The Meissner Effect

1
The Meissner Effect

• An unintuitive property of supercondctors

2
Outline

• What is the Meissner Effect?
• Superconductors as diamagnets.
• Meissner effect in perfect conductors?
• Meissner effect in superconductors.

3
The Meissner Effect

• A diamagnetic property exhibited by
  superconductors.
• End result is the exclusion of magnetic field
  from the interior of a superconductor.
• What is diamagnetism?

4
Diamagnetism?

• A superconductor is not only a perfect conductor
  (R0), but a perfect diamagnet.
• It will tend to repel a magnet.

5
So, Superconductors are Perfect Diamagnets?

• If a superconductor was only a perfect conductor,
  would there be a Meissner Effect?
• Recall Faradays Law of Induction.

6
Faradays Law of Induction
7
Faradays Law of Induction

• A change in magnetic flux will induce an emf in a
  conductor.
• There will be no induced emf if the magnetic flux
  is constant with respect to time.

8
The Minus Sign

• What does the minus sign imply physically?
• The direction of the induced emf will be such
  that the magnetic field produced by the induced
  emf resists the change in magnetic flux.
• The presence of the minus sign is referred to as
  Lenzs Law

9
Lenzs Law

• If the magnetic flux is decreasing out of the
  page, which way will the induced emf be directed?
  (Note the induced emf has the same direction as
  the induced current.)

10

• The direction of the induced emf (or current),
  will be counterclockwise.
• This will generate an induced magnetic field out
  of the page, counteracting the decrease in flux.
  (Found from the right-hand rule for current
  carrying wires.)

11
Perfect Conductor

• Move this perfect conductor into a magnetic
  field.
• By Faradays Law of Induction, a current is
  induced.
• The magnetic field generated by this current
  would oppose the change of the applied field.

12

• How long will the induced current flow? Recall P
  I2R.
• The induced current would flow indefinitely.
  There is no I2R power loss. The induced magnetic
  field will continue to oppose the change in the
  applied field.
• Conversely, if the conductor is in a magnetic
  field which is then removed, an induced current
  and corresponding magnetic field would tend to
  oppose the removal of the applied field.

13
What Do We See?

• Would a magnet levitate over the surface of a
  perfect conductor?
• No, if a magnet is placed on top of a material
  which becomes a perfect conductor, there would be
  no effect on the magnet.
• There would only be an opposing force if the
  magnet was removed.

14
In Superconductors

• Faradays Law does not explain magnetic repulsion
  by superconductors.
• Below its critical temperature (Tc) a
  superconductor does not allow any magnetic field
  to enter it.

15

• Circulating currents on the surface of the
  superconductor induce microscopic magnetic
  dipoles that oppose the applied field.
• The induced field repels the applied field, and
  the magnet associated with it.
• If a magnet is on top of a superconductor as it
  is cooled below its Tc, it would exclude the
  magnetic field of the magnet.

16
The Result
17
References

• Image 1 http//www.physics.ubc.ca/outreach/p420_
  97/bruce/ybco.html
• Image 2 http//www.sci.kun.nl/hfml/froglev.html
• Image 3 http//physicsweb.org/article/world/11/12
  /6
• Image 4 http//www.phys.warwick.ac.uk/supermag/Re
  search/Superconductors/body_superconductors.html