MadSci Network: Physics

Re: At what speed do electrons travel in a superconductor?

Date: Wed Sep 1 09:38:46 1999
Posted By: Michael L. Roginsky, Staff, Avionics, Honeywell Defense Avionics
Area of science: Physics
ID: 935929262.Ph

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:
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:]

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