0 Members and 1 Guest are viewing this topic.
In the early 20th century, experiments by Ernest Rutherford established that atoms consisted of a diffuse cloud of negatively charged electrons surrounding a small, dense, positively charged nucleus. Given this experimental data, Rutherford naturally considered a planetary model of the atom, the Rutherford model of 1911 – electrons orbiting a solar nucleus – however, the said planetary model of the atom has a technical difficulty: the laws of classical mechanics (i.e. the Larmor formula) predict that the electron will release electromagnetic radiation while orbiting a nucleus. Because the electron would lose energy, it would rapidly spiral inwards, collapsing into the nucleus on a timescale of around 16 picoseconds. This atom model is disastrous, because it predicts that all atoms are unstable.
An extremely sensitive method for the purpose, pioneered by Onnes himself, is thetechnique of estimating the upper limit of the resistivity by studying the decay rate of thepersistent current in a superconducting ring. Once established, the time dependence of thecurrent I(t) through the ring is given by I(t) = I0 e – (R/L) t where I0 is the current at t = 0, R is theresistance and L is the inductance of the ring. If the superconductor had zero resistance, thecurrent would not decay even for infinitely long times. However, an experiment can beperformed only over a limited amount of time. In a number of such experiments no detectable decay of the current was found for periods of time extending to several years.
In a minor variation of the experiment, after the loop became superconducting, thesource current was switched off, the superconducting loop being driven into the persistentcurrent mode. It was observed that even now the field generated by coil B remained muchlarger than the value in the normal state, indicating that the resistances in the two paths areexactly zero. This provides additional evidence that no extraneous effects such as differentialterminal resistances have any role to play. In summary, we have demonstrated that the dc resistance of a superconducting wireis indeed zero and not just unmeasurably small, thus resolving the uncertainty that had lingeredon for nearly a century after the discovery of the phenomenon of superconductivity.
But experiments using superconductor ring show that circulating electrical current can go on indefinitely, which means they don't lose energy through radiation.https://arxiv.org/ftp/cond-mat/papers/0506/0506426.pdf
But experiments using superconductor ring show that circulating electrical current can go on indefinitely, which means they don't lose energy through radiation.
Is the case of a simple superconducting loop equivalent to a simple magnet?
If so, would the wiggly wire loop be equivalent to an array of magnets?
- For an MRI machine with a superconductor loop 2m in diameter, this is something like 0.0001 Hz.
@alancalverd knows all about superconductors in an MRI machine - how often do you have to recharge them? - And did you need to take into account radiation from the superconductor coils when doing EMI tests?- Or was it impossible to measure against all of the intentional EMI required to align all our protons?
If you consider the supercon as nothing more or less than an ideal wire carrying a constant current, classical Maxwell electromagnetism says that it doesn't radiate because di/dt = 0.
But why early 20th century scientists objected to Rutherford's planetary atomic model arguing that circular motion of electrons around nucleus must radiate energy,
I think the classical argument would be that the energy lost by electrons on one side of the "loop" is gained by the electrons on the other side.
The original model was for hydrogen, with just one electron.