29-03-2011, 12:01 PM
SUBMITTED BY:-
GAURAV KUKRETI
[attachment=11238]
INTRODUCTION:-
The concept of superconductivity was first discovered by H.Kamerlingh Onnes in 1911.He discovered that the electrical resistivity of certain metals and alloys suddenly drops down to zero when their specimen are cooled to sufficiently low temperature called the critical temperature(Tc). This phenomenon is called superconductivity and the material showing such a behaviour is called a superconductor. This phenomenon was first discovered in mercury(Hg) and the critical temperature of Hg was found out to be 4.2 K. Different materials have different critical temperatures at which they become superconductors.For example lead(Au) has critical temperature of 7.25 K while niobium has a critical temperature of 9.2 K. An electric current set up in asuperconductor persists for a very long time. In traditional conductors like copper or aluminium the transmission is asscoiated with the wastage of energy due to resistance drop.Typical copper wire generators are only 50% efficient in generating electricity. Due to this there is loss of billions of dollars. Super conductivity is a solution to all those problems because the electrical resistance is zero. If the national grid were made of super conductors rather than Al, savings would be enormous. Super conducting electricity generators are about 99% efficient. Cellular transmission involving super conductors provides clearer signals as it improves range, receiver sensitivity and frequency stability and also decreases the costs of cellular phone service.
Super conductivity is referred as a "macroscopic quantum phenomenon”. In earlier days, it existed at low temperatures. Now it has crossed halfway from room temperature. Since these superconductors occur at high temperature than 0 degree K, these are called high temperature superconductors. Below Tc, superconducting materials exhibit two characteristic properties.
1. Zero electrical resistance
2. Perfect diamagnetism (The Meissner effect)
Zero electrical resistance means that no electricity. This has many applications. Energy is lost as heat as material conducts. Due to zero electrical resistance there is no wastage of energy. The second of these properties, perfect diamagnetism, means that the superconducting material will exclude a magnetic field-this is known as Meissner effect and can be used to display extraordinary physical effects. The Meissner effect is explained below.
MEISSNER EFFECT
The Meissner effect is the expulsion of a magnetic fieldfrom a superconductor during its transition to the superconducting state.This phenomenon was discovered by Meissner and Ochsenfeld in 1933. They found out that if a conductor is cooled in a magnetic field below the transition temperature(Tc) then at transition the lines of induction B are pushed out as shown in the below figure.
The above figure shows the Meissner effect. Magnetic field lines, represented as arrows, are excluded from a superconductor when it is below its critical temperature.Here ‘T’ and ‘Tc’ represent normal and critical temperatures respectively. Superconductors in the Meissner state exhibit perfect diamagnetism, or superdiamagnetism, meaning that the total magnetic field is very close to zero deep inside them (many penetration depths from the surface). This means that their magnetic susceptibility, χv
FUNDAMENTALS OF SUPERCONDUCTORS:
Superconductors have the ability to conduct electricity without the loss of energy. When current flows in an ordinary conductor,for example copper wire then some energy is lost. In a light bulb or electric heater, the electrical resistance creates light and heat.In metals such as copper and aluminium,electricity is conducted when outer energy level electrons migrate as individuals from one atom to another. These atoms form a vibrating lattice within the metal conductor,the warmer the metal the more it vibrates.As the electrons begin to move through the maze they collide with tiny impurities or imperfections in the lattice. When the electrons bump into these obstacles they fly off in all directions and lose energy in the form of heat.
Inside a superconductor the behavior of electrons is vastly different. The impurities and
lattice are still there, but the movement of the superconducting electrons through the
obstacle course is quite different. As the superconducting electrons travel through the
conductor they pass unobstructed through the complex lattice. Since they bump into nothing
and create no friction they can transmit electricity with no appreciable loss in the current and no loss of energy.
The understanding of superconductivity was advanced in 1957 by three American
physicists-John Bardeen, Leon Cooper, and John Schrieffer, through their Theories of
Superconductivity, know as the BCS Theory. The BCS theory explains superconductivity
at temperatures close to absolute zero. Cooper realized that atomic lattice vibrations
were directly responsible for unifying the entire current. They forced the electrons to pair
up into teams that could pass all of the obstacles which caused resistance in the
conductor. These teams of electrons are known as Cooper pairs. Electrons normally repel one another must feel an overwhelming attraction in superconductors. The answer to this problem was found to be in phonons, packets of sound waves present in the lattice as it vibrates. Although this lattice vibration cannot be heard, its role as a moderator is indispensable.
According to the theory, as one negatively charged electron passes by positively
charged ions in the lattice of the superconductor, the lattice distorts. This in turn causes
phonons to be emitted which forms a trough of positive charges around the electron.
