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Physicist Chuck Agosta and the students in his research lab study how a material's ability to conduct electricity is affected by changes in temperature, pressure and magnetic field. |
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The path of least resistance
Professor Chuck Agosta's research
Considering the cost of household electricity, it's discouraging to know that you don't use everything you pay for. That's because the copper wire commonly used to conduct electricity through a house is not as efficient as scientists would like it to be. Some of that electrical energy dissipates as heat before it ever reaches the lamp on your desk.
To use all the electricity we purchase, we'd need wire made from superconducting material. Physicist Chuck Agosta and his research team have pioneered a new technique for exploring the properties of these materials.
What are superconductors?
Superconductors are materials that, under specific environmental conditions, allow electricity to flow through them without resistance*. In other words, the amount of electricity coming out one end equals the amount that went in. No energy is lost as heat. The copper wire we currently use is rather like a leaky garden hose--some of the water dribbles out before it reaches the tree being watered. A superconductor would be more like an intact hose--all the water coming in one end reaches the tree at the other end.
Limitations of superconductors
Unfortunately, the superconductors developed to date can only exist in a super-cold environment, far below room temperature. This requirement makes them impractical to use in many situations and very difficult to study. In addition, the materials that currently lend themselves to becoming superconductors don't as yet have the flexibility that makes electrical wire so convenient. Often the materials developed are available only in very tiny quantities.
Despite these limitations, physicists like Chuck Agosta are excited about the future potential of superconductors. Currently, the most well known use of superconductors is in Magnetic Resonance Imaging (MRI), a technology used by hospitals for patient diagnosis. Scientists hope eventually to develop superconductors that can, for example, speed up computer circuitry or make possible frictionless transportation systems.
Frigid temperatures and magnetic fields at Clark
Since superconductors were first discovered in 1911, physicists have been creating new superconducting materials with varying properties, and trying to understand how they work at the atomic level. The physics laboratory at Clark has two pieces of equipment that make it an ideal place to study superconductivity. The first can produce the very cold temperatures conducive to superconductivity, while the second can create extremely intense magnetic fields, specifically, pulsed magnetic fields. This latter piece of equipment is important because physicists know that surrounding a superconductor with a magnetic field of high enough magnitude can actually destroy the superconductivity, even when the temperature stays constant.
A new way to study very tiny superconductors
Physicists often have to think creatively to devise methods for studying difficult questions. Agosta and his team recently published a paper announcing a new method of studying superconductivity in very tiny samples.
In order to study electrical conductivity, the scientist must be able to compare the input of electricity with the output after it passes through a material. Usually this is accomplished by creating an electrical circuit that involves mounting a lead on two sides of the material being studied, and then measuring directly the amount of electrical conductivity. However, the superconducting crystal (composed of carbon, sulphur, and selenium) being studied by Agosta's team is so tiny--about 1/10th of a millimeter, that the use of leads is not physically possible. So the team devised a way of measuring the electrical conductivity and magnetic properties of a superconductor using radio waves. Their method exploits an interesting property of any superconductor: its ability to create a "hole" in a magnetic field by creating an equal and opposite magnetic field inside the superconductor. (This property is called the Meissner Effect.)
Agosta's technique uses a tank circuit consisting of two components, an inductor and a capacitor. An inductor is simply a coil of wire that generates a magnetic field (mostly inside the coil) when electricity passes through the wire. A capacitor is a storage device. It stores electrical energy similar to the way a person might collect an electrical charge when walking across a rug on a dry day. The use of a capacitor and an inductor in combination creates electrical oscillations and, consequently, radio frequency waves inside the coil. The waves oscillate at a frequency that is determined by the capacitance and inductance values of the components.
When a superconducting material is placed inside the inductor, a "hole" is created in the coil's magnetic field, changing the frequency of the radio waves. When the external magnetic field is increased to a level called the critical magnetic field, it destroys the superconductivity of the material in the coil and the hole disappears. The magnetic field in the coil (inductor) can then assume its normal shape. Agosta measures the change in resonant frequency of the circuit to find the strength of the magnetic field at which superconductivity in the crystal is destroyed, and thus is able to compare different superconductors to see which ones are more robust.
* Resistance is a term physicists and electricians use to describe how easily a material allows for the passage of electricity through it. A material that electricity passes though easily is said to be characterized by low resistance.
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Additional Resources
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Destroying superconductivity by
increasing the magnetic field.
A) A tiny (about .01 mm) sample of superconducing material (indicated in pink) is placed in a coil.
B) An electrical current passes through the coil, generating a magnetic field. The superconductor creates a "hole" in the field.
C) When the strength of the magnetic field reaches a critical level, the superconductivity is destroyed and the "hole" disappears. |
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