[sdiy] Dr strange no volt!

[Magnus Danielson]

From: Glen <mclilith at charter.net>
Subject: Re: [sdiy] Dr strange no volt!
Date: Fri, 06 Aug 2004 19:30:13 -0700
Message-ID: <4.1.20040806191811.0095c450 at mail.charter.net>

> At 03:26 PM 8/6/04 , M.A. Koot wrote:
> 
> >Interesting discussion, I've heard about having 0 volts and still having
> >currents before, really weird.
> >But when one does look at it straight to the point, Mister Ohm still says I
> >= U/R.
> >So no matter the resistance: if U = 0, I is also 0!
> >Period. ;)
> 
> Not so fast, you're actually defining "division by zero". I've always been
> told that the world has yet to come to a definite conclusion as to what
> division by zero would yield. One suggestion that I've heard is division by
> zero results in infinity, but I don't know how many people tend to agree
> with that hypothesis. People can't even decide on exactly what the
> ramifications of infinity are, for one thing.
> 
> Remember, part of the discussion was whether you could have current flow
> with zero voltage, inside a superconductive loop. Ohm's law sort of breaks
> down in this case, I would think.

Well, it doesn't break down, but it depends on which way you are using it.

Ohm's law gives you an indication of how the potential difference is expressed
in the current thought some device and the resistance of that device. That is:

U = R * I

Now, if you have a superconductor at your hands, the resistance has fallen to
zero (R = 0) and Ohm's law still works correctly in the forward direction.
However, if you tries to use it backwards you get silly results, but that 
should not come as a supprice since you had a potential of 0V and where then
dividing that with the know zero resistance, now the current can be anything
really, you just can't know since there is no data to support it! You can
figure it out, but not by using Ohm's law, since the zero resistance will
uncouple the current from voltage observations (the way we are used to).

> So, if you truly had zero resistance and zero voltage, could/would there be
> current flow in a superconductive loop? If so, how much current? Infinite?

Yes, there can be current flowing in the loop and infact, I've done the
exercise, so it's not absurd either.

The amount of current you have in the loop is as I said not observable by the
means of voltage differances. It will not be infinite, that is you just doing
the math wrong and confusing yourself by not being able to separate the
strangeness of physics by the strangeness of math.

How can there be a current flowing in the loop then?
Well, since there is no resistance, if one has created a current, it can go on
infinitly long since there is no loss due to resistance. OK, so once we got it
there, it stays. How did it get there then? Well, we have a closed loop and as
any closed loop we can get a current going inside of it by inducing it into the
loop. So, by having a magnet, electromagnet or any other source of a magnetic
field one can induce a current into it. Naturally, this current loop will as
any other current loop create a magnetic field. Infact, this is the ultimate
eddy current loop and it will counter-act any magnetic field applied to it
(within certain limits). If you have a superconductor and then brings a small
magnet above it, the magnetic field of the magnet will induce a mirror in the
superconductor and the magnet will float above the superconductor. This is a
fun way to show superconductivity and is known as the Meissner effect.

Now, can there be an inifint current in the loop? No! Superconductors break
down from superconductivity when the current density (A/m^2) becomes to high.
This is infact more prominent in so called high temperature superconductors
than in traditional metal superconductors. I have a history with 
superconductors way back in time, so that's why I just know these things.

Cheers,
Magnus - not much of a superconductor, infact I never conducts!