Before the electron passes by and before the lattice springs back to its normal position,
a second electron is drawn into the trough. It is through this process that two electrons,
which should repel one another, page link up. The forces exerted by the phonons overcome
the electrons' natural repulsion. The electron pairs are coherent with one another as they
pass through the conductor in unison. The electrons are screened by the phonons and
are separated by some distance. When one of the electrons that make up a Cooper pair
and passes close to an ion in the crystal lattice, the attraction between the negative
electron and the positive ion cause a vibration to pass from ion to ion until the other
electron of the pair absorbs the vibration. The net effect is that the electron has emitted
a phonon and the other electron has absorbed the phonon. It is this exchange that keeps
the Cooper pairs together.
The electrons in the superconducting state are like an array of rapidly moving vehicles.
The BCS theory successfully shows that electrons can be attracted to one another
through interactions with the crystalline lattice. This occurs despite the fact that electrons
have the same charge. When the atoms of the lattice oscillate as positive and negative
regions, the electron pair is alternatively pulled together and pushed apart without a
collision. The electron pairing is favorable because it has the effect of putting the material
into a lower energy state. When electrons are linked together in pairs, they move through
the superconductor in an orderly fashion.
TYPES OF SUPERCONDUCTORS:
There are two types of superconductors, Type I and Type II. Very pure samples of lead,mercury
and tin are examples of Type I superconductors. The identifying characteristics are zero electrical resistivity below a critical temperature, zero internal magnetic field (Meissner effect), and a critical magnetic field above which superconductivity ceases. While important for understanding superconductivity, the Type I superconductors have been of limited practical usefulness since the critical magnetic fields are very small and the superconducting state disappears suddenly at that temperature. Type I superconductors are sometimes called "soft" superconductors.The below figure shows the graph of induced magnetic field of a Type I superconductor versus applied field. It shows that when an external magnetic field (horizontal abscissa) is applied to a Type I superconductor the induced magnetic field (vertical ordinate) exactly cancels that applied field until there is an abrupt change from the superconducting state to the normal state. Type I superconductors are very pure metals that typically have critical fields too low for use in superconducting magnets. Magnetic field strength is measured in units of gauss. The earths magnetic field is approximately 0.5 gauss.The strongest type-I superconductor, pure lead has a critical field of about 800 gauss. The unit of a gauss is a very small unit. A much larger unit of field strength is the tesla (T).
GAURAV KUKRETI
[attachment=11238]
INTRODUCTION:-
The concept of superconductivity was first discovered by H.Kamerlingh Onnes in 1911.He discovered that the electrical resistivity of certain metals and alloys suddenly drops down to zero when their specimen are cooled to sufficiently low temperature called the critical temperature(Tc). This phenomenon is called superconductivity and the material showing such a behaviour is called a superconductor. This phenomenon was first discovered in mercury(Hg) and the critical temperature of Hg was found out to be 4.2 K. Different materials have different critical temperatures at which they become superconductors.For example lead(Au) has critical temperature of 7.25 K while niobium has a critical temperature of 9.2 K. An electric current set up in asuperconductor persists for a very long time. In traditional conductors like copper or aluminium the transmission is asscoiated with the wastage of energy due to resistance drop.Typical copper wire generators are only 50% efficient in generating electricity. Due to this there is loss of billions of dollars. Super conductivity is a solution to all those problems because the electrical resistance is zero. If the national grid were made of super conductors rather than Al, savings would be enormous. Super conducting electricity generators are about 99% efficient. Cellular transmission involving super conductors provides clearer signals as it improves range, receiver sensitivity and frequency stability and also decreases the costs of cellular phone service.
Super conductivity is referred as a "macroscopic quantum phenomenon”. In earlier days, it existed at low temperatures. Now it has crossed halfway from room temperature. Since these superconductors occur at high temperature than 0 degree K, these are called high temperature superconductors. Below Tc, superconducting materials exhibit two characteristic properties.
1. Zero electrical resistance
2. Perfect diamagnetism (The Meissner effect)
Zero electrical resistance means that no electricity. This has many applications. Energy is lost as heat as material conducts. Due to zero electrical resistance there is no wastage of energy. The second of these properties, perfect diamagnetism, means that the superconducting material will exclude a magnetic field-this is known as Meissner effect and can be used to display extraordinary physical effects. The Meissner effect is explained below.
MEISSNER EFFECT
The Meissner effect is the expulsion of a magnetic fieldfrom a superconductor during its transition to the superconducting state.This phenomenon was discovered by Meissner and Ochsenfeld in 1933. They found out that if a conductor is cooled in a magnetic field below the transition temperature(Tc) then at transition the lines of induction B are pushed out as shown in the below figure.
