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Physics Project 2011

Superconductivity Homework

on 6 September 2011

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Transcript of Superconductivity

Superconductivity Emma Fox 12BY What is superconductivity? Superconductivity is the flow of electric current without resistance in certain metals, alloys and ceramics at temperatures near absolute zero. Absolute zero is the temperature at which no more heat can be removed. This corresponds to 0K or -273.15°C. The temperature at which the transition to superconductivity occurs is known as the critical temperature, Tc, and each substance has a different one. Temperatures are given in Kelvins. An increase or decrease in 1 Kelvin is the same amount of temperature change as 1 degree Celcius, but 0K is the same as -273.15 degrees C. This avoids lots of negative numbers. Superconductivity was discovered by H. Kamerlingh Onnes at the University of Leiden, Netherlands in 1911. Just three years earlier, Onnes developed a method of liquifying helium gas, which provided a medium to experiment at supercool temperatures just above absolute zero. Whilst investigating the electrical resistance of mercury at low temperatures, he discovered that the resistance dropped to nothing at 4.2K. In 1913, Onnes was awarded the Nobel Prize in Physics for his work. Here are the critical temperatures of a few common substances: Material Critical temp.(K)
Aluminum 1.20
Cadmium 0.56
Lead 7.2
Mercury 4.16
Niobium 8.70
Thorium 1.37
Tin 3.72
Titanium 0.39
Uranium 1.0
Zinc 0.91
Niobium/Tin 18.1 The periodic table below shows the elements that are known to have superconducting properties... How does superconductivity work? Electrical resistance in metals is caused by electrons being scattered by either impurities or the vibrations in the lattice structure. In a superconductor, as a negatively charged electron moves between two rows of positively charged atoms, it pulls inwards on the atoms of the lattice. This distortion attracts a second electron to move in behind it, creating a 'Cooper pair'. Magnetism and Superconductivity Superconducting materials also interact in interesting ways with magnetic fields. While in the superconducting state, a superconducting material will tend to exclude all magnetic fields, a phenomenon known as the Meissner Effect The next video shows the Meissner Effect in action with a magnet hovering above a superconductor. In 1986, a breakthrough discovery was made in the field of superconductivity. Alex Müller and Georg Bednorz created a brittle ceramic compound that superconducted at the highest temperature then known: 30K. This was a remarkable discovery since ceramics are normally insulators, which don't conduct electricity and at low temperatures, they were behaving in not-yet-understood ways.

The two men won a Nobel Prize the following year for discovering the first superconducting copper-oxides. This discovery triggered a flurry of activity to experiment with more ceramics to find a higher critical temperature, Tc. In May 2009, the world record for Tc was set at 254K by a ceramic with a formula of (Tl4Ba)Ba2Ca2Cu7O13+. Applications of superconductivity One of the main uses of superconductivity at the moment is in Magnetic Resonance Imaging (MRI scanners). Magnets and radio waves are used to scan the body in a non-invasive procedure, helping to diagnose problems like multiple sclerosis, cancer and torn ligaments. Another use of superconductivity is Nuclear Magnetics Resonance (NMR). This is an analytical chemistry technique used in quality control. Superconductivity is used in particle accelerators that speed up a particle up to the speed of light, collide it with another particle and thereby discover its internal parts. Maglev trains also use superconductivity. Friction is minimised by powerful onboard magnets that suspend the train above the tracks. They are now in commercial development in Japan. MRI Scanners Hydrogen nuclei naturally spin on their own axis, but when they are caught in a strong magnetic field, the hydrogen nuclei line up, like compass needles, and spin on an aligned axis. If the protons are hit with short bursts of radio waves, they briefly spin around. When the nuclei return to their natural positions, they resound a radio signal of their own. The intensity of the responding radio signal reflects the numbers of hydrogen atoms in a given area. This can be built up into an image showing dense areas in white. The Future of Superconductivity? If conductor resistance could be eliminated entirely, there would be no power losses or inefficiencies in electric power systems. Electric motors could be made almost perfectly100% efficient. Components such as capacitors and inductors, whose ideal characteristics are normally spoiled by resistances, could be made ideal in a practical sense. Already, some practical superconducting conductors, motors, and capacitors have been developed, but their use at this present time is limited due to the practical problems in maintaining super-cold temperatures. Furthermore, superconducting materials will lose their superconductivity (no matter how cold you make them) if exposed to too strong of a magnetic field. In fact, the presence of any magnetic field tends to lower the critical temperature of any superconducting material: the more magnetic field present, the colder you have to make the material before it will superconduct.

This is another practical limitation to superconductors in circuit design, since electric current through any conductor produces a magnetic field. Even though a superconducting wire would have zero resistance to oppose current, there will still be a limit of how much current could practically go through that wire due to its critical magnetic field limit. Risks of MRI
Since the 1970s, there has been no evidence to suggest that the magnetic waves used during MRI scans pose any health risks.
There is also no evidence that MRI scans pose a risk during pregnancy. However, as a precaution, the use of MRI scans is not recommended during the first trimester of pregnancy.

There have been a number of accidents where unsecured metal objects, such as mops or oxygen cylinders, were pulled towards the MRI scanner when the magnetic field was turned on, resulting in the person in the scanner being injured. These types of accidents are known as projectile accidents.
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