Electron flow vs current flow? (Repost)
Current is rate of flow of electronic charge. Simply, flow of electrons. In the presence of electrical potential, electrons flow from negative terminal to positive one. In , prior to electricity being identified with the electron, Ben Franklin chose a convention re- garding the direction of current flow. Franklin It is important to realize that the difference between conventional current flow and electron flow. Electric current is the flow of electrons from the negative terminal to the positive called, “conventional current flow” and is used when drawing circuit diagrams. The potential difference provided by cells connected in series is the sum of the.
How can there by a current on the order of 1 or 2 ampere in a circuit if the drift speed is only about 1 meter per hour? Current is the rate at which charge crosses a point on a circuit. A high current is the result of several coulombs of charge crossing over a cross section of a wire on a circuit. If the charge carriers are densely packed into the wire, then there does not have to be a high speed to have a high current.
That is, the charge carriers do not have to travel a long distance in a second, there just has to be a lot of them passing through the cross section. Current does not have to do with how far charges move in a second but rather with how many charges pass through a cross section of wire on a circuit. To illustrate how densely packed the charge carriers are, we will consider a typical wire found in household lighting circuits - a gauge copper wire.
Each copper atom has 29 electrons; it would be unlikely that even the 11 valence electrons would be in motion as charge carriers at once.
Electric current - Wikipedia
If we assume that each copper atom contributes just a single electron, then there would be as much as 56 coulombs of charge within a thin 0. With that much mobile charge within such a small space, a small drift speed could lead to a very large current. To further illustrate this distinction between drift speed and current, consider this racing analogy. Suppose that there was a very large turtle race with millions and millions of turtles on a very wide race track.
Turtles do not move very fast - they have a very low drift speed. Suppose that the race was rather short - say 1 meter in length - and that a large percentage of the turtles reached the finish line at the same time - 30 minutes after the start of the race.
In such a case, the current would be very large - with millions of turtles passing a point in a short amount of time.
In this analogy, speed has to do with how far the turtles move in a certain amount of time; and current has to do with how many turtles cross the finish line in a certain amount of time. The Nature of Charge Flow Once it has been established that the average drift speed of an electron is very, very slow, the question soon arises: Why does the light in a room or in a flashlight light immediately after the switched is turned on?
Wouldn't there be a noticeable time delay before a charge carrier moves from the switch to the light bulb filament?
The answer is NO! As mentioned abovecharge carriers in the wires of electric circuits are electrons. These electrons are simply supplied by the atoms of copper or whatever material the wire is made of within the metal wire. Once the switch is turned to on, the circuit is closed and there is an electric potential difference is established across the two ends of the external circuit. The electric field signal travels at nearly the speed of light to all mobile electrons within the circuit, ordering them to begin marching.
As the signal is received, the electrons begin moving along a zigzag path in their usual direction. Thus, the flipping of the switch causes an immediate response throughout every part of the circuit, setting charge carriers everywhere in motion in the same net direction. While the actual motion of charge carriers occurs with a slow speed, the signal that informs them to start moving travels at a fraction of the speed of light.
Electron flow vs. current flow
The electrons that light the bulb in a flashlight do not have to first travel from the switch through 10 cm of wire to the filament.
Rather, the electrons that light the bulb immediately after the switch is turned to on are the electrons that are present in the filament itself. As the switch is flipped, all mobile electrons everywhere begin marching; and it is the mobile electrons present in the filament whose motion are immediately responsible for the lighting of its bulb. As those electrons leave the filament, new electrons enter and become the ones that are responsible for lighting the bulb.
The electrons are moving together much like the water in the pipes of a home move. When a faucet is turned on, it is the water in the faucet that emerges from the spigot. These regions may be initiated by field electron emissionbut are then sustained by localized thermionic emission once a vacuum arc forms. These small electron-emitting regions can form quite rapidly, even explosively, on a metal surface subjected to a high electrical field.
Vacuum tubes and sprytrons are some of the electronic switching and amplifying devices based on vacuum conductivity. Superconductivity Superconductivity is a phenomenon of exactly zero electrical resistance and expulsion of magnetic fields occurring in certain materials when cooled below a characteristic critical temperature. Like ferromagnetism and atomic spectral linessuperconductivity is a quantum mechanical phenomenon.
It is characterized by the Meissner effectthe complete ejection of magnetic field lines from the interior of the superconductor as it transitions into the superconducting state.
The occurrence of the Meissner effect indicates that superconductivity cannot be understood simply as the idealization of perfect conductivity in classical physics.
Semiconductor In a semiconductor it is sometimes useful to think of the current as due to the flow of positive " holes " the mobile positive charge carriers that are places where the semiconductor crystal is missing a valence electron.
This is the case in a p-type semiconductor.
A semiconductor has electrical conductivity intermediate in magnitude between that of a conductor and an insulator. In the classic crystalline semiconductors, electrons can have energies only within certain bands i. Energetically, these bands are located between the energy of the ground state, the state in which electrons are tightly bound to the atomic nuclei of the material, and the free electron energy, the latter describing the energy required for an electron to escape entirely from the material.
The energy bands each correspond to a large number of discrete quantum states of the electrons, and most of the states with low energy closer to the nucleus are occupied, up to a particular band called the valence band. Semiconductors and insulators are distinguished from metals because the valence band in any given metal is nearly filled with electrons under usual operating conditions, while very few semiconductor or virtually none insulator of them are available in the conduction band, the band immediately above the valence band.
The ease of exciting electrons in the semiconductor from the valence band to the conduction band depends on the band gap between the bands. The size of this energy band gap serves as an arbitrary dividing line roughly 4 eV between semiconductors and insulators.
With covalent bonds, an electron moves by hopping to a neighboring bond. The Pauli exclusion principle requires that the electron be lifted into the higher anti-bonding state of that bond. For delocalized states, for example in one dimension — that is in a nanowirefor every energy there is a state with electrons flowing in one direction and another state with the electrons flowing in the other.
For a net current to flow, more states for one direction than for the other direction must be occupied. For this to occur, energy is required, as in the semiconductor the next higher states lie above the band gap. Often this is stated as: However, as a semiconductor's temperature rises above absolute zerothere is more energy in the semiconductor to spend on lattice vibration and on exciting electrons into the conduction band.