Superconductors in magnetic fields

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Discussion Overview

The discussion centers on the behavior of superconductors in magnetic fields, particularly focusing on the forces they experience and the implications of the Meissner effect. It includes theoretical considerations and conceptual clarifications related to type II superconductors and their interaction with magnetic fields.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants propose that superconductors do experience forces when passed through a magnetic field, referencing levitation as evidence.
  • One participant questions whether type II superconductors, with flux lines penetrating the material, would experience a Lorentz force due to an internal current, suggesting it may be calculated using the formula (##F_L = IL \times B##).
  • Another participant explains that the magnetic field is expelled from superconductors due to the Meissner effect, which results in surface currents that cancel the applied field within the superconductor, characterizing it as a perfect diamagnet.

Areas of Agreement / Disagreement

Participants express differing views on the nature of forces acting on superconductors in magnetic fields, with some agreeing on the existence of forces while others explore the implications of the Meissner effect and the behavior of type II superconductors. The discussion remains unresolved regarding the specifics of the forces and their calculations.

Contextual Notes

There are limitations regarding the assumptions made about the behavior of type II superconductors and the conditions under which the Lorentz force is applicable. The discussion does not resolve the mathematical implications of these assumptions.

Danyon
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Does a superconductor experience any forces acting upon it when it is passed through a magnetic field?
 
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Danyon said:
Does a superconductor experience any forces acting upon it when it is passed through a magnetic field?

Yes, as evidenced by those videos of levitating superconductors.
 
Adding to that, if type II superconductors would have some of the flux lines passing through the material:
Flux_Pinning_Field_Diagram.jpg

Let's assume there is current flowing within the conductor will it experience a Lorentz force(##F_L##)? Is it equal to(##IL \times B##)?
 
The magnetic field will be expelled from the superconductor. It can only propagate within a distance called the London penetration depth. This is the meissner effect. The applied field in the interior is canceled by surface currents. In this sense, an sc is a perfect diamagnet, but the diamagnetism is different from ordinary materials.
 

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