Shielding magnetism with superconductors

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Discussion Overview

The discussion revolves around the behavior of superconductors in relation to magnetic fields, particularly whether a superconductor can block the magnetic field generated by a current-carrying wire. Participants explore different scenarios of how superconductors interact with magnetic fields, including the timing of the superconducting state relative to the current flow.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants propose that surrounding a current-carrying wire with a superconductor does not block the magnetic field if the superconductor is introduced after the current is flowing.
  • Others argue that if the superconductor surrounds the circuit before the current is initiated, it will block the magnetic field due to the properties of superconductivity.
  • A participant emphasizes the need for the superconductor to be cooled after surrounding the circuit to enter the superconducting state, which is necessary for effective shielding.
  • Another participant introduces the concept of the Meissner effect, suggesting that superconductors can shield magnetic fields regardless of the initial presence of a magnetic field, provided they are Type I or below the lower critical field.
  • There is a discussion about the role of Eddy currents in superconductors versus normal conductors, highlighting that Eddy currents in superconductors do not dissipate, allowing them to maintain a constant magnetic flux.
  • One participant mentions the potential for creating a flux trap when surrounding an existing magnetic field with a superconducting shield, referencing applications in electron microscopy.
  • A question is raised about the applicability of Ampere's force law to superconductors, indicating further inquiry into the topic.

Areas of Agreement / Disagreement

Participants express differing views on the conditions under which superconductors can shield magnetic fields, with no consensus reached on the specifics of the scenarios discussed. The discussion remains unresolved regarding the implications of the Meissner effect and the timing of the superconducting state.

Contextual Notes

Some limitations include the dependence on the type of superconductor (Type I vs. Type II) and the critical fields, as well as the assumptions about the initial magnetic flux present when the superconductor transitions to its superconducting state.

Who May Find This Useful

This discussion may be of interest to those studying superconductivity, magnetism, and their applications in technology, particularly in fields like materials science and electrical engineering.

Danyon
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If I surrounded a wire carrying current with a superconductor would the magnetic field from the wire be blocked from the outside?
 
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If while the current is flowing, you surround it with a superconducting material, then no, it doesn't block the magnetic field. But if you surround the circuit with the superconducting material first and then start the current flow, then yes, it will block the magnetic field.
 
Shyan said:
If while the current is flowing, you surround it with a superconducting material, then no, it doesn't block the magnetic field. But if you surround the circuit with the superconducting material first and then start the current flow, then yes, it will block the magnetic field.

What's the difference between the two scenarios that causes this?
 
At first I should correct that the material shouldn't be in the superconducting state before placing it there. So how it should be done, is first surrounding the circuit with the material, then cooling the material for it to make transition to the superconducting state and then start the current flowing.
Drakkith said:
What's the difference between the two scenarios that causes this?
A material in the superconducting state tends to keep the magnetic flux through it, constant. So if, when enters the superconducting phase, there is no flux through it, its going to keep the flux zero. So when you start the current flow and the magnetic field is created, the material will induce currents in itself to cancel the magnetic field from the circuit and keep the flux constant(which is zero in this case).
 
Shyan said:
At first I should correct that the material shouldn't be in the superconducting state before placing it there. So how it should be done, is first surrounding the circuit with the material, then cooling the material for it to make transition to the superconducting state and then start the current flowing.

A material in the superconducting state tends to keep the magnetic flux through it, constant. So if, when enters the superconducting phase, there is no flux through it, its going to keep the flux zero. So when you start the current flow and the magnetic field is created, the material will induce currents in itself to cancel the magnetic field from the circuit and keep the flux constant(which is zero in this case).

This is confusing. It appears that you are mixing up superconductivity with perfect diamagnetism.

It doesn't matter if the there is already a magnetic a field or not. A superconductor will shield off the magnetic so field via the Meisner effect, IF the superconductor is a a Type I or below the lower critical field.

Zz.
 
ZapperZ said:
This is confusing. It appears that you are mixing up superconductivity with perfect diamagnetism.

It doesn't matter if the there is already a magnetic a field or not. A superconductor will shield off the magnetic so field via the Meisner effect, IF the superconductor is a a Type I or below the lower critical field.

Zz.

The point is, when a flux through a conducting material is turned on, Eddy currents are induced in it that, by Lenz's law, oppose the changing of the flux from zero to that finite value. But those currents are dissipated away in a finite time in a conductor. But if you do the same thing to a material in the superconducting phase, the Eddy currents won't go away so they will preserve the previous amount of flux.
 
One way of showing it in most superconductivity books, is through the following diagram:
fig4.gif
 
I think surrounding a existing magnetic field with a superconducting magnetic shield can create a flux trap pinning the existing field. I remember reading something about it when looking at shielding methods for electron microscopes.

Did a google search and found something similar: Superconducting Shields
 
Does Amperes force law apply to superconductors?
 
  • #10
ZapperZ said:
This is confusing. It appears that you are mixing up superconductivity with perfect diamagnetism.

It doesn't matter if the there is already a magnetic a field or not. A superconductor will shield off the magnetic so field via the Meisner effect, IF the superconductor is a a Type I or below the lower critical field.

Zz.

I studied superconductivity a while ago and it seems I don't remember it well. Sorry for the wrong information, to others in the thread as well.
 

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