PHY_10_51_V2_DM_Supercond
Оценка 4.6

# PHY_10_51_V2_DM_Supercond

Оценка 4.6
docx
07.05.2020
PHY_10_51_V2_DM_Supercond.docx

Questions:

What is superconductivity?

How do we know the resistance of a superconductor is zero?

What is a Critical Temperature?

Who discovered these phenomena?

 If mercury is cooled below 4.2 K, it loses all electric resistance - it becomes a superconductor.. This was made by H. Kammerlingh Onnes in 1911. Observations were then made that other metals also exhibit zero resistivity below a certain critical temperature.   How do we know the resistance of a superconductor is zero?   The fact that the resistance is zero has been shown by the observation that currents in superconducting lead rings remains constant for many years with no measurable reduction in value. Resistance to a current is rather like the friction experienced by a moving object. If there was no friction and you set an object moving it would theoretically continue at the same speed for ever. The same is true with current. Once you set the charge moving it would continue to move at a sateady rate ( constant current). This has been observed in supercooled metals - they become super conductors. An induced current in an ordinary metal ring would decay rapidly from the dissipation of heat energy resulting from ordinary resistance, but superconducting rings have exhibited a 'decay constant' for the current of over a billion years!   Critical Temperature   The critical temperature for a superconductor is the temperature at which the electrical resistivity of the substance drops to zero. The transition is so sudden and so complete that it appears to be a transition to a different phase of matter.. Several materials exhibit superconducting phase transitions at low temperatures. The highest critical temperature was about 23 K until the discovery in 1986 of some high temperature superconductors. The 'high' temperatures are still low (125K is considered 'very high'!). These materials with critical temperatures in the range 120 K have received a great deal of attention because they can be maintained in the superconducting state with liquid nitrogen (77 K).They can therefore have easier practical applications. The surprising thing about them is that they are not metals - but ceramics! -----------------------------------------------------

TASK 2: Applications of superconducting metals

Questions:

What is a superconducting magnet?

Applications of a superconducting magnet?

What is a Superconducting power cables

Applications of a Superconducting power cables

superconducting electromagnet is an electromagnet that is built using coils of superconducting wire. They must be cooled to cryogenic temperatures during operation. Their advantages are that they can produce stronger magnetic fields than ordinary iron-core electromagnets, and can be cheaper to operate, since no enegy is lost as heat because of ohmic resistance of the windings. During operation, the magnet windings must be cooled below their critical temperature; the temperature at which the winding material changes from the normal resistive state and becomes a superconductor. Liquid helium is used as a coolant for most superconductive coils.

Superconducting magnets are widely used in MRI machines, NMR equipment, mass spectrometers, magnetic separation processes, and particle accelerators.

They are preferred to ordinary electromagnets because:

·           They can achieve field that is 10x stronger than ordinary ferromagnetic-core electromagnets.

·           The field is generally more stable, resulting in less noisy measurements.

·           They can be smaller, and the area at the center of the magnet is empty rather than being occupied by an iron core.

·           Most importantly, for large magnets they can consume much less power. (Once set up and stable the only power the magnet consumes is that needed for any refrigeration equipment to preserve the cryogenic temperature).

Superconducting power cables

High-temperature superconductors promise to revolutionize power distribution by providing lossless transmission of electrical power.

The development of superconductors with transition temperatures higher than the boiling point of liquid nitrogen has made the concept of superconducting power lines commercially feasible, at least for high-load applications. It has been estimated that the waste would be halved using this method, since the necessary refrigeration equipment would consume about half the power saved by the elimination of the majority of resistive losses.

In one hypothetical future system called a Super Grid, the cost of cooling would be eliminated by coupling the transmission line with a liquid hydrogen pipeline.

Superconducting cables are particularly suited to high load density areas such as the business district of large cities, where purchase of an easement for cables (permission to put cables in has to be bought!) would be very costly.

Questions:

What is a maglev train?

How does it works?

Maglev-short for magnetic levitation trains can trace their roots to technology pioneered at Brookhaven National Laboratory. James Powell and Gordon Danby of Brookhaven received the first patent for a magnetically levitated train design in the late 1960s. The idea came to Powell as he sat in a traffic jam, thinking that there must be a better way to travel on land than cars or traditional trains. He dreamed up the idea of using superconducting magnets to levitate a train car. Superconducting magnets are electromagnets that are cooled to extreme temperatures during use, which dramatically increases the power of the magnetic field.

The first commercially operated high-speed superconducting maglev train opened in Shanghai in 2004, while others are in operation in Japan and South Korea. In the United States, a number of routes are being explored to connect cities such as Baltimore and Washington, D.C.

In maglev, superconducting magnets suspend a train car above a U-shaped concrete guideway. Like ordinary magnets, these magnets repel one another when matching poles face each other.

Credit: Carly Wilkins

"A maglev train car is just a box with magnets on the four corners," says Jesse Powell, the son of the maglev inventor, who now works with his father. It's a bit more complex than that, but the concept is simple. The magnets employed are superconducting, which means that when they are cooled to less than 450 degrees Fahrenheit below zero, they can generate magnetic fields up to 10 times stronger than ordinary electromagnets, enough to suspend and propel a train.

These magnetic fields interact with simple metallic loops set into the concrete walls of the maglev guideway. The loops are made of conductive materials, like aluminum, and when a magnetic field moves past, it creates an electric current that generates another magnetic field.

Three types of loops are set into the guideway at specific intervals to do three important tasks: one creates a field that makes the train hover about 5 inches above the guideway; a second keeps the train stable horizontally. Both loops use magnetic repulsion to keep the train car in the optimal spot; the further it gets from the center of the guideway or the closer to the bottom, the more magnetic resistance pushes it back on track.

The third set of loops is a propulsion system run by alternating current power. Here, both magnetic attraction and repulsion are used to move the train car along the guideway. Imagine the box with four magnets—one on each corner. The front corners have magnets with north poles facing out, and the back corners have magnets with south poles outward. Electrifying the propulsion loops generates magnetic fields that both pull the train forward from the front and push it forward from behind.

This floating magnet design creates a smooth trip. Even though the train can travel up to 375 miles per hour, a rider experiences less turbulence than on traditional steel wheel trains because the only source of friction is air.

Another big benefit is safety. Maglev trains are "driven" by the powered guideway. Any two trains traveling the same route cannot catch up and crash into one another because they're all being powered to move at the same speed. Similarly, traditional train derailments that occur because of cornering too quickly can't happen with maglev. The further a maglev train gets from its normal position between the guideway walls, the stronger the magnetic force pushing it back into place becomes.

This core feature is what's most exciting to Jesse Powell. "With maglev, there is no driver. The vehicles have to move where the network sends them. That's basic physics. So now that we have computer algorithms for routing things very efficiently, we could change the scheduling of the entire network on the fly. It leads to a much more flexible transportation system in the future," he said.

While this exciting technology isn't deployed in the United States today, if Powell and his team get their way, you could someday be floating your way to your next destination.

Multiple Choice questions: Superconductivity

 1.The current in a superconductor produces zero, voltage drop across it a small voltage drop across it a large voltage drop across it a strong electric field around it

 2.Superconductivity was first observed by Ohm Ampere H.K. Onnes Schrieffer

 3.Super conductivity is exhibited by hydrogen at 4.2 K mercury at 4.0 K mercury at 4.2 K potassium at 4.2 K

 4.The first successful theory on superconductivity was due to Schrieffer Onnes Ampere and Schrieffer Bardeen Cooper and Schrieffer

 5.At the critical temperature, the resistance of a super conductor increase rapidly decrease rapidly remains constant increase slowly

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