|MadSci Network: Physics|
Dear Klaus in Korea, One thing to remember is that metal used in common cables allows the creation of "free electrons" that can act as carriers of electric charge. What does that mean? It means that the electrons are not bound in a specific atomic state of the metal atoms, but are freely randomized throughout the metal material. You've probably heard of the "conduction band" in conductors or semiconductors. It is in this high energy band where electron current resides. While free electrons (modeled as what's known as a "Fermi free electron gas") exhibit magnetic and orbital spin character, their spins are also randomized. The primary momenta of charge carriers that generate common current are linear, not spin. The quantum mechanical momentum of a free electron is mv = ©¤k. What happens when we apply an electromagnetic field to a system of free electrons? They feel a "Lorentz" force described by the following relation: If we have only an electrical field exerting the electromotive force (as in common cable), the magnetic field is zero (B = 0) and, Over a time period, t, That results in, Now, with this simple theory, the longer we leave the field on, the faster and faster the electrons start to move (the k-values, which are proportional to the momenta, keep on increasing in the x-direction) This would mean that if you apply a field to a copper wire, and create an electrical current (movement of electrons), the current would grow as a function of time, apparently without a limit. What stops the electrons from moving faster and faster in this electric field? (they are accelerating under this force). Collisions. These occur because of impurities in the cable, lattice defects or defomitites, and phonons (acoustic and optical particles characteristic of the cable material). Experiments have shown that at room temperature (300 K), the electrical resistivity is dominated by electron collisions with phonons. Here is the URL of a website with a nice pictorial view of current at the atomic level in a copper cable with a great discussion of drift velocity... => http://hyperphysics.phy-astr.gsu.edu/hbase/electric/ohmmic.html Another thing to consider is if a cable conducts high-frequency alternating current then the effective cross sectional area of the wire available for current conduction is diminished. The "skin effect" is the tendency of an alternating electric current to distribute itself within a conductor so that the current density near the surface of the conductor is greater than that at its core. That is, the electric current tends to flow at the "skin" of the conductor. A type of cable called litz wire (from the German Litzendraht, woven wire) is used to mitigate the skin effect for frequencies up to about one or two MHz. It consists of a number of insulated wire strands woven together in a carefully designed pattern, so that the overall magnetic field acts equally on all the wires and causes the total current to be distributed equally among them. Litz wire is often used in the windings of high-frequency transformers, to increase their efficiency. Interesting new research on using individual atoms as conducting "cables" has given fresh insight into charge carrier flow on a nanometer scale. Conduction properties of these "cables" at the atomic scale are very different from the common cable we've discussed here. It turns out to be possible to send extremely strong currents (corresponding to current densities of ~10e14 A/m2) through such devices without damaging them. See the website at the URL => http://www.physics.leidenuniv.nl/sections/ Hope this helps Klaus in your investigation of electric current, ---* Dr. Ken Beck
Try the links in the MadSci Library for more information on Physics.