|MadSci Network: Physics|
Hello, Craig. The type of question you're asking is extremely valuable to the pursuit of all sciences: what happens when we push a current idea to the extreme? Most ideas in science are applicable only if certain conditions are met, and trying to find the boundaries of these conditions is an important part of discovery. In your particular case, the question can be restated: what are the boundaries of the Meissner Effect? It turns out that superconductivity has a phase diagram, much like water does. [I'll explain what a phase diagram is in just a minute.] For a superconductor, applying too much magnetic field will eventually ruin the effect that causes superconductivity, thus destroying the Meissner Effect. So, as you bring a magnet closer and closer to a superconductor, eventually the magnetic field will be strong enough to destroy the superconductivity and wipe out the repulsive force. That's really the answer. To understand it fully, we would want to do two things: 1) describe what we mean by a phase diagram, and 2) explain why the phase diagram looks like it does. The first item isn't difficult to explain, so I'll go ahead and do that here. If you're interested in why a superconductor phase diagram looks the way it does, we'll have to save that for a later question (which is another way of saying that I don't know :) ) A phase diagram outlines the boundaries of specific phases that a material can achieve. Water, for example, can exist as in solid, liquid, or gas phases. Under what conditions in temperature and pressure will water take a particular form? That's what a phase diagram will tell us. We know from everyday experience that water will boil into a gas if it gets too hot, and will freeze into ice if it gets too cold. At a normal, everyday atmospheric pressure, water freezes at about 0 degrees C and boils at about 100 degrees C. This changes, however, if you change the pressure of the air around water. Up in the mountains, where the atmospheric pressure is lower, water boils at a much lower temperature - this actually makes cooking difficult for mountain climbers. A phase diagram outlines careful measurements which outline as closely as possible the actual boundaries of the phases. It will track the change in the temperature at which water boils (and freezes) as a function of pressure. This is important to your question because superconductivity is another phase of matter for some materials. There are even different phases within superconductivity! Therefore, there are boundaries beyond which the same material will stop being superconductors. Temperature is an important parameter for superconductors, just like for water. It turns out that magnetic fields are important, too. Just as changing the pressure around water will change boiling points, changing the magnetic field changes the maximum temperature at which a superconductor will stay a superconductor. For each temperature, there is a "critical magnetic field" beyond which a material cannot stay superconducting. The colder the superconductor is, the higher the critical field, but even if you cool the superconductor to near zero degrees, there will still be a field strength that can stop the superconductivity. Good job, and keep asking questions! - Guy PS Turns out there aren't that many easily-found examples of superconductor phase diagrams. Here's one: http://www- unix.mcs.anl.gov/superconductivity/phase.html For an introduction to superconductors, you can look at: http://superconductors.org/ or http://www.ame ricanmagnetics.com/tutorial/basics.html
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