|MadSci Network: Physics|
Hello Jonathan: Electrons travel close to the speed of light, somewhere in the range of 186,000 miles/sec, Any physical system containing equal numbers of positive and negative charges is neutral. Charge is a conserved quantity; the net electric charge in a closed physical system is constant. Although charge is conserved, it can be transferred from one body to another. Electric current is the flow of charge through a conductor. Conduction of electricity consists of the flow of charges (electrons). Metals are good conductors of both heat and electricity because they have a high free-electron density. Even the best conductor still has resistance to the flow of current so electromotive force (voltage) must be sustained to keep the flow from stopping. Resistance translates to loss in the passage of current. This loss is expressed in heat and costs money. Superconductors have no resistance, so the electrons are free to travel without any resistance. In a normal copper wire, a current will flow if a voltage is applied down the length of the wire. The voltage must be maintained or the current will stop (when your battery runs out, it is no longer supplying voltage, so you get no current). The ratio of voltage to current is called R, the electrical resistance of the wire. In a superconducting wire, a current can flow with no applied voltage-- that is, R=0. The state of zero electrical resistance is a bit like the state of zero friction. Imagine a really slick patch of ice with no surface friction. Put a sled on it very carefully so there is no initial motion. The sled will remain still: it can't spontaneously start moving. However, if you give it just the slightest push, it will slide forever without slowing down because there is no friction to take energy from its motion. Similarly, in the superconducting state, a material can pass a current without dissipation, as long as you "give it a push" by applying some initial voltage so the current can start flowing. A second characteristic of the superconducting state is that a superconductor will repel a magnetic field (as long as the field is below a certain limit). Metals in the normal state (like room-temperature copper) happily allow a static magnetic field to penetrate them-- you can try this with a thin sheet of copper foil and a strong refrigerator magnet. You'll find that the magnet will still stick to the fridge even with the foil in between. But a metal in the superconducting state will force the magnetic field to bend around it, something like the aerodynamic lines you've seen passing by an airplane wing or high-speed car in wind simulations. The superconductor does this by running small currents over its surface-- it can do that all by itself because it requires no voltage. (And, of course, the net current over the surface will be zero; as some flows over one area of the superconductor, some compensating current will flow over another area). Electrical currents produce magnetic fields, and the superconductor uses its surface currents to "cancel out" the external field it feels exactly, so there is no net field inside. All this sounds marvelous, but there are severe drawbacks in practical use of superconductors. I found a website for you to investigate more about superconductivity. It is: http://superconductors.org/ If I can be of help understanding more about superconductors, let me know. Have fun while you learn! MAD.SCI Micro. [Note added by MadSci Admin: also see the following previously-posted answer: http://www.madsci.org/posts/archives/oct98/904621812.Eg.r.html]
Try the links in the MadSci Library for more information on Physics.