Superconductors

2. What are superconductors?
 Some materials offer zero resistance to the flow of current below a specific
temperature. They are called superconductors.
 Mostly they are metals, alloys and ceramics.
 This specific temperature is called the critical temperature or the
transition temperature(at which the transition from normal to
superconducting state of the material takes place).
 This phenomenon is termed as superconductivity.

3. Discovery
 Super conductivity was first discovered in 1911 by the Dutch physicist, Heike
Kammerlingh Onnes.
 Onnes felt that a wire’s resistance would vanish if it is cooled too much. This
suggested that there would be a steady decrease in electrical resistance,
allowing for better conduction of electricity.
 Onnes passed a current through a pure mercury wire and measured its
resistance as he gradually decreased its temperature. To his surprise there
was little or no resistance at 4.2 K.

4. At 4.2 K the electrical resistance of mercury vanished, meaning
extremely good conduction of electricity that is now known as
Super conductivity.

6. General Properties of Super Conductors.
 Virtually Zero electrical resistance
 Effect of impurities: Critical temperature is lowered by the addition
of impurities.
 Magnetic field effect: Application of strong magnetic field beyond
critical value destroys the superconductivity resulting in the
transition to the normal state again.
 Phase transition: phase transition from normal to superconducting
state occurs when it is cooled below the critical temperature Tc

7. Meissner Effect

9. Reason for Meissner Effect:
It has been explained by the London equation which depicts that the magnetic
field inside the superconductor decreases exponentially than that around its
surface.
The applied magnetic field is expelled from the inside of the superconductor and
bent around it. These magnetic fields are expelled because under the influence
of a magnetic field, surface currents develop to create magnetization within the
superconductor. This magnetization is equal and opposite to the magnetic field,
resulting in cancelling out the magnetic field everywhere within the
superconductor. Superconductors are strongly diamagnetic. Since diamagnetics
have a magnetization that opposes any applied magnetic field, the
superconductor is repelled by the magnetic field. When a magnet is placed
above a superconductor, this repelling force can be stronger than gravity,
allowing the magnet to levitate above the superconductor.

10. Important Factors controlling
superconductivity
1. Critical Temperature: The temperature below which resistance of a
superconductor becomes zero.
2. Critical Magnetic Field: Minimum magnetic field required to destroy
the superconducting state
3. Critical Current density: The minimum amount of current that can
pass through a superconductor without transitioning it back to the
normal state.

11. Magnetic Levitation
 Magnetic levitation is a method by which an object is suspended with no
support other than the magnetic fields. Magnetic force is used to
counteract the effects of the gravitational field and any other
accelerations.
 The two main points in magnetic levitation are lifting force: so that the
object lifts upwards against gravity and stability: so that the object does
not flip or fly off
 Magnetic levitation is the result of Meissner effect.

13. Applications

14. Maglev Trains

15. Maglev trains
 Maglev is short for magnetic levitation.
 Maglev trains float over a guide way using the basic principles of magnets
to replace the old steel wheel and track trains.
 There is no physical contact between the train and the track, so the only
resistance offered to them is by air. Friction is greatly reduced and so these
trains can travel at very high speeds.
 Currently, Japan and South Korea have operational maglev systems.
 Their speed can be as much as 500 km/h
 They work on the principle of EDS and EMS.

16. Working:
Electrodynamic Suspension(EDS)
 This is the technique used by the Japanese system of maglev train.
 The train has superconductor electromagnets fitted in its base. The track
has electromagnetic coils. The sideways have electromagnets plus
propulsion and guidance coils.
 The reason for the levitation is the Meissner effect.

17.  To propel the vehicle, the electromagnets are placed on the sides of the
guide-way. They are energized according to the time when the train
reaches that particular spot and are de-energized the rest of the time.
 The train is pushed due to repulsion of like poles and is pulled due to
attraction of unlike poles. So the train moves forward. Now the next set of
pole comes into action and thereby a continuous motion is produced. The
polarity of the electromagnets is constantly changing because of an
alternating current. So the train moves in only one direction.

18.  Superconducting electromagnets are used as the coils of these
electromagnets can carry huge amount of current as the resistivity is
almost zero. Hence, the magnetic force is higher and hence higher speed
is achieved.
 Propulsion coils also provide guidance. The emf induced in coils on both
sides of the train(along the sideways) are equal in magnitude but opposite
in direction which cancel out each other. Hence, the train keeps moving in
the centre.

20. Disadvantage of EDS
At lower speeds (at the start of motion), levitation cannot be achieved and
therefore, wheels also have to be provided. When the speed is sufficient
enough to induce enough emf and produce magnetic field, then the train
levitates and wheels are not required then.

21. Thank you!