The above figure shows the Meissner effect. Magnetic field lines, represented as arrows, are excluded from a superconductor when it is below its critical temperature.Here ‘T’ and ‘Tc’ represent normal and critical temperatures respectively. Superconductors in the Meissner state exhibit perfect diamagnetism, or superdiamagnetism, meaning that the total magnetic field is very close to zero deep inside them (many penetration depths from the surface). This means that their magnetic susceptibility, χv
FUNDAMENTALS OF SUPERCONDUCTORS:
Superconductors have the ability to conduct electricity without the loss of energy. When current flows in an ordinary conductor,for example copper wire then some energy is lost. In a light bulb or electric heater, the electrical resistance creates light and heat.In metals such as copper and aluminium,electricity is conducted when outer energy level electrons migrate as individuals from one atom to another. These atoms form a vibrating lattice within the metal conductor,the warmer the metal the more it vibrates.As the electrons begin to move through the maze they collide with tiny impurities or imperfections in the lattice. When the electrons bump into these obstacles they fly off in all directions and lose energy in the form of heat.
Inside a superconductor the behavior of electrons is vastly different. The impurities and
lattice are still there, but the movement of the superconducting electrons through the
obstacle course is quite different. As the superconducting electrons travel through the
conductor they pass unobstructed through the complex lattice. Since they bump into nothing
and create no friction they can transmit electricity with no appreciable loss in the current and no loss of energy.
The understanding of superconductivity was advanced in 1957 by three American
physicists-John Bardeen, Leon Cooper, and John Schrieffer, through their Theories of
Superconductivity, know as the BCS Theory. The BCS theory explains superconductivity
at temperatures close to absolute zero. Cooper realized that atomic lattice vibrations
were directly responsible for unifying the entire current. They forced the electrons to pair
up into teams that could pass all of the obstacles which caused resistance in the
conductor. These teams of electrons are known as Cooper pairs. Electrons normally repel one another must feel an overwhelming attraction in superconductors. The answer to this problem was found to be in phonons, packets of sound waves present in the lattice as it vibrates. Although this lattice vibration cannot be heard, its role as a moderator is indispensable.
According to the theory, as one negatively charged electron passes by positively
charged ions in the lattice of the superconductor, the lattice distorts. This in turn causes
phonons to be emitted which forms a trough of positive charges around the electron.
Before the electron passes by and before the lattice springs back to its normal position,
a second electron is drawn into the trough. It is through this process that two electrons,
which should repel one another, page link up. The forces exerted by the phonons overcome
the electrons' natural repulsion. The electron pairs are coherent with one another as they
pass through the conductor in unison. The electrons are screened by the phonons and
are separated by some distance. When one of the electrons that make up a Cooper pair
and passes close to an ion in the crystal lattice, the attraction between the negative
electron and the positive ion cause a vibration to pass from ion to ion until the other
electron of the pair absorbs the vibration. The net effect is that the electron has emitted
a phonon and the other electron has absorbed the phonon. It is this exchange that keeps
the Cooper pairs together.
The electrons in the superconducting state are like an array of rapidly moving vehicles.
The BCS theory successfully shows that electrons can be attracted to one another
through interactions with the crystalline lattice. This occurs despite the fact that electrons
have the same charge. When the atoms of the lattice oscillate as positive and negative
regions, the electron pair is alternatively pulled together and pushed apart without a
collision. The electron pairing is favorable because it has the effect of putting the material
into a lower energy state. When electrons are linked together in pairs, they move through
the superconductor in an orderly fashion.
TYPES OF SUPERCONDUCTORS:
There are two types of superconductors, Type I and Type II. Very pure samples of lead,mercury
and tin are examples of Type I superconductors. The identifying characteristics are zero electrical resistivity below a critical temperature, zero internal magnetic field (Meissner effect), and a critical magnetic field above which superconductivity ceases. While important for understanding superconductivity, the Type I superconductors have been of limited practical usefulness since the critical magnetic fields are very small and the superconducting state disappears suddenly at that temperature. Type I superconductors are sometimes called "soft" superconductors.The below figure shows the graph of induced magnetic field of a Type I superconductor versus applied field. It shows that when an external magnetic field (horizontal abscissa) is applied to a Type I superconductor the induced magnetic field (vertical ordinate) exactly cancels that applied field until there is an abrupt change from the superconducting state to the normal state. Type I superconductors are very pure metals that typically have critical fields too low for use in superconducting magnets. Magnetic field strength is measured in units of gauss. The earths magnetic field is approximately 0.5 gauss.The strongest type-I superconductor, pure lead has a critical field of about 800 gauss. The unit of a gauss is a very small unit. A much larger unit of field strength is the tesla (T).
