On Wed, Feb 5, 2014 at 1:42 PM, David Roberson <[email protected]> wrote:
> > > While looking at reviews for Caver A. Mead's book, I read a review that > said he made a mistake including voltage in a calculation for > superconductors. > > Now I think that there must be voltage of a type in superconductors, > there are 2 types of voltage. > > One is the voltage drop across a conductor. This is similar to the > voltage on a charged capacitor. > > But there are other type is kinetic voltage, this is where a charge is > moving at a given velocity as it used in particle accelerators. > > >Voltage of this type can be compared to (or come from) inertia, and if > electrons are moving then there will be some persistence even if impedance > is removed since electrons still have mass.< > > There is no need to apply a voltage across the leads of a superconducting > loop for current to flow. Any current present will continue indefinitely. > Well obviously. Although a voltage would be required to initiate a current flow however minimal, superconductors still generally manifest magnetic fields which is why they are used in super powerful magnets. This means the establishment of a magnetic field, additionally even if it was somehow perfectly non-inductive it still requires some force to get electrons to move in the first place however minimal this may be, electrons are light but not massless. > And, if you do apply a voltage, the current will ramp up as long as the > voltage is applied. The ramp rate is established by the voltage you apply > and the inductance of the loop. > Agreed, but voltage is still required to get it moving. > > >If a superconducting ring that carried a current was suddenly opened, > the electrons are still moving and must compress slightly as they come to a > stop leaving the ends momentarily charged to some degree.< > > All of the energy stored within the magnetic field must be either > converted into heat by arcing across the open circuit and heating the air, > or by charging the effective capacitance formed by the open leads. The > energy given to the capacitor will be returned to the loop inductance when > the current reverses and this process can ring indefinitely as long as the > loss is zero. > Yes, but here there is clearly voltage. Also a lossless resonant superconductor might be impossible unless radiation resistance is somehow zero. A tiny tiny bit of ohmic resistance stops a copper ring from behaving like a superconductor so I doubt it take radiation resistance to do quench the oscillation. > > >Additionally imagine a superconductive loop in an alternating EM field, > there is a voltage induced by the changing magnetic field (or > relativistically distorted electric field) and this does not lead to a > voltage drop, but there is still a voltage, if this loop was opened and a > normal circuit inserted you would indeed see a voltage.< > > There is a voltage drop in this case due to the AC current induced within > the loop flowing through the loop inductance. It does not lead to heat > because the voltage and current are at right angles to each other. > > >Indeed even if we use a resistive wire in such a loop, no voltage drop > is noted, and yet there is still a voltage present to overcome the > resistance, and the resistance is still impeding the flow of electrons. But > would it be correct to say that this is happening with no voltage, even > though none can be read by any instrument?< > > Perhaps I do not understand what you are saying here as I would expect to > see a voltage drop measured across the ends of any resistor carrying > current. > There are no ends, I said a loop, and the entire loop is in a uniform voltage field and the entire loop is uniformly resistive. This produces no measurable voltage unless there is an imperfection of uniformity of these 2 factors.
