When a voltage difference is applied across a conductor, it creates an electric field within the material. The electric field exerts a force on the free electrons within the conductor, causing them to move from areas of high potential energy to areas of lower potential energy. The flow of electrons in response to the applied electric field is what we refer to as an electric current.

In conductors, the valence electrons are essentially free and strongly repel each other. Any external influence which moves one of them will cause a repulsion of other electrons, which propagates “domino fashion” through the conductor.

How fast does electricity flow? – Speed of electricity

The word electricity refers generally to the movement of electrons (or other charge carriers) through a conductor in the presence of a potential difference or an electric field. The speed of this flow has multiple meanings. If we are going to deal with the question of how fast electricity flows, then we have to distinguish two basic types of speeds. 

• Wave propagation speed. 
• Drift velocity

In everyday electrical and electronic devices, the signals travel as electromagnetic waves typically at 50%–99% of the speed of light in vacuum, while the electrons themselves move much more slowly.

Wave Propagation Speed

Wave propagation speed also called the velocity factor or velocity of propagation of a transmission medium is the ratio of the speed at which a wavefront (of an electromagnetic signal, a radio signal, a light pulse in an optical fibre or a change of the electrical voltage on a copper wire) passes through the medium, to the speed of light in vacuum.

The dimensions of the wire and electrical properties like its inductance affect the exact propagation speed, but usually it will be around 90 percent of the speed of light – about 270,000 km/s.

In everyday electrical and electronic devices, the signals travel as electromagnetic waves typically at 50%–99% of the speed of light in a vacuum, while the electrons themselves move much more slowly; see drift velocity and electron mobility.

For example, the velocity factor for coaxial cable is typically around 0.66 to 0.85, meaning that the velocity of an electromagnetic wave in the cable is only about two-thirds to four-fifths of the velocity of light in a vacuum. This can cause signals to experience a delay when transmitted through the cable since the velocity of the wave is slower than it would be in a vacuum. The delay can be significant for high-frequency signals, which can experience phase shifts or distortion due to the velocity factor.

Drift Velocity

In electricity, drift velocity refers to the average velocity of the charge carriers, usually electrons, as they move through a conductor under the influence of an electric field.

When a voltage is applied across a conductor, an electric field is established, which causes the electrons to move in a certain direction. However, the electrons do not move in a straight line but rather undergo a random motion due to collisions with the atoms of the conductor losing energy and scattering in random directions. This creates resistance to the flow of electrons and causes some of the energy of the electric field to be converted into heat.

This random motion causes the electrons to have an average velocity, which is called the drift velocity.

The drift velocity of electrons in a conductor is typically quite slow, on the order of a few millimeters per second, even though the current in the conductor may be quite high. This is because the electrons are constantly colliding with the atoms of the conductor, which slows down their overall motion. Drift velocity is proportional to current. In a resistive material, it is also proportional to the magnitude of an external electric field.

While the drift velocity is relatively slow, it is still an important concept in understanding the behavior of electric currents in conductors. The overall flow of electric charge in a conductor is determined by the combination of the drift velocity and the number of charge carriers moving through the conductor.

The drift velocity of electrons in a conductor can be calculated using the following formula:

v_d = (I / nAq)

where:

• v_d is the drift velocity of electrons in meters per second (m/s)
• I is the current flowing through the conductor in amperes (A)
• n is the number of charge carriers per unit volume in the conductor (in m^-3)
• A is the cross-sectional area of the conductor in square meters (m^2)
• q is the charge of a single electron, which is approximately 1.602 x 10^-19 Coulombs (C)

This formula is derived from the equation for electrical current (I = nAqv_d), which relates the current flowing through a conductor to the number of charge carriers, their velocity, and the cross-sectional area of the conductor.

For example, when a DC voltage is applied, the electron drift velocity will increase in speed proportionally to the strength of the electric field. The drift velocity in a 2 mm diameter copper wire in 1 ampere current is approximately 8 cm per hour. AC voltages cause no net movement; the electrons oscillate back and forth in response to the alternating electric field (over a distance of a few micrometers).

Drift Velocity and Electron Mobility

Drift velocity and electron mobility are two related concepts in the study of electricity and conductors, but they refer to different aspects of the behavior of charge carriers, such as electrons, in a material.

Drift velocity refers to the average velocity of charge carriers, such as electrons, as they move through a conductor under the influence of an electric field. This velocity is affected by factors such as the density of the charge carriers, the cross-sectional area of the conductor, and the strength of the electric field. The drift velocity is typically quite slow, on the order of a few millimeters per second, due to the frequent collisions between charge carriers and the atoms of the conductor.

Electron mobility, on the other hand, is a measure of how easily electrons can move through a material under the influence of an electric field. It is defined as the ratio of the drift velocity of the electrons to the electric field strength. In other words, electron mobility is a measure of how efficiently the electrons can move through the material, taking into account the resistance to their motion due to collisions with the atoms of the conductor. The unit of electron mobility is meters squared per volt-second (m^2/Vs).

While drift velocity and electron mobility are related, they are not interchangeable. The drift velocity is a physical quantity that describes the motion of charge carriers in a conductor, while electron mobility is a material property that characterizes how easily electrons can move through a specific material under the influence of an electric field.