Critical field for a superconductor - why?

In summary, a Type I superconducting cylinder will expel magnetic flux up until a critical field strength is reached due to an energy balance between the superconducting state and the expelled flux. This is not the case for a zero-resistance (non-superconducting) cylinder, as it is a theoretical discussion and there is no critical field for such a material. The behavior of the Meissner effect (superconducting) is different from a perfect conductor.
  • #1
postscript07
2
0
Cool a cylinder made from a Type I superconducting material below the superconducting transition temperature.

Apply a magnetic field parallel to the cylinder. The cylinder expels the flux.. up until the field reaches a critical value.

Why does a critical value exist? That is, why doesn't the cylinder expel magnetic flux independent of the field strength?

Many thanks.
 
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  • #2
In short, it is a question of energy balance. The superconducting state has lower energy due to "condensation" of electrons into Cooper pairs. In the absence of a strong field, it is energetically favorable to be in the superconducting state.

The expelled flux, however, exerts a pressure inwards and this represents an unfavorable energy situation. As the field strength rises, the energy penalty to exclude flux exceeds the energy savings of condensation, and the material goes normal.
 
  • #3
Thank you marcusl, that is very helpful.

Does a critical field exist for a zero-resistance (not superconducting) cylinder? Hall (1st ed Manchester) p264 suggests this is the case. Why?
 
  • #4
postscript07 said:
Thank you marcusl, that is very helpful.

Does a critical field exist for a zero-resistance (not superconducting) cylinder? Hall (1st ed Manchester) p264 suggests this is the case. Why?
Since a non-SC zero resistance material doesn't exist, this is a theoretical discussion. There is no critical field so far as I know.

Do you know the difference in behavior between the Meissner effect (SC) and a perfect conductor?
 

FAQ: Critical field for a superconductor - why?

1. What is the critical field for a superconductor?

The critical field for a superconductor is the maximum magnetic field that a superconductor can withstand before it loses its superconducting properties.

2. Why is the critical field important for superconductors?

The critical field is important because it determines the practical applications of superconductors. Superconductors are used in various technologies such as MRI machines, particle accelerators, and power transmission lines, and the critical field determines their maximum performance and efficiency.

3. What factors affect the critical field for a superconductor?

The critical field for a superconductor is affected by the material properties of the superconductor, such as its critical temperature, critical current, and critical magnetic field. It is also influenced by the temperature, pressure, and external magnetic field.

4. How is the critical field measured for a superconductor?

The critical field for a superconductor is typically measured using a technique called magnetometry, which involves applying an increasing magnetic field to the superconductor and measuring the critical field at which the material transitions from superconducting to normal state.

5. What happens when the critical field is exceeded for a superconductor?

When the critical field is exceeded for a superconductor, it undergoes a phase transition from superconducting to normal state. This results in the loss of its zero resistance and perfect diamagnetism properties, and the material starts to exhibit resistance and repel magnetic fields.